If a key's refcount is dropped to zero between key_lookup() peeking at
the refcount and subsequently attempting to increment it, refcount_inc()
will see a zero refcount. Here, refcount_inc() will WARN_ONCE(), and
will *not* increment the refcount, which will remain zero.
Once key_lookup() drops key_serial_lock, it is possible for the key to
be freed behind our back.
This patch uses refcount_inc_not_zero() to perform the peek and
increment atomically. A helper with lockdep annotation is added to
document why this is safe.
Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to
refcount_t")
Signed-off-by: Mark Rutland <[email protected]>
Cc: David Howells <[email protected]>
Cc: David Windsor <[email protected]>
Cc: Elena Reshetova <[email protected]>
Cc: Hans Liljestrand <[email protected]>
Cc: James Morris <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Peter Zijlstra <[email protected]>
---
security/keys/key.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/security/keys/key.c b/security/keys/key.c
index 455c04d..150f51d 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -632,6 +632,12 @@ void key_put(struct key *key)
}
EXPORT_SYMBOL(key_put);
+static bool key_get_not_free(struct key *key)
+{
+ lockdep_assert_held(&key_serial_lock);
+ return refcount_inc_not_zero(&key->usage);
+}
+
/*
* Find a key by its serial number.
*/
@@ -660,14 +666,12 @@ struct key *key_lookup(key_serial_t id)
goto error;
found:
- /* pretend it doesn't exist if it is awaiting deletion */
- if (refcount_read(&key->usage) == 0)
- goto not_found;
-
- /* this races with key_put(), but that doesn't matter since key_put()
- * doesn't actually change the key
+ /*
+ * Pretend it doesn't exist if it is awaiting deletion. This races with
+ * key_put(), but we can peek at the key until we drop key_serial_lock.
*/
- __key_get(key);
+ if (!key_get_not_free(key))
+ goto not_found;
error:
spin_unlock(&key_serial_lock);
--
1.9.1
