[PATCH net] kcm: do not attach PF_KCM sockets to avoid deadlock

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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 <eduma...@google.com>
Reported-by: Dmitry Vyukov <dvyu...@google.com>
---
 net/kcm/kcmsock.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 
48e993b2dbcf1afae04968ed840e2e98c2cf6772..af4e76ac88ff0817398d1d7460a41f0cd5fe6f30
 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1387,6 +1387,10 @@ 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;