[PATCH] net: tulip: xircom_cb: drop runt frames before skb copy

["Pablo Vallespín Aranguren" <pablopva014@gmail.com>]

investigate_read_descriptor() takes the 11-bit packet length straight
from the device-written descriptor and subtracts 4 for the CRC. A
length below 4 underflows pkt_len to a negative value, which is then
passed to skb_put_data() as a huge unsigned length and corrupts memory
past the end of the freshly allocated skb.

Drop the frame as a length error if pkt_len is negative after CRC
removal.

Signed-off-by: Pablo Vallespín Aranguren <pablopva014@gmail.com>
Assisted-by: gkh_clanker_t1000
---
 drivers/net/ethernet/dec/tulip/xircom_cb.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ethernet/dec/tulip/xircom_cb.c b/drivers/net/ethernet/dec/tulip/xircom_cb.c
index e5d2ede13845..62c64da1c8fa 100644
--- a/drivers/net/ethernet/dec/tulip/xircom_cb.c
+++ b/drivers/net/ethernet/dec/tulip/xircom_cb.c
@@ -1110,6 +1110,11 @@ investigate_read_descriptor(struct net_device *dev, struct xircom_private *card,
 					/* minus 4, we don't want the CRC */
 		struct sk_buff *skb;
 
+		if (pkt_len < 0) {
+			dev->stats.rx_length_errors++;
+			goto out;
+		}
+
 		if (pkt_len > 1518) {
 			netdev_err(dev, "Packet length %i is bogus\n", pkt_len);
 			pkt_len = 1518;
-- 
2.55.0