netlink: split up copies in the ack construction

Clean up the use of unsafe_memcpy() by adding a flexible array
at the end of netlink message header and splitting up the header
and data copies.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 0 deletions

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 4418b1981e31..784b4688fc6f 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -932,6 +932,27 @@ static inline struct nlmsghdr *nlmsg_put(struct sk_buff *skb, u32 portid, u32 se
 }
 
 /**
+ * nlmsg_append - Add more data to a nlmsg in a skb
+ * @skb: socket buffer to store message in
+ * @size: length of message payload
+ *
+ * Append data to an existing nlmsg, used when constructing a message
+ * with multiple fixed-format headers (which is rare).
+ * Returns NULL if the tailroom of the skb is insufficient to store
+ * the extra payload.
+ */
+static inline void *nlmsg_append(struct sk_buff *skb, u32 size)
+{
+	if (unlikely(skb_tailroom(skb) < NLMSG_ALIGN(size)))
+		return NULL;
+
+	if (NLMSG_ALIGN(size) - size)
+		memset(skb_tail_pointer(skb) + size, 0,
+		       NLMSG_ALIGN(size) - size);
+	return __skb_put(skb, NLMSG_ALIGN(size));
+}
+
+/**
  * nlmsg_put_answer - Add a new callback based netlink message to an skb
  * @skb: socket buffer to store message in
  * @cb: netlink callback