dvb-core: Proper handling ULE SNDU length of 0 (CVE-2006-4623)

ULE (Unidirectional Lightweight Encapsulation RFC 4326) decapsulation
code has a bug that allows an attacker to send a malformed ULE packet
with SNDU length of 0 and bring down the receiving machine. This patch
fix the bug and has been tested on version 2.6.17.11. This bug is 100%
reproducible and the modified source code (GPL) used to produce this bug
will be posted on http://nrg.cs.usm.my/downloads.htm shortly.  The
kernel will produce a dump during CRC32 checking on faulty ULE packet.
Signed-off-by: Ang Way Chuang <wcang@nrg.cs.usm.my>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

drivers/media/dvb/dvb-core/dvb_net.c

@@ -492,7 +492,8 @@ static void dvb_net_ule( struct net_device *dev, const u8 *buf, size_t buf_len )
 				} else
 					priv->ule_dbit = 0;
 
-				if (priv->ule_sndu_len > 32763) {
+				if (priv->ule_sndu_len > 32763 ||
+				    priv->ule_sndu_len < ((priv->ule_dbit) ? 4 : 4 + ETH_ALEN)) {
 					printk(KERN_WARNING "%lu: Invalid ULE SNDU length %u. "
 					       "Resyncing.\n", priv->ts_count, priv->ule_sndu_len);
 					priv->ule_sndu_len = 0;