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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: fff292914d ("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>
zero-colors
Mark Rutland 2017-06-08 14:47:41 +01:00 committed by James Morris
parent 7cbe0932c2
commit 92347cfd62
1 changed files with 4 additions and 7 deletions
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11
security/keys/key.c
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@ -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);