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

From: Wei Wang <redacted>
Date: Fri, 10 Aug 2018 11:14:56 -0700

From: Wei Wang <redacted>

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 <redacted>
Signed-off-by: Martin KaFai Lau <redacted>

 ...

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v1->v2: Removed dst_clone() as Guillaume Nault suggested

Applied and queued up for -stable, thank you.