[PATCH bpf v4 1/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[Qi Tang <tpluszz77@gmail.com>]

check_mem_access() matches PTR_TO_BUF via base_type() which strips
PTR_MAYBE_NULL, allowing direct dereference without a null check.

Map iterator ctx->key and ctx->value are PTR_TO_BUF | PTR_MAYBE_NULL.
On stop callbacks these are NULL, causing a kernel NULL dereference.

Add a type_may_be_null() guard to the PTR_TO_BUF branch, matching the
existing PTR_TO_BTF_ID pattern.

Fixes: 20b2aff4bc15 ("bpf: Introduce MEM_RDONLY flag")
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 937c9adf1b3d..17850836943b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7905,7 +7905,8 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 	} else if (reg->type == CONST_PTR_TO_MAP) {
 		err = check_ptr_to_map_access(env, regs, regno, off, size, t,
 					      value_regno);
-	} else if (base_type(reg->type) == PTR_TO_BUF) {
+	} else if (base_type(reg->type) == PTR_TO_BUF &&
+		   !type_may_be_null(reg->type)) {
 		bool rdonly_mem = type_is_rdonly_mem(reg->type);
 		u32 *max_access;
 
-- 
2.43.0