From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751161AbdEaMkn (ORCPT ); Wed, 31 May 2017 08:40:43 -0400 Received: from foss.arm.com ([217.140.101.70]:43568 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751038AbdEaMkm (ORCPT ); Wed, 31 May 2017 08:40:42 -0400 Date: Wed, 31 May 2017 13:39:57 +0100 From: Mark Rutland To: David Howells Cc: keyrings@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, David Windsor , Elena Reshetova , Hans Liljestrand , James Morris , Kees Cook , Peter Zijlstra Subject: Re: [PATCH] KEYS: fix refcount_inc() on zero Message-ID: <20170531123957.GA10351@leverpostej> References: <1495820254-6651-1-git-send-email-mark.rutland@arm.com> <26155.1496233244@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <26155.1496233244@warthog.procyon.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 31, 2017 at 01:20:44PM +0100, David Howells wrote: > Mark Rutland 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 > 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 > Signed-off-by: David Howells > Cc: David Windsor > Cc: Elena Reshetova > Cc: Hans Liljestrand > Cc: James Morris > Cc: Kees Cook > Cc: Peter Zijlstra > > 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);