[NET]: Fix signedness issues in net/core/filter.c

This is the code to load packet data into a register:

                        k = fentry->k;
                        if (k < 0) {
...
                        } else {
                                u32 _tmp, *p;
                                p = skb_header_pointer(skb, k, 4, &_tmp);
                                if (p != NULL) {
                                        A = ntohl(*p);
                                        continue;
                                }
                        }

skb_header_pointer checks if the requested data is within the
linear area:

        int hlen = skb_headlen(skb);

        if (offset + len <= hlen)
                return skb->data + offset;

When offset is within [INT_MAX-len+1..INT_MAX] the addition will
result in a negative number which is <= hlen.

I couldn't trigger a crash on my AMD64 with 2GB of memory, but a
coworker tried on his x86 machine and it crashed immediately.

This patch fixes the check in skb_header_pointer to handle large
positive offsets similar to skb_copy_bits. Invalid data can still
be accessed using negative offsets (also similar to skb_copy_bits),
anyone using negative offsets needs to verify them himself.

Thanks to Thomas Vögtle <thomas.voegtle@coreworks.de> for verifying the
problem by crashing his machine and providing me with an Oops.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1211,7 +1211,7 @@ static inline void *skb_header_pointer(const struct sk_buff *skb, int offset,
 {
 	int hlen = skb_headlen(skb);
 
-	if (offset + len <= hlen)
+	if (hlen - offset >= len)
 		return skb->data + offset;
 
 	if (skb_copy_bits(skb, offset, buffer, len) < 0)