Re: [PATCH net v2] l2tp: use sk_dst_check() to avoid race on sk->sk_dst_cache

[David Miller <davem@davemloft.net>]

From: Wei Wang <weiwan@google.com>
Date: Fri, 10 Aug 2018 11:14:56 -0700

> From: Wei Wang <weiwan@google.com>
> 
> In l2tp code, if it is a L2TP_UDP_ENCAP tunnel, tunnel->sk points to a
> UDP socket. User could call sendmsg() on both this tunnel and the UDP
> socket itself concurrently. As l2tp_xmit_skb() holds socket lock and call
> __sk_dst_check() to refresh sk->sk_dst_cache, while udpv6_sendmsg() is
> lockless and call sk_dst_check() to refresh sk->sk_dst_cache, there
> could be a race and cause the dst cache to be freed multiple times.
> So we fix l2tp side code to always call sk_dst_check() to garantee
> xchg() is called when refreshing sk->sk_dst_cache to avoid race
> conditions.
> 
> Syzkaller reported stack trace:
 ...
> 
> Fixes: 71b1391a4128 ("l2tp: ensure sk->dst is still valid")
> Reported-by: syzbot+05f840f3b04f211bad55@syzkaller.appspotmail.com
> Signed-off-by: Wei Wang <weiwan@google.com>
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
 ...
> ---
> v1->v2: Removed dst_clone() as Guillaume Nault suggested

Applied and queued up for -stable, thank you.