xfrm: espintcp: fix UAF during close

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>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>