[IPV6]: IPV6_CHECKSUM socket option can corrupt kernel memory

So here is a patch that introduces skb_store_bits -- the opposite of
skb_copy_bits, and uses them to read/write the csum field in rawv6.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -985,6 +985,94 @@ fault:
 	return -EFAULT;
 }
 
+/**
+ *	skb_store_bits - store bits from kernel buffer to skb
+ *	@skb: destination buffer
+ *	@offset: offset in destination
+ *	@from: source buffer
+ *	@len: number of bytes to copy
+ *
+ *	Copy the specified number of bytes from the source buffer to the
+ *	destination skb.  This function handles all the messy bits of
+ *	traversing fragment lists and such.
+ */
+
+int skb_store_bits(const struct sk_buff *skb, int offset, void *from, int len)
+{
+	int i, copy;
+	int start = skb_headlen(skb);
+
+	if (offset > (int)skb->len - len)
+		goto fault;
+
+	if ((copy = start - offset) > 0) {
+		if (copy > len)
+			copy = len;
+		memcpy(skb->data + offset, from, copy);
+		if ((len -= copy) == 0)
+			return 0;
+		offset += copy;
+		from += copy;
+	}
+
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
+		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+		int end;
+
+		BUG_TRAP(start <= offset + len);
+
+		end = start + frag->size;
+		if ((copy = end - offset) > 0) {
+			u8 *vaddr;
+
+			if (copy > len)
+				copy = len;
+
+			vaddr = kmap_skb_frag(frag);
+			memcpy(vaddr + frag->page_offset + offset - start,
+			       from, copy);
+			kunmap_skb_frag(vaddr);
+
+			if ((len -= copy) == 0)
+				return 0;
+			offset += copy;
+			from += copy;
+		}
+		start = end;
+	}
+
+	if (skb_shinfo(skb)->frag_list) {
+		struct sk_buff *list = skb_shinfo(skb)->frag_list;
+
+		for (; list; list = list->next) {
+			int end;
+
+			BUG_TRAP(start <= offset + len);
+
+			end = start + list->len;
+			if ((copy = end - offset) > 0) {
+				if (copy > len)
+					copy = len;
+				if (skb_store_bits(list, offset - start,
+						   from, copy))
+					goto fault;
+				if ((len -= copy) == 0)
+					return 0;
+				offset += copy;
+				from += copy;
+			}
+			start = end;
+		}
+	}
+	if (!len)
+		return 0;
+
+fault:
+	return -EFAULT;
+}
+
+EXPORT_SYMBOL(skb_store_bits);
+
 /* Checksum skb data. */
 
 unsigned int skb_checksum(const struct sk_buff *skb, int offset,