[PATCH bpf v1 7/8] bpf: Reject untrusted allocated-object pointers

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

From: Ning Ding <dingning04@gmail.com>

When the final RCU read-side critical section ends, a local kptr is demoted
to PTR_UNTRUSTED but retains MEM_ALLOC. The pointer may be NULL or may refer
to an object whose lifetime is no longer protected.

type_is_ptr_alloc_obj() nevertheless recognizes any PTR_TO_BTF_ID with
MEM_ALLOC as a live allocated object. In particular, a refcount-only local
kptr never carries NON_OWN_REF, so it still passes the
bpf_refcount_acquire() argument check after RCU protection ends. The kfunc
can then dereference NULL or stale memory.

Make type_is_ptr_alloc_obj() reject PTR_UNTRUSTED pointers. Since
type_is_non_owning_ref() is based on the same predicate, graph kfunc
arguments obey the same live-object requirement. Fault-protected reads of
the demoted pointer remain valid: writes are already rejected, and read
fixups use bpf_may_fault_on_deref() rather than this predicate.

Fixes: 1b12171533a9 ("bpf: Mark direct ld of stashed bpf_{rb,list}_node as non-owning ref")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Ning Ding <dingning04@gmail.com>
[ kkd: Rewrote commit log ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf_verifier.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 1339c2f028db..36b65797877d 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1381,7 +1381,9 @@ 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_non_owning_ref(u32 type)
-- 
2.53.0