[PATCH] KEYS: fix refcount_inc() on zero
From: Kees Cook <hidden>
Date: 2017-05-26 19:05:57
Also in:
keyrings, lkml
On Fri, May 26, 2017 at 10:37 AM, Mark Rutland [off-list ref] wrote:
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 <redacted>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Hans Liljestrand <redacted>
Cc: James Morris <redacted>
Cc: Kees Cook <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>Thanks for catching this! Reviewed-by: Kees Cook <redacted> -Kees
quoted hunk ↗ jump to hunk
--- 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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