Commit 92347cfd authored Jun 08, 2017 by Mark Rutland Committed by James Morris Jun 09, 2017

KEYS: fix refcount_inc() on zero



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.

Fixes: fff29291 ("security, keys: convert key.usage from atomic_t to refcount_t")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: David Windsor <dwindsor@gmail.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Hans Liljestrand <ishkamiel@gmail.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

parent 7cbe0932

security/keys/key.c

+4 −7

Original line number	Diff line number	Diff line
		@@ -660,14 +660,11 @@ not_found:
		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
		/* A key is allowed to be looked up only if someone still owns a
		* reference to it - otherwise it's awaiting the gc.
		*/
		__key_get(key);
		if (!refcount_inc_not_zero(&key->usage))
		goto not_found;

		error:
		spin_unlock(&key_serial_lock);

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