[PATCH 4.12 21/52] kcm: do not attach PF_KCM sockets to avoid deadlock

[Greg Kroah-Hartman]

4.12-stable review patch.  If anyone has any objections, please let me know.

------------------

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


[ 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 <eduma...@google.com>
Reported-by: Dmitry Vyukov <dvyu...@google.com>
Acked-by: Tom Herbert <t...@quantonium.net>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/kcm/kcmsock.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1383,6 +1383,10 @@ static int kcm_attach(struct socket *soc
        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;