selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 93 insertions, 0 deletions

diff --git a/drivers/md/dm-vdo/thread-registry.c b/drivers/md/dm-vdo/thread-registry.c
new file mode 100644
index 000000000..d4a077d58
--- /dev/null
+++ b/drivers/md/dm-vdo/thread-registry.c
@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2023 Red Hat
+ */
+
+#include "thread-registry.h"
+
+#include <asm/current.h>
+#include <linux/rculist.h>
+
+#include "permassert.h"
+
+/*
+ * We need to be careful when using other facilities that may use thread registry functions in
+ * their normal operation. For example, we do not want to invoke the logger while holding a lock.
+ */
+
+void vdo_initialize_thread_registry(struct thread_registry *registry)
+{
+	INIT_LIST_HEAD(&registry->links);
+	spin_lock_init(&registry->lock);
+}
+
+/* Register the current thread and associate it with a data pointer. */
+void vdo_register_thread(struct thread_registry *registry,
+			 struct registered_thread *new_thread, const void *pointer)
+{
+	struct registered_thread *thread;
+	bool found_it = false;
+
+	INIT_LIST_HEAD(&new_thread->links);
+	new_thread->pointer = pointer;
+	new_thread->task = current;
+
+	spin_lock(&registry->lock);
+	list_for_each_entry(thread, &registry->links, links) {
+		if (thread->task == current) {
+			/* There should be no existing entry. */
+			list_del_rcu(&thread->links);
+			found_it = true;
+			break;
+		}
+	}
+	list_add_tail_rcu(&new_thread->links, &registry->links);
+	spin_unlock(&registry->lock);
+
+	VDO_ASSERT_LOG_ONLY(!found_it, "new thread not already in registry");
+	if (found_it) {
+		/* Ensure no RCU iterators see it before re-initializing. */
+		synchronize_rcu();
+		INIT_LIST_HEAD(&thread->links);
+	}
+}
+
+void vdo_unregister_thread(struct thread_registry *registry)
+{
+	struct registered_thread *thread;
+	bool found_it = false;
+
+	spin_lock(&registry->lock);
+	list_for_each_entry(thread, &registry->links, links) {
+		if (thread->task == current) {
+			list_del_rcu(&thread->links);
+			found_it = true;
+			break;
+		}
+	}
+	spin_unlock(&registry->lock);
+
+	VDO_ASSERT_LOG_ONLY(found_it, "thread found in registry");
+	if (found_it) {
+		/* Ensure no RCU iterators see it before re-initializing. */
+		synchronize_rcu();
+		INIT_LIST_HEAD(&thread->links);
+	}
+}
+
+const void *vdo_lookup_thread(struct thread_registry *registry)
+{
+	struct registered_thread *thread;
+	const void *result = NULL;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(thread, &registry->links, links) {
+		if (thread->task == current) {
+			result = thread->pointer;
+			break;
+		}
+	}
+	rcu_read_unlock();
+
+	return result;
+}