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

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>

2 files changed, 95 insertions, 1 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5c0e6809024f..62377bcb66fd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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);
 
diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index e526315c718a..79a328276805 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -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";