Messages in this thread Patch in this message | | | From | Mark Rutland <> | Subject | [PATCH] KEYS: fix refcount_inc() on zero | Date | Fri, 26 May 2017 18:37:34 +0100 |
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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 <mark.rutland@arm.com> Cc: 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> --- 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
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