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: 627d2d6b ("udp: enable MSG_PEEK at non-zero offset")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>