seccomp: don't leak memory when filter install races

In seccomp_set_mode_filter() with TSYNC | NEW_LISTENER, we first initialize
the listener fd, then check to see if we can actually use it later in
seccomp_may_assign_mode(), which can fail if anyone else in our thread
group has installed a filter and caused some divergence. If we can't, we
partially clean up the newly allocated file: we put the fd, put the file,
but don't actually clean up the *memory* that was allocated at
filter->notif. Let's clean that up too.

To accomplish this, let's hoist the actual "detach a notifier from a
filter" code to its own helper out of seccomp_notify_release(), so that in
case anyone adds stuff to init_listener(), they only have to add the
cleanup code in one spot. This does a bit of extra locking and such on the
failure path when the filter is not attached, but it's a slow failure path
anyway.

Fixes: 7f9dda3b2c2f ("seccomp: allow TSYNC and USER_NOTIF together")
Reported-by: syzbot+3ad9614a12f80994c32e@syzkaller.appspotmail.com
Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200902014017.934315-1-tycho@tycho.pizza
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 3ee59ce0a323dcad450961a2de31845646f57c1f..bb0dd9ae699a4dd0b252f947de761a36a08194d1 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1109,13 +1109,12 @@ out:
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
-static int seccomp_notify_release(struct inode *inode, struct file *file)
+static void seccomp_notify_detach(struct seccomp_filter *filter)
 {
-       struct seccomp_filter *filter = file->private_data;
        struct seccomp_knotif *knotif;
 
        if (!filter)
-               return 0;
+               return;
 
        mutex_lock(&filter->notify_lock);
 
@@ -1142,6 +1141,13 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
        kfree(filter->notif);
        filter->notif = NULL;
        mutex_unlock(&filter->notify_lock);
+}
+
+static int seccomp_notify_release(struct inode *inode, struct file *file)
+{
+       struct seccomp_filter *filter = file->private_data;
+
+       seccomp_notify_detach(filter);
        __put_seccomp_filter(filter);
        return 0;
 }
@@ -1581,6 +1587,7 @@ out_put_fd:
                        listener_f->private_data = NULL;
                        fput(listener_f);
                        put_unused_fd(listener);
+                       seccomp_notify_detach(prepared);
                } else {
                        fd_install(listener, listener_f);
                        ret = listener;