[PATCH 1/1] media: uvcvideo: Fix 'type' check leading to overflow

[Paolo Pisati]

From: Alistair Strachan <astrachan at google.com>

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.

Avoid this problem by checking for the MSB in the wTerminalType field.
If the bit is set, assume the descriptor is bad, and abort parsing it.

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 at googlegroups.com>
Signed-off-by: Alistair Strachan <astrachan at google.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung at kernel.org>

CVE-2019-2101

(cherry picked from commit 47bb117911b051bbc90764a8bff96543cbd2005f)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 drivers/media/usb/uvc/uvc_driver.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
index 28b91b7d756f..1778fdeded94 100644
--- a/drivers/media/usb/uvc/uvc_driver.c
+++ b/drivers/media/usb/uvc/uvc_driver.c
@@ -1054,11 +1054,19 @@ static int uvc_parse_standard_control(struct uvc_device *dev,
 			return -EINVAL;
 		}
 
-		/* Make sure the terminal type MSB is not null, otherwise it
-		 * could be confused with a unit.
+		/*
+		 * Reject invalid terminal types that would cause issues:
+		 *
+		 * - The high byte must be non-zero, otherwise it would be
+		 *   confused with a unit.
+		 *
+		 * - Bit 15 must be 0, as we use it internally as a terminal
+		 *   direction flag.
+		 *
+		 * Other unknown types are accepted.
 		 */
 		type = get_unaligned_le16(&buffer[4]);
-		if ((type & 0xff00) == 0) {
+		if ((type & 0x7f00) == 0 || (type & 0x8000) != 0) {
 			uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
 				"interface %d INPUT_TERMINAL %d has invalid "
 				"type 0x%04x, skipping\n", udev->devnum,
-- 
2.17.1