bpf: check_cond_jmp_op(): properly infer if register is null

Nicholas Carlini reported a bug when verifier can incorrectly infer
that a pointer is non-null. The bug occurs when two pointers are
compared and one of them has a type w/o PTR_MAYBE_NULL flag,
but which allows a value to be NULL at runtime.
Here is an example:

  // `a` is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED
  // `a` is 0 at runtime.
  // `b` is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL
  void *a = bpf_rdonly_cast(0, 0);
  int  *b = bpf_map_lookup_elem(...);

  if (a == b)
    *b = 42;  // verifier does not catch null pointer dereference

This happens because of a special case in check_cond_jmp_op(),
which attempts to strip PTR_MAYBE_NULL flags from pointer types,
when processing comparisons like `rA == rB`, if either rA or rB can't
be null.

The non-null property is derived based on the absence of
PTR_MAYBE_NULL flag on rA's or rB's type. But that is not sufficient
for types like PTR_TO_MEM, as in the example.

This patch replaces type_may_be_null() call with reg_not_null(),
which contains an allowlist of types for which absence of
PTR_MAYBE_NULL actually means that the value can't be NULL at runtime.

At the moment, the list in the reg_not_null() omits two types for
which PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF.
In order to remain backward compatible, and assuming that only
comparison between pointers of the same type makes sense,
this commit extends reg_not_null(). W/o such an extension e.g.
verifier_jeq_infer_not_null/null_ptr_to_map_value fails.

reg_not_null() can be extended further, but I deem that out of scope
for the fix at hand. Explicit base_type(...) != PTR_TO_BTF_ID
checks in the check_cond_jmp_op() can be removed with migration to
reg_not_null(), but that is a behavioural change, as the special case
would start matching for PTR_TO_BTF_ID that is also is_trusted_reg().
I omit the behavioural change from this commit.

Fixes: befae75856ab ("bpf: propagate nullness information for reg to reg comparisons")
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260826-bug-029-bad-non-null-inference-v2-1-136789ace9e9@localhost
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5e37ca75e5c4..e64035683795 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -355,6 +355,8 @@ static bool reg_not_null(struct bpf_verifier_env *env, const struct bpf_reg_stat
 	type = base_type(type);
 	return type == PTR_TO_SOCKET ||
 		type == PTR_TO_TCP_SOCK ||
+		type == PTR_TO_XDP_SOCK ||
+		type == PTR_TO_BUF ||
 		type == PTR_TO_MAP_VALUE ||
 		type == PTR_TO_MAP_KEY ||
 		type == PTR_TO_SOCK_COMMON ||
@@ -16968,7 +16970,6 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	 */
 	if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X &&
 	    __is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) &&
-	    type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type) &&
 	    base_type(src_reg->type) != PTR_TO_BTF_ID &&
 	    base_type(dst_reg->type) != PTR_TO_BTF_ID) {
 		eq_branch_regs = NULL;
@@ -16984,9 +16985,11 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 			break;
 		}
 		if (eq_branch_regs) {
-			if (type_may_be_null(src_reg->type))
+			/* src == dst && dst != NULL => src != NULL */
+			if (reg_not_null(env, dst_reg) && type_may_be_null(src_reg->type))
 				mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]);
-			else
+			/* src == dst && src != NULL => dst != NULL */
+			if (reg_not_null(env, src_reg) && type_may_be_null(dst_reg->type))
 				mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]);
 		}
 	}