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

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

    From: Eric Dumazet <edumazet@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 <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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    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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