kcm: Check if sk_user_data already set in kcm_attach

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+114b15f2be420a8886c3@syzkaller.appspotmail.com
Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Tom Herbert <tom@quantonium.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 2 deletions

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 7632797fb68e..4a8d407f8902 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1410,9 +1410,18 @@ 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;
@@ -1420,8 +1429,11 @@ 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;