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Registered in England and Wales under Company Registration No. 3798903 Subject: [PATCH 03/23] KEYS: fix refcount_inc() on zero From: David Howells To: jmorris@namei.org Cc: Mark Rutland , Kees Cook , Peter Zijlstra , linux-kernel@vger.kernel.org, dhowells@redhat.com, linux-security-module@vger.kernel.org, keyrings@vger.kernel.org, James Morris , Hans Liljestrand , Elena Reshetova , David Windsor Date: Thu, 08 Jun 2017 14:47:41 +0100 Message-ID: <149692966132.11452.11344589657708880218.stgit@warthog.procyon.org.uk> In-Reply-To: <149692963884.11452.7673998701432248814.stgit@warthog.procyon.org.uk> References: <149692963884.11452.7673998701432248814.stgit@warthog.procyon.org.uk> User-Agent: StGit/0.17.1-dirty MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 08 Jun 2017 13:47:44 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Mark Rutland 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 --- security/keys/key.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-) 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);