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. After this, the existing
barrier() is no longer needed.

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>
---
v2: remove the unnecessary barrier()
v1: https://lore.kernel.org/netdev/50e2ab4348eb8177581058f0152394cfae6a8d27.1783071494.git.sd@queasysnail.net/ (local)

 net/xfrm/espintcp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/espintcp.c b/net/xfrm/espintcp.c
index 374e1b964438..cd817b855ba1 100644
--- a/net/xfrm/espintcp.c
+++ b/net/xfrm/espintcp.c

@@ -515,7 +515,8 @@ static void espintcp_close(struct sock *sk, long timeout)
 	strp_stop(&ctx->strp);
 
 	sk->sk_prot = &tcp_prot;
-	barrier();
+
+	synchronize_rcu();
 
 	disable_work_sync(&ctx->work);
 	strp_done(&ctx->strp);

-- 
2.54.0