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

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

From: Tom Herbert <t...@quantonium.net>

commit e5571240236c5652f3e079b1d5866716a7ad819c upstream.

This is needed to prevent sk_user_data being overwritten.
The check is done under the callback lock. This should prevent
a socket from being attached twice to a KCM mux. It also prevents
a socket from being attached for other use cases of sk_user_data
as long as the other cases set sk_user_data under the lock.
Followup work is needed to unify all the use cases of sk_user_data
to use the same locking.

Reported-by: syzbot+114b15f2be420a888...@syzkaller.appspotmail.com
Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Tom Herbert <t...@quantonium.net>
Reviewed-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 net/kcm/kcmsock.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1405,9 +1405,18 @@ static int kcm_attach(struct socket *soc
                return err;
        }
 
-       sock_hold(csk);
-
        write_lock_bh(&csk->sk_callback_lock);
+
+       /* Check if sk_user_data is aready by KCM or someone else.
+        * Must be done under lock to prevent race conditions.
+        */
+       if (csk->sk_user_data) {
+               write_unlock_bh(&csk->sk_callback_lock);
+               strp_done(&psock->strp);
+               kmem_cache_free(kcm_psockp, psock);
+               return -EALREADY;
+       }
+
        psock->save_data_ready = csk->sk_data_ready;
        psock->save_write_space = csk->sk_write_space;
        psock->save_state_change = csk->sk_state_change;
@@ -1415,8 +1424,11 @@ static int kcm_attach(struct socket *soc
        csk->sk_data_ready = psock_data_ready;
        csk->sk_write_space = psock_write_space;
        csk->sk_state_change = psock_state_change;
+
        write_unlock_bh(&csk->sk_callback_lock);
 
+       sock_hold(csk);
+
        /* Finished initialization, now add the psock to the MUX. */
        spin_lock_bh(&mux->lock);
        head = &mux->psocks;


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