[PATCH 2/2] media: uvcvideo: Fix out-of-bounds read in uvc_ctrl_status_event()

[Wei Jie Law <98lawweijie@gmail.com>]

An out-of-bounds read exists in v6.12.105 and upstream. KASAN crash log:

  BUG: KASAN: slab-use-after-free in uvc_get_le_value+0x314/0x350
  Read of size 1 at addr ffff88802df955e4 by task kworker/0:2/128
  Workqueue: events uvc_ctrl_status_event_work
   uvc_ctrl_status_event+0x128/0x280
   uvc_ctrl_status_event_work+0x82/0x240
  The buggy address belongs to the object at ffff88802df955e0
   which belongs to the cache kmalloc-16 of size 16

uvc_ctrl_add_mapping() validates mapping->offset against the control
data buffer, sized from the device's GET_LEN answer.
uvc_ctrl_status_event() applies the same offset to the 11 byte bValue
field of a control change event, so uvc_get_le_value() reads past the
16 byte struct uvc_status and the value is reported to userspace as a
V4L2_EVENT_CTRL change.  offset is a u8 bit offset, so the read reaches
24 bytes into the neighbouring objects; KASAN calls it a use-after-free
because the neighbour it read had just been freed.

Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
Cc: stable@vger.kernel.org
Signed-off-by: Wei Jie Law <98lawweijie@gmail.com>
Assisted-by: Claude:claude-opus-5
---
 drivers/media/usb/uvc/uvc_ctrl.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
index 21802f9b1b61..d082bcb8d98d 100644
--- a/drivers/media/usb/uvc/uvc_ctrl.c
+++ b/drivers/media/usb/uvc/uvc_ctrl.c
@@ -2156,11 +2156,16 @@ void uvc_ctrl_status_event(struct uvc_video_chain *chain,
 		uvc_ctrl_clear_handle(ctrl);
 
 	list_for_each_entry(mapping, &ctrl->info.mappings, list) {
-		s32 value;
+		s32 value = 0;
 
-		if (uvc_ctrl_mapping_is_compound(mapping))
-			value = 0;
-		else
+		/*
+		 * The offset is validated against the control size, not
+		 * against the event payload, so a mapping can reach past
+		 * bValue[].
+		 */
+		if (!uvc_ctrl_mapping_is_compound(mapping) &&
+		    mapping->offset + mapping->size <=
+			8 * sizeof_field(struct uvc_status_control, bValue))
 			value = uvc_mapping_get_s32(mapping, UVC_GET_CUR, data);
 
 		/*
-- 
2.43.0