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Subject[PATCH 4.9 12/78] kcm: do not attach PF_KCM sockets to avoid deadlock
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Eric Dumazet <>

[ 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 <>
Reported-by: Dmitry Vyukov <>
Acked-by: Tom Herbert <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/kcm/kcmsock.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1381,6 +1381,10 @@ 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;

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