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