[PATCH 5.13 229/300] bpf: Fix possible out of bound write in narrow load handling

Greg Kroah-Hartman
2021-09-13T14:16:37Z

From: Andrey Ignatov <rdna@fb.com>

[ Upstream commit d7af7e497f0308bc97809cc48b58e8e0f13887e1 ]

Fix a verifier bug found by smatch static checker in [0].

This problem has never been seen in prod to my best knowledge. Fixing it
still seems to be a good idea since it's hard to say for sure whether
it's possible or not to have a scenario where a combination of
convert_ctx_access() and a narrow load would lead to an out of bound
write.

When narrow load is handled, one or two new instructions are added to
insn_buf array, but before it was only checked that

cnt >= ARRAY_SIZE(insn_buf)

And it's safe to add a new instruction to insn_buf[cnt++] only once. The
second try will lead to out of bound write. And this is what can happen
if `shift` is set.

Fix it by making sure that if the BPF_RSH instruction has to be added in
addition to BPF_AND then there is enough space for two more instructions
in insn_buf.

The full report [0] is below:

kernel/bpf/verifier.c:12304 convert_ctx_accesses() warn: offset 'cnt' incremented past end of array
kernel/bpf/verifier.c:12311 convert_ctx_accesses() warn: offset 'cnt' incremented past end of array

kernel/bpf/verifier.c
12282
12283 insn->off = off & ~(size_default - 1);
12284 insn->code = BPF_LDX | BPF_MEM | size_code;
12285 }
12286
12287 target_size = 0;
12288 cnt = convert_ctx_access(type, insn, insn_buf, env->prog,
12289 &target_size);
12290 if (cnt == 0 || cnt >= ARRAY_SIZE(insn_buf) ||
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Bounds check.

12291 (ctx_field_size && !target_size)) {
12292 verbose(env, "bpf verifier is misconfigured\n");
12293 return -EINVAL;
12294 }
12295
12296 if (is_narrower_load && size < target_size) {
12297 u8 shift = bpf_ctx_narrow_access_offset(
12298 off, size, size_default) * 8;
12299 if (ctx_field_size <= 4) {
12300 if (shift)
12301 insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
^^^^^
increment beyond end of array

12302 insn->dst_reg,
12303 shift);
--> 12304 insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
^^^^^
out of bounds write

12305 (1 << size * 8) - 1);
12306 } else {
12307 if (shift)
12308 insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH,
12309 insn->dst_reg,
12310 shift);
12311 insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
^^^^^^^^^^^^^^^
Same.

12312 (1ULL << size * 8) - 1);
12313 }
12314 }
12315
12316 new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
12317 if (!new_prog)
12318 return -ENOMEM;
12319
12320 delta += cnt - 1;
12321
12322 /* keep walking new program and skip insns we just inserted */
12323 env->prog = new_prog;
12324 insn = new_prog->insnsi + i + delta;
12325 }
12326
12327 return 0;
12328 }

[0] https://lore.kernel.org/bpf/20210817050843.GA21456@kili/

v1->v2:
- clarify that problem was only seen by static checker but not in prod;

Fixes: 46f53a65d2de ("bpf: Allow narrow loads with offset > 0")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210820163935.1902398-1-rdna@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/verifier.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c07126558bb7..d810f9e0ed9d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11982,6 +11982,10 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
if (is_narrower_load && size < target_size) {
u8 shift = bpf_ctx_narrow_access_offset(
off, size, size_default) * 8;
+ if (shift && cnt + 1 >= ARRAY_SIZE(insn_buf)) {
+ verbose(env, "bpf verifier narrow ctx load misconfigured\n");
+ return -EINVAL;
+ }
if (ctx_field_size <= 4) {
if (shift)
insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
--
2.30.2