[PATCH v2] rndis_wlan: Prevent buffer overflow in rndis_query_oid

[Szymon Heidrich]

Since resplen and respoffs are signed integers sufficiently
large values of unsigned int len and offset members of RNDIS
response will result in negative values of prior variables.
This may be utilized to bypass implemented security checks
to either extract memory contents by manipulating offset or
overflow the data buffer via memcpy by manipulating both
offset and len.

Additionally assure that sum of resplen and respoffs does not
overflow so buffer boundaries are kept.

Fixes: 80f8c5b434f9 ("rndis_wlan: copy only useful data from rndis_command respond")
Signed-off-by: Szymon Heidrich <szymon.heidrich@xxxxxxxxx>
---
V1 -> V2: Use size_t and min macro, fix netdev_dbg format

 drivers/net/wireless/rndis_wlan.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/net/wireless/rndis_wlan.c b/drivers/net/wireless/rndis_wlan.c
index 82a7458e0..bf72e5fd3 100644
--- a/drivers/net/wireless/rndis_wlan.c
+++ b/drivers/net/wireless/rndis_wlan.c
@@ -696,8 +696,8 @@ static int rndis_query_oid(struct usbnet *dev, u32 oid, void *data, int *len)
 		struct rndis_query	*get;
 		struct rndis_query_c	*get_c;
 	} u;
-	int ret, buflen;
-	int resplen, respoffs, copylen;
+	int ret;
+	size_t buflen, resplen, respoffs, copylen;
 
 	buflen = *len + sizeof(*u.get);
 	if (buflen < CONTROL_BUFFER_SIZE)
@@ -732,22 +732,15 @@ static int rndis_query_oid(struct usbnet *dev, u32 oid, void *data, int *len)
 
 		if (respoffs > buflen) {
 			/* Device returned data offset outside buffer, error. */
-			netdev_dbg(dev->net, "%s(%s): received invalid "
-				"data offset: %d > %d\n", __func__,
-				oid_to_string(oid), respoffs, buflen);
+			netdev_dbg(dev->net,
+				   "%s(%s): received invalid data offset: %zu > %zu\n",
+				   __func__, oid_to_string(oid), respoffs, buflen);
 
 			ret = -EINVAL;
 			goto exit_unlock;
 		}
 
-		if ((resplen + respoffs) > buflen) {
-			/* Device would have returned more data if buffer would
-			 * have been big enough. Copy just the bits that we got.
-			 */
-			copylen = buflen - respoffs;
-		} else {
-			copylen = resplen;
-		}
+		copylen = min(resplen, buflen - respoffs);
 
 		if (copylen > *len)
 			copylen = *len;
-- 
2.39.0