Subject: [PATCH 12/25] kcm: do not attach PF_KCM sockets to avoid deadlock

From: Eric Dumazet <[email-masked]>
Date: Wed, 30 Aug 2017 09:29:31 -0700

[ Upstream commit 351050ecd6523374b370341cc29fe61e2201556b ]

syzkaller had no problem to trigger a deadlock, attaching a KCM socket
to another one (or itself). (original syzkaller report was a very
confusing lockdep splat during a sendmsg())

It seems KCM claims to only support TCP, but no enforcement is done,
so we might need to add additional checks.

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: Dmitry Vyukov <[email-masked]>
Acked-by: Tom Herbert <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/kcm/kcmsock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index fecad1098cf8..7eb0e8fe3ca8 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1381,6 +1381,10 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 	if (!csk)
 		return -EINVAL;
 
+	/* We must prevent loops or risk deadlock ! */
+	if (csk->sk_family == PF_KCM)
+		return -EOPNOTSUPP;
+
 	psock = kmem_cache_zalloc(kcm_psockp, GFP_KERNEL);
 	if (!psock)
 		return -ENOMEM;
-- 
2.13.5