[PATCH net-next] tcp: drop skb dst in tcp_rcv_established()

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

In commit f84af32cbca7 ("net: ip_queue_rcv_skb() helper")
I dropped the skb dst in tcp_data_queue().

This only dealt with so-called TCP input slow path.

When fast path is taken, tcp_rcv_established() calls
tcp_queue_rcv() while skb still has a dst.

This was mostly fine, because most dsts at this point
are not refcounted (thanks to early demux)

However, TCP packets sent over loopback have refcounted dst.

Then commit 68822bdf76f1 ("net: generalize skb freeing
deferral to per-cpu lists") came and had the effect
of delaying skb freeing for an arbitrary time.

If during this time the involved netns is dismantled, cleanup_net()
frees the struct net with embedded net->ipv6.ip6_dst_ops.

Then when eventually dst_destroy_rcu() is called,
if (dst->ops->destroy) ... triggers an use-after-free.

It is not clear if ip6_route_net_exit() lacks a rcu_barrier()
as syzbot reported similar issues before the blamed commit.

( https://groups.google.com/g/syzkaller-bugs/c/CofzW4eeA9A/m/009WjumTAAAJ )

Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp_input.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cc3de8dc57970c97316ad1591cac0ca5f1a24c47..97cfcd85f84e6f873c3e60c388e6c27628451a7d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5928,6 +5928,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);
 
 			/* Bulk data transfer: receiver */
+			skb_dst_drop(skb);
 			__skb_pull(skb, tcp_header_len);
 			eaten = tcp_queue_rcv(sk, skb, &fragstolen);
 
-- 
2.36.0.464.gb9c8b46e94-goog