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

[Mark Rutland]

On Wed, May 31, 2017 at 01:20:44PM +0100, David Howells wrote:
> 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.

Fair enough, on all counts.

> I've adjusted your patch - see attached.

That looks fine to me, thanks!

Mark.

> 
> 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);