[PATCH] Input: ims-psu - check if CDC union descriptor is sane

[Colin King]

From: Dmitry Torokhov <dmitry.torokhov at gmail.com>

CVE-2017-16645

Before trying to use CDC union descriptor, try to validate whether that it
is sane by checking that intf->altsetting->extra is big enough and that
descriptor bLength is not too big and not too small.

Reported-by: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov at gmail.com>
(clean cherry pick from upstream commit ea04efee7635c9120d015dcdeeeb6988130cb67a)
Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 drivers/input/misc/ims-pcu.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/input/misc/ims-pcu.c b/drivers/input/misc/ims-pcu.c
index 6bf82ea..ae47312 100644
--- a/drivers/input/misc/ims-pcu.c
+++ b/drivers/input/misc/ims-pcu.c
@@ -1635,13 +1635,25 @@ static void ims_pcu_buffers_free(struct ims_pcu *pcu)
 		return NULL;
 	}
 
-	while (buflen > 0) {
+	while (buflen >= sizeof(*union_desc)) {
 		union_desc = (struct usb_cdc_union_desc *)buf;
 
+		if (union_desc->bLength > buflen) {
+			dev_err(&intf->dev, "Too large descriptor\n");
+			return NULL;
+		}
+
 		if (union_desc->bDescriptorType == USB_DT_CS_INTERFACE &&
 		    union_desc->bDescriptorSubType == USB_CDC_UNION_TYPE) {
 			dev_dbg(&intf->dev, "Found union header\n");
-			return union_desc;
+
+			if (union_desc->bLength >= sizeof(*union_desc))
+				return union_desc;
+
+			dev_err(&intf->dev,
+				"Union descriptor to short (%d vs %zd\n)",
+				union_desc->bLength, sizeof(*union_desc));
+			return NULL;
 		}
 
 		buflen -= union_desc->bLength;
-- 
1.9.1