Patch "af-unix: fix use-after-free with concurrent readers while splicing" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

af-unix: fix use-after-free with concurrent readers while splicing

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
af-unix-fix-use-after-free-with-concurrent-readers-while-splicing.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hannes Frederic Sowa <[email protected]>
Date: Tue, 10 Nov 2015 16:23:15 +0100
Subject: af-unix: fix use-after-free with concurrent readers while splicing

From: Hannes Frederic Sowa <[email protected]>

[ Upstream commit 73ed5d25dce0354ea381d6dc93005c3085fae03d ]

During splicing an af-unix socket to a pipe we have to drop all af-unix socket locks. While doing so we allow another reader to enter unix_stream_read_generic which can read, copy and finally free another skb. If exactly this skb is just in process of being spliced we get a use-after-free report by kasan.

First, we must make sure to not have a free while the skb is used during the splice operation. We simply increment its use counter before unlocking the reader lock.

Stream sockets have the nice characteristic that we don't care about zero length writes and they never reach the peer socket's queue. That said, we can take the UNIXCB.consumed field as the indicator if the skb was already freed from the socket's receive queue. If the skb was fully consumed after we locked the reader side again we know it has been dropped by a second reader. We indicate a short read to user space and abort the current splice operation.

This bug has been found with syzkaller
(http://github.com/google/syzkaller) by Dmitry Vyukov.

---
 net/unix/af_unix.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

@@ -440,6 +440,7 @@ static void unix_release_sock(struct soc
 		if (state == TCP_LISTEN)
 			unix_release_sock(skb->sk, 1);
 		/* passed fds are erased in the kfree_skb hook	      */
+		UNIXCB(skb).consumed = skb->len;
 		kfree_skb(skb);
 	}
 

@@ -2071,6 +2072,7 @@ static int unix_stream_read_generic(stru
 
 	do {
 		int chunk;
+		bool drop_skb;
 		struct sk_buff *skb, *last;
 
 		unix_state_lock(sk);

@@ -2151,7 +2153,11 @@ unlock:
 		}
 
 		chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
+		skb_get(skb);
 		chunk = state->recv_actor(skb, skip, chunk, state);
+		drop_skb = !unix_skb_len(skb);
+		/* skb is only safe to use if !drop_skb */
+		consume_skb(skb);
 		if (chunk < 0) {
 			if (copied == 0)
 				copied = -EFAULT;

@@ -2160,6 +2166,18 @@ unlock:
 		copied += chunk;
 		size -= chunk;
 
+		if (drop_skb) {
+			/* the skb was touched by a concurrent reader;
+			 * we should not expect anything from this skb
+			 * anymore and assume it invalid - we can be
+			 * sure it was dropped from the socket queue
+			 *
+			 * let's report a short read
+			 */
+			err = 0;
+			break;
+		}
+
 		/* Mark read part of skb as used */
 		if (!(flags & MSG_PEEK)) {
 			UNIXCB(skb).consumed += chunk;

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/ipv6-add-complete-rcu-protection-around-np-opt.patch queue-4.3/ipv6-check-rt-dst.from-for-the-dst_nocache-route.patch queue-4.3/af-unix-passcred-support-for-sendpage.patch queue-4.3/af_unix-don-t-append-consumed-skbs-to-sk_receive_queue.patch queue-4.3/ipv6-avoid-creating-rtf_cache-from-a-rt-that-is-not-managed-by-fib6-tree.patch queue-4.3/net-scm-fix-pax-detected-msg_controllen-overflow-in-scm_detach_fds.patch queue-4.3/ipv6-check-expire-on-dst_nocache-route.patch queue-4.3/af-unix-fix-use-after-free-with-concurrent-readers-while-splicing.patch queue-4.3/ip_tunnel-disable-preemption-when-updating-per-cpu-tstats.patch queue-4.3/af_unix-take-receive-queue-lock-while-appending-new-skb.patch