[RFC PATCH v2 01/11] KEYS: Add key_gc_flush() for draining RCU callbacks

[Jarkko Sakkinen <jarkko@kernel.org>]

keyring_clear() and key_invalidate() release keyring links via call_rcu(),
and key_put() defers final key destruction to key_gc_work.

Once live keys pin the key type until key_gc_unused_keys(), callers that
clear an internal cache before module unload need a synchronous barrier so
module references are dropped before delete_module() checks the refcount.

Add key_gc_flush() for draining RCU callbacks and flush key_gc_work until
no further garbage collection passes remain queued.

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
 include/linux/key.h |  2 ++
 security/keys/gc.c  | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/key.h b/include/linux/key.h
index 420412aab926..0f408255f740 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -439,6 +439,7 @@ extern int restrict_link_reject(struct key *keyring,
 				struct key *restriction_key);
 
 extern int keyring_clear(struct key *keyring);
+void key_gc_flush(void);
 
 extern key_ref_t keyring_search(key_ref_t keyring,
 				struct key_type *type,
@@ -526,6 +527,7 @@ extern void key_init(void);
 #define key_free_user_ns(ns)		do { } while(0)
 #define key_remove_domain(d)		do { } while(0)
 #define key_lookup(k)			NULL
+#define key_gc_flush()			do { } while (0)
 
 #endif /* CONFIG_KEYS */
 #endif /* __KERNEL__ */
diff --git a/security/keys/gc.c b/security/keys/gc.c
index eda445f815d4..3cfbf00085b3 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -21,6 +21,20 @@ unsigned key_gc_delay = 5 * 60;
 static void key_garbage_collector(struct work_struct *work);
 DECLARE_WORK(key_gc_work, key_garbage_collector);
 
+/**
+ * key_gc_flush - Wait for pending key unlinks and garbage collection.
+ *
+ * Drain RCU callbacks from keyring edits and wait for the key garbage
+ * collector work item to finish reaping unreferenced keys.
+ */
+void key_gc_flush(void)
+{
+	do {
+		rcu_barrier();
+	} while (flush_work(&key_gc_work));
+}
+EXPORT_SYMBOL_GPL(key_gc_flush);
+
 /*
  * Reaper for links from keyrings to dead keys.
  */
-- 
2.47.3