ZDI reported and analyzed a race condition during close for espintcp
sockets:

    espintcp_close() frees emsg->skb via kfree_skb() without holding
    any socket lock. Concurrently, the xfrm_trans_reinject work queue
    invokes esp_output_tcp_finish() -> espintcp_push_skb() ->
    espintcp_push_msgs() -> skb_send_sock_locked(), which reads the
    same skb as a data source.

Fix this by adding a synchronize_rcu() call after resetting sk_prot,
since esp_output_tcp_finish() runs under RCU and won't use a socket
with sk_prot == &tcp_prot.  Simply taking the socket lock in
espintcp_close() could lead to leaks, if esp_output_tcp_finish()
re-adds an skb in the slot we just freed.

Cc: stable@vger.kernel.org
Fixes: e27cca96cd68 ("xfrm: add espintcp (RFC 8229)")
Reported-by: zdi-disclosures@trendmicro.com
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
---
 net/xfrm/espintcp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/xfrm/espintcp.c b/net/xfrm/espintcp.c
index 374e1b964438..f09b5dd85db8 100644
--- a/net/xfrm/espintcp.c
+++ b/net/xfrm/espintcp.c

@@ -517,6 +517,8 @@ static void espintcp_close(struct sock *sk, long timeout)
 	sk->sk_prot = &tcp_prot;
 	barrier();
 
+	synchronize_rcu();
+
 	disable_work_sync(&ctx->work);
 	strp_done(&ctx->strp);
 

-- 
2.54.0