kcm: do not attach PF_KCM sockets to avoid deadlock

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 <edumazet@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index da49191f7ad0d7edfda0c10762ef5f7471a8f6ac..4abf6287d7e1c29314db5c846acd16c3a2a377db 100644 (file)
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1383,6 +1383,10 @@ 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;