[PATCH bpf-next v4 3/4] bpf: Reject untrusted allocated-object pointers

[Ning Ding <dingning04@gmail.com>]

type_is_ptr_alloc_obj() currently treats PTR_UNTRUSTED pointers as valid
allocated objects. This allows a pointer that is no longer protected by
RCU to be passed to bpf_refcount_acquire(). If the object was freed and
its address reused, the verifier may acquire a reference through stale
memory.

Make type_is_ptr_alloc_obj() reject PTR_UNTRUSTED. Add
type_is_untrusted_ptr_alloc_obj() for the read path, where these pointers
are still allowed for BPF_PROBE_MEM reads. This keeps checks strict
without breaking safe reads.

Fixes: 1b12171533a9 ("bpf: Mark direct ld of stashed bpf_{rb,list}_node as non-owning ref")
Reported-by: sashiko-bot@kernel.org
Link: https://lore.kernel.org/r/20260726021304.97ED91F000E9@smtp.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Ning Ding <dingning04@gmail.com>
---
 include/linux/bpf_verifier.h | 11 ++++++++++-
 kernel/bpf/verifier.c        |  8 +++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 27b43fda9b17..22fcd01c3cce 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1341,7 +1341,16 @@ static inline bool bpf_type_has_unsafe_modifiers(u32 type)
 
 static inline bool type_is_ptr_alloc_obj(u32 type)
 {
-	return base_type(type) == PTR_TO_BTF_ID && type_flag(type) & MEM_ALLOC;
+	return base_type(type) == PTR_TO_BTF_ID &&
+	       type_flag(type) & MEM_ALLOC &&
+	       !(type_flag(type) & PTR_UNTRUSTED);
+}
+
+static inline bool type_is_untrusted_ptr_alloc_obj(u32 type)
+{
+	return base_type(type) == PTR_TO_BTF_ID &&
+	       type_flag(type) & MEM_ALLOC &&
+	       type_flag(type) & PTR_UNTRUSTED;
 }
 
 static inline bool type_is_non_owning_ref(u32 type)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bc021c8c8fbf..c3a11c98cd45 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5820,7 +5820,13 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 			return -EACCES;
 		}
 
-		if (type_is_alloc(reg->type) && !type_is_non_owning_ref(reg->type) &&
+		/*
+		 * Skip this referenced-ID sanity check for untrusted allocated objects;
+		 * the access check above already rejects writes through them.
+		 */
+		if (type_is_alloc(reg->type) &&
+		    !type_is_untrusted_ptr_alloc_obj(reg->type) &&
+		    !type_is_non_owning_ref(reg->type) &&
 		    !(reg->type & MEM_RCU) && !reg_is_referenced(env, reg)) {
 			verifier_bug(env, "allocated object must have a referenced id");
 			return -EFAULT;
-- 
2.43.0