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Merge branch 'bpf-fix-unsound-scalar-forking-for-bpf_or'

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Daniel Wade says:

====================
bpf: Fix unsound scalar forking for BPF_OR

maybe_fork_scalars() unconditionally sets the pushed path dst register
to 0 for both BPF_AND and BPF_OR.  For AND this is correct (0 & K == 0),
but for OR it is wrong (0 | K == K, not 0).  This causes the verifier to
track an incorrect value on the pushed path, leading to a verifier/runtime
divergence that allows out-of-bounds map value access.

v4: Use block comment style for multi-line comments in selftests (Amery Hung)
    Add Reviewed-by/Acked-by tags
v3: Use single-line comment style in selftests (Alexei Starovoitov)
v2: Use push_stack(env, env->insn_idx, ...) to re-execute the insn
    on the pushed path (Eduard Zingerman)
====================

Link: https://patch.msgid.link/20260314021521.128361-1-danjwade95@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Alexei Starovoitov 2026-03-21 13:14:28 -07:00
commit 06880982c6
2 changed files with 95 additions and 1 deletions
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2
kernel/bpf/verifier.c
View File

@ -15999,7 +15999,7 @@ static int maybe_fork_scalars(struct bpf_verifier_env *env, struct bpf_insn *ins
else
return 0;
branch = push_stack(env, env->insn_idx + 1, env->insn_idx, false);
branch = push_stack(env, env->insn_idx, env->insn_idx, false);
if (IS_ERR(branch))
return PTR_ERR(branch);

94
tools/testing/selftests/bpf/progs/verifier_bounds.c
View File

@ -2037,4 +2037,98 @@ __naked void signed_unsigned_intersection32_case2(void *ctx)
: __clobber_all);
}
SEC("socket")
__description("maybe_fork_scalars: OR with constant rejects OOB")
__failure __msg("invalid access to map value")
__naked void or_scalar_fork_rejects_oob(void)
{
asm volatile (" \
r1 = 0; \
*(u64*)(r10 - 8) = r1; \
r2 = r10; \
r2 += -8; \
r1 = %[map_hash_8b] ll; \
call %[bpf_map_lookup_elem]; \
if r0 == 0 goto l0_%=; \
r9 = r0; \
r6 = *(u64*)(r9 + 0); \
r6 s>>= 63; \
r6 |= 8; \
/* r6 is -1 (current) or 8 (pushed) */ \
if r6 s< 0 goto l0_%=; \
/* pushed path: r6 = 8, OOB for value_size=8 */ \
r9 += r6; \
r0 = *(u8*)(r9 + 0); \
l0_%=: r0 = 0; \
exit; \
" :
: __imm(bpf_map_lookup_elem),
__imm_addr(map_hash_8b)
: __clobber_all);
}
SEC("socket")
__description("maybe_fork_scalars: AND with constant still works")
__success __retval(0)
__naked void and_scalar_fork_still_works(void)
{
asm volatile (" \
r1 = 0; \
*(u64*)(r10 - 8) = r1; \
r2 = r10; \
r2 += -8; \
r1 = %[map_hash_8b] ll; \
call %[bpf_map_lookup_elem]; \
if r0 == 0 goto l0_%=; \
r9 = r0; \
r6 = *(u64*)(r9 + 0); \
r6 s>>= 63; \
r6 &= 4; \
/* \
* r6 is 0 (pushed, 0&4==0) or 4 (current) \
* both within value_size=8 \
*/ \
if r6 s< 0 goto l0_%=; \
r9 += r6; \
r0 = *(u8*)(r9 + 0); \
l0_%=: r0 = 0; \
exit; \
" :
: __imm(bpf_map_lookup_elem),
__imm_addr(map_hash_8b)
: __clobber_all);
}
SEC("socket")
__description("maybe_fork_scalars: OR with constant allows in-bounds")
__success __retval(0)
__naked void or_scalar_fork_allows_inbounds(void)
{
asm volatile (" \
r1 = 0; \
*(u64*)(r10 - 8) = r1; \
r2 = r10; \
r2 += -8; \
r1 = %[map_hash_8b] ll; \
call %[bpf_map_lookup_elem]; \
if r0 == 0 goto l0_%=; \
r9 = r0; \
r6 = *(u64*)(r9 + 0); \
r6 s>>= 63; \
r6 |= 4; \
/* \
* r6 is -1 (current) or 4 (pushed) \
* pushed path: r6 = 4, within value_size=8 \
*/ \
if r6 s< 0 goto l0_%=; \
r9 += r6; \
r0 = *(u8*)(r9 + 0); \
l0_%=: r0 = 0; \
exit; \
" :
: __imm(bpf_map_lookup_elem),
__imm_addr(map_hash_8b)
: __clobber_all);
}
char _license[] SEC("license") = "GPL";