[PATCH] media: em28xx: fix v4l2 use-after-free after failed init

[Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>]

em28xx_v4l2_init() publishes the new object in dev->v4l2 before the
video device is registered. If a later initialization step fails -past
v4l2_device_register-,  the error path unregisters the device nodes and
drops the initial v4l2_device reference, but previously left dev->v4l2
pointing at the freed object.

syzbot reported:

BUG: KASAN: slab-use-after-free in v4l2_device_disconnect+0xa4/0xc0
		drivers/media/v4l2-core/v4l2-device.c:78

em28xx_v4l2_fini() is called later from USB disconnect. It reads
dev->v4l2 before taking dev->lock, and on the failed-init path it owns
no reference. The last close of the video node can therefore drop the
final v4l2_device reference and free the object between that read and
its use in v4l2_device_disconnect(), causing a use-after-free.

Fixing this by clearing dev->v4l2 under dev->lock in the init error path
before dropping the initial reference, and read it under dev->lock in fini.
This makes fini see NULL after a failed init, while the successful path
continues to hold the initial reference until fini is done.

Fixes: 288254383674 ("media: em28xx: use v4l2_device release callback")
Reported-by: syzbot+1e71765881a6fc43185f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1e71765881a6fc43185f
Cc: stable@vger.kernel.org
Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>
---
 drivers/media/usb/em28xx/em28xx-video.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
index 84424284d486..572e6835b851 100644
--- a/drivers/media/usb/em28xx/em28xx-video.c
+++ b/drivers/media/usb/em28xx/em28xx-video.c
@@ -2414,7 +2414,7 @@ static int em28xx_v4l2_open(struct file *filp)
  */
 static int em28xx_v4l2_fini(struct em28xx *dev)
 {
-	struct em28xx_v4l2 *v4l2 = dev->v4l2;
+	struct em28xx_v4l2 *v4l2;
 
 	if (dev->is_audio_only) {
 		/* Shouldn't initialize IR for this interface */
@@ -2426,13 +2426,15 @@ static int em28xx_v4l2_fini(struct em28xx *dev)
 		return 0;
 	}
 
-	if (!v4l2)
-		return 0;
-
 	dev_info(&dev->intf->dev, "Closing video extension\n");
 
 	mutex_lock(&dev->lock);
 
+	/* matches em28xx_v4l2_init error paths */
+	v4l2 = dev->v4l2;
+	if (!v4l2)
+		return 0;
+
 	v4l2_device_disconnect(&v4l2->v4l2_dev);
 
 	em28xx_uninit_usb_xfer(dev, EM28XX_ANALOG_MODE);
@@ -3129,6 +3131,9 @@ static int em28xx_v4l2_init(struct em28xx *dev)
 		vb2_video_unregister_device(&v4l2->vdev);
 	}
 
+	/* matches check in em28xx_v4l2_fini (under the same lock) */
+	dev->v4l2 = NULL;
+
 	mutex_unlock(&dev->lock);
 
 	/*
-- 
2.43.0