[PATCH net-next 2/2] kcm: Check if sk_user_data already set in kcm_attach

[Tom Herbert]

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

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 474ea2200592..5ae02a5b0838 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1414,9 +1414,18 @@ static int kcm_attach(struct socket *sock, struct socket 
*csock,
                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;
@@ -1424,8 +1433,11 @@ static int kcm_attach(struct socket *sock, struct socket 
*csock,
        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;
-- 
2.11.0