[PATCH] media: uvcvideo: Fix 'type' check leading to overflow

[Alistair Strachan]

When initially testing the Camera Terminal Descriptor wTerminalType
field (buffer[4]), no mask is used. Later in the function, the MSB is
overloaded to store the descriptor subtype, and so a mask of 0x7fff
is used to check the type.

If a descriptor is specially crafted to set this overloaded bit in the
original wTerminalType field, the initial type check will fail (falling
through, without adjusting the buffer size), but the later type checks
will pass, assuming the buffer has been made suitably large, causing an
overflow.

This problem could be resolved in a few different ways, but this fix
applies the same initial type check as used by UVC_ENTITY_TYPE (once we
have a 'term' structure.) Such crafted wTerminalType fields will then be
treated as *generic* Input Terminals, not as CAMERA or
MEDIA_TRANSPORT_INPUT, avoiding an overflow.

Originally reported here:
https://groups.google.com/forum/#!topic/syzkaller/Ot1fOE6v1d8
A similar (non-compiling) patch was provided at that time.

Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Alistair Strachan <astrachan@xxxxxxxxxx>
Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxxxxxxx
---
 drivers/media/usb/uvc/uvc_driver.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
index bc369a0934a3..279a967b8264 100644
--- a/drivers/media/usb/uvc/uvc_driver.c
+++ b/drivers/media/usb/uvc/uvc_driver.c
@@ -1082,11 +1082,11 @@ static int uvc_parse_standard_control(struct uvc_device *dev,
 		p = 0;
 		len = 8;
 
-		if (type == UVC_ITT_CAMERA) {
+		if ((type & 0x7fff) == UVC_ITT_CAMERA) {
 			n = buflen >= 15 ? buffer[14] : 0;
 			len = 15;
 
-		} else if (type == UVC_ITT_MEDIA_TRANSPORT_INPUT) {
+		} else if ((type & 0x7fff) == UVC_ITT_MEDIA_TRANSPORT_INPUT) {
 			n = buflen >= 9 ? buffer[8] : 0;
 			p = buflen >= 10 + n ? buffer[9+n] : 0;
 			len = 10;
-- 
2.20.0.405.gbc1bbc6f85-goog