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,