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bpf: Reject untrusted allocated-object pointers

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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: 1b12171533 ("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>
Link: https://lore.kernel.org/r/20260904084325.52250-8-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Ning Ding 2026-09-04 10:43:20 +02:00 committed by Alexei Starovoitov
parent 6668ed271e
commit 7441ee8276
1 changed files with 3 additions and 1 deletions
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4
include/linux/bpf_verifier.h
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@ -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)