[PATCH bpf v2 8/8] selftests/bpf: Reject refcount acquisition after RCU unlock

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

From: Ning Ding <dingning04@gmail.com>

Add a sleepable verifier test that loads a refcount-only local kptr in an
explicit RCU read-side critical section, ends the section, and passes the
pointer to bpf_refcount_acquire().

The loaded pointer never carries NON_OWN_REF. After RCU unlock it retains
MEM_ALLOC while becoming PTR_UNTRUSTED, which previously made the kfunc
argument check accept it as a live allocated object. Expect verification to
reject the untrusted argument instead.

Signed-off-by: Ning Ding <dingning04@gmail.com>
[ kkd: Rewrote commit log ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../bpf/progs/refcounted_kptr_fail.c          | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
index f787ecf189d8..338e43822ffe 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
@@ -189,6 +189,33 @@ long rbtree_remove_after_rcu_unlock(void *ctx)
 	return 0;
 }
 
+SEC("?syscall")
+__failure __msg("R1 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 *node, *ref;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_refcount_only, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_rcu_read_lock();
+	node = mapval->node;
+	if (!node) {
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+	bpf_rcu_read_unlock();
+
+	ref = bpf_refcount_acquire(node);
+	if (ref)
+		bpf_obj_drop(ref);
+
+	return 0;
+}
+
 SEC("?syscall")
 __failure __msg("invalid mem access 'scalar'")
 long graph_kptr_after_spin_unlock(void *ctx)
-- 
2.53.0