[PATCH bpf v3 4/4] selftests/bpf: Test untrusted allocated-object pointers

[Ning Ding]

The verifier previously allowed pointers used after RCU protection ended
to reach bpf_refcount_acquire() and, for one object layout, a direct write.
If the object was freed and reused, these operations could access stale
memory.

Add tests that keep BPF_PROBE_MEM reads accepted but reject reference
acquisition and direct writes after RCU protection ends. Cover both tested
object layouts.

Reported-by: sashiko-bot@xxxxxxxxxx
Link: https://lore.kernel.org/r/20260726021304.97ED91F000E9@xxxxxxxxxxxxxxx
Assisted-by: Codex:gpt-5
Signed-off-by: Ning Ding <dingning04@xxxxxxxxx>
---
 .../selftests/bpf/progs/refcounted_kptr.c     | 100 ++++++++++++++++++
 .../bpf/progs/refcounted_kptr_fail.c          |  27 +++++
 2 files changed, 127 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
index fd35093285c0d..b70be8b52ff80 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
@@ -893,6 +893,106 @@ long refcount_acquire_rcu_map_kptr_null_checked(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__success
+long map_kptr_read_after_rcu_unlock(void *ctx)
+{
+	struct map_value_refcount_only *mapval;
+	struct node_refcount_only *n;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_refcount_only, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+	bpf_rcu_read_unlock();
+
+	return n->key;
+}
+
+SEC("?tc")
+__failure __msg("is neither owning or non-owning ref")
+long refcount_acquire_graph_after_rcu_unlock(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data *n, *m;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_nodes, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+	bpf_rcu_read_unlock();
+
+	m = bpf_refcount_acquire(n);
+	if (m)
+		bpf_obj_drop(m);
+
+	return 0;
+}
+
+SEC("?tc")
+__failure __msg("only read is supported")
+long graph_map_kptr_write_after_rcu_unlock(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data *n;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_nodes, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	bpf_rcu_read_unlock();
+
+	n->key = 1;
+	return 0;
+}
+
+SEC("?tc")
+__success
+long graph_map_kptr_read_after_spin_unlock(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data *n;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_nodes, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+	bpf_rcu_read_unlock();
+
+	bpf_spin_lock(&lock);
+	bpf_spin_unlock(&lock);
+
+	return n->key;
+}
+
 static long __stash_map_empty_xchg(struct node_data *n, int idx)
 {
 	struct map_value *mapval = bpf_map_lookup_elem(&stashed_nodes, &idx);
diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
index acd3e81a39168..3408f68ad444d 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
@@ -127,6 +127,33 @@ long refcount_acquire_rcu_map_kptr_unchecked_drop(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__failure __msg("is neither owning or non-owning ref")
+long refcount_acquire_after_rcu_unlock(void *ctx)
+{
+	struct map_value_refcount_only *mapval;
+	struct node_refcount_only *n, *m;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_refcount_only, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	bpf_rcu_read_unlock();
+
+	m = bpf_refcount_acquire(n);
+	if (m)
+		bpf_obj_drop(m);
+
+	return 0;
+}
+
 SEC("?tc")
 __failure __msg("Unreleased reference id=3 alloc_insn={{[0-9]+}}")
 long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
-- 
2.43.0