Patch "udp: on peeking bad csum, drop packets even if not at head" has been added to the 4.12-stable tree

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

udp: on peeking bad csum, drop packets even if not at head

to the 4.12-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:
udp-on-peeking-bad-csum-drop-packets-even-if-not-at-head.patch
and it can be found in the queue-4.12 subdirectory.

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

From: Eric Dumazet <[email protected]>
Date: Tue, 22 Aug 2017 09:39:28 -0700
Subject: udp: on peeking bad csum, drop packets even if not at head

From: Eric Dumazet <[email protected]>

[ Upstream commit fd6055a806edc4019be1b9fb7d25262599bca5b1 ]

When peeking, if a bad csum is discovered, the skb is unlinked from the queue with __sk_queue_drop_skb and the peek operation restarted.

__sk_queue_drop_skb only drops packets that match the queue head.

This fails if the skb was found after the head, using SO_PEEK_OFF socket option. This causes an infinite loop.

We MUST drop this problematic skb, and we can simply check if skb was already removed by another thread, by looking at skb->next :

This pointer is set to NULL by the __skb_unlink() operation, that might have happened only under the spinlock protection.

Many thanks to syzkaller team (and particularly Dmitry Vyukov who provided us nice C reproducers exhibiting the lockup) and Willem de Bruijn who provided first version for this patch and a test program.

---
 net/core/datagram.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -345,7 +345,7 @@ int __sk_queue_drop_skb(struct sock *sk,
 	if (flags & MSG_PEEK) {
 		err = -ENOENT;
 		spin_lock_bh(&sk->sk_receive_queue.lock);
-		if (skb == skb_peek(&sk->sk_receive_queue)) {
+		if (skb->next) {
 			__skb_unlink(skb, &sk->sk_receive_queue);
 			atomic_dec(&skb->users);
 			if (destructor)

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

queue-4.12/ipv6-fix-typo-in-fib6_net_exit.patch queue-4.12/ipv6-fix-sparse-warning-on-rt6i_node.patch queue-4.12/udp-on-peeking-bad-csum-drop-packets-even-if-not-at-head.patch queue-4.12/ipv6-add-rcu-grace-period-before-freeing-fib6_node.patch queue-4.12/kcm-do-not-attach-pf_kcm-sockets-to-avoid-deadlock.patch