Re: [PATCH] KEYS: fix refcount_inc() on zero

From: David Howells
Date: Wed May 31 2017 - 08:20:53 EST


Mark Rutland <mark.rutland@xxxxxxx> 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@xxxxxxx>
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@xxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Cc: David Windsor <dwindsor@xxxxxxxxx>
Cc: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Cc: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Cc: James Morris <james.l.morris@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

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);