From fd6055a806edc4019be1b9fb7d25262599bca5b1 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 22 Aug 2017 09:39:28 -0700 Subject: [PATCH] udp: on peeking bad csum, drop packets even if not at head 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. Fixes: 627d2d6b5500 ("udp: enable MSG_PEEK at non-zero offset") Signed-off-by: Eric Dumazet Reported-by: Dmitry Vyukov Cc: Willem de Bruijn Acked-by: Paolo Abeni Acked-by: Willem de Bruijn Signed-off-by: David S. Miller --- net/core/datagram.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/core/datagram.c b/net/core/datagram.c index a21ca8dee5ea..8c2f4489ff8f 100644 --- a/net/core/datagram.c +++ b/net/core/datagram.c @@ -362,7 +362,7 @@ int __sk_queue_drop_skb(struct sock *sk, struct sk_buff_head *sk_queue, if (flags & MSG_PEEK) { err = -ENOENT; spin_lock_bh(&sk_queue->lock); - if (skb == skb_peek(sk_queue)) { + if (skb->next) { __skb_unlink(skb, sk_queue); refcount_dec(&skb->users); if (destructor) -- 2.30.2