From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Fri, 26 May 2017 18:37:34 +0100 Subject: [PATCH] KEYS: fix refcount_inc() on zero Message-ID: <1495820254-6651-1-git-send-email-mark.rutland@arm.com> To: linux-security-module@vger.kernel.org List-Id: linux-security-module.vger.kernel.org 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 Cc: David Howells Cc: David Windsor Cc: Elena Reshetova Cc: Hans Liljestrand Cc: James Morris Cc: Kees Cook Cc: Peter Zijlstra --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html