linux-kernel - [PATCH 4.13 63/85] FS-Cache: fix dereference of NULL user_key

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Date:   Tue, 24 Oct 2017 15:07:37 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, James Morris <james.l.morris@...cle.com>,
        Eric Biggers <ebiggers@...gle.com>,
        David Howells <dhowells@...hat.com>
Subject: [PATCH 4.13 63/85] FS-Cache: fix dereference of NULL user_key_payload

4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@...gle.com>

commit d124b2c53c7bee6569d2a2d0b18b4a1afde00134 upstream.

When the file /proc/fs/fscache/objects (available with
CONFIG_FSCACHE_OBJECT_LIST=y) is opened, we request a user key with
description "fscache:objlist", then access its payload.  However, a
revoked key has a NULL payload, and we failed to check for this.
request_key() *does* skip revoked keys, but there is still a window
where the key can be revoked before we access its payload.

Fix it by checking for a NULL payload, treating it like a key which was
already revoked at the time it was requested.

Fixes: 4fbf4291aa15 ("FS-Cache: Allow the current state of all objects to be dumped")
Reviewed-by: James Morris <james.l.morris@...cle.com>
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/fscache/object-list.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/fscache/object-list.c
+++ b/fs/fscache/object-list.c
@@ -330,6 +330,13 @@ static void fscache_objlist_config(struc
 	rcu_read_lock();

 	confkey = user_key_payload_rcu(key);
+	if (!confkey) {
+		/* key was revoked */
+		rcu_read_unlock();
+		key_put(key);
+		goto no_config;
+	}
+
 	buf = confkey->data;

 	for (len = confkey->datalen - 1; len >= 0; len--) {