Messages in this thread Patch in this message | | | From | David Howells <> | Subject | Re: [PATCH] KEYS: fix refcount_inc() on zero | Date | Wed, 31 May 2017 13:20:44 +0100 |
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Mark Rutland <mark.rutland@arm.com> wrote:
> This patch uses refcount_inc_not_zero() to perform the peek and > increment atomically.
I think the helper is unnecessary. Better to adjust the comment if you really want to explain it. Anyone editing the code should be that this is inside a critical section.
> A helper with lockdep annotation is added to document why this is safe.
This doesn't explain why this is safe.
> + /* > + * 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. > */
With your change, there is no race with key_put() - so the second sentence is unnecessary.
I've adjusted your patch - see attached.
David --- commit f66bf831c45306ebbc28aecd407e238983457251 Author: Mark Rutland <mark.rutland@arm.com> Date: Fri May 26 18:37:34 2017 +0100
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: fff292914d3a2f1e ("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>
diff --git a/security/keys/key.c b/security/keys/key.c index 455c04d80bbb..d84ee2a87da6 100644 --- a/security/keys/key.c +++ b/security/keys/key.c @@ -660,14 +660,11 @@ 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 + /* 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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