FAILED: patch "[PATCH] media: dvb-core: Fix UAF due to refcount races at releasing" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

fd3d91ab1c6a ("media: dvb-core: Fix UAF due to refcount races at releasing")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From fd3d91ab1c6ab0628fe642dd570b56302c30a792 Mon Sep 17 00:00:00 2001
From: Takashi Iwai <[email protected]>
Date: Mon, 31 Oct 2022 11:02:45 +0100
Subject: [PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

The dvb-core tries to sync the releases of opened files at dvb_dmxdev_release() with two refcounts: dvbdev->users and dvr_dvbdev->users. A problem is present in those two syncs: when yet another dvb_demux_open() is called during those sync waits, dvb_demux_open() continues to process even if the device is being closed. This includes the increment of the former refcount, resulting in the leftover refcount after the sync of the latter refcount at dvb_dmxdev_release(). It ends up with use-after-free, since the function believes that all usages were gone and releases the resources.

This patch addresses the problem by adding the check of dmxdev->exit flag at dvb_demux_open(), just like dvb_dvr_open() already does. With the exit flag check, the second call of dvb_demux_open() fails, hence the further corruption can be avoided.

Also for avoiding the races of the dmxdev->exit flag reference, this patch serializes the dmxdev->exit set up and the sync waits with the dmxdev->mutex lock at dvb_dmxdev_release(). Without the mutex lock, dvb_demux_open() (or dvb_dvr_open()) may run concurrently with dvb_dmxdev_release(), which allows to skip the exit flag check and continue the open process that is being closed.

CVE-2022-41218 is assigned to those bugs above.

index f6ee678107d3..9ce5f010de3f 100644
@@ -790,6 +790,11 @@ static int dvb_demux_open(struct inode *inode, struct file *file)
 	if (mutex_lock_interruptible(&dmxdev->mutex))
 		return -ERESTARTSYS;
 
+	if (dmxdev->exit) {
+		mutex_unlock(&dmxdev->mutex);
+		return -ENODEV;
+	}
+
 	for (i = 0; i < dmxdev->filternum; i++)
 		if (dmxdev->filter[i].state == DMXDEV_STATE_FREE)
 			break;

@@ -1448,7 +1453,10 @@ EXPORT_SYMBOL(dvb_dmxdev_init);
 
 void dvb_dmxdev_release(struct dmxdev *dmxdev)
 {
+	mutex_lock(&dmxdev->mutex);
 	dmxdev->exit = 1;
+	mutex_unlock(&dmxdev->mutex);
+
 	if (dmxdev->dvbdev->users > 1) {
 		wait_event(dmxdev->dvbdev->wait_queue,
 				dmxdev->dvbdev->users == 1);