In preparation for run-time memcpy() bounds checking, split the nlmsg
copying for error messages (which crosses a previous unspecified flexible
array boundary) in half. Avoids the future run-time warning:

memcpy: detected field-spanning write (size 32) of single field "&errmsg->msg" (size 16)

Creates an explicit flexible array at the end of nlmsghdr for the payload,
named "nlmsg_payload". There is no impact on UAPI; the sizeof(struct
nlmsghdr) does not change, but now the compiler can better reason about
where things are being copied.

Fixed-by: Rasmus Villemoes [off-list ref]
Link: https://lore.kernel.org/lkml/d7251d92-150b-5346-6237-52afc154bb00@rasmusvillemoes.dk (local)
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Rich Felker <redacted>
Cc: Eric Dumazet <edumazet@google.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <redacted>
---
 include/uapi/linux/netlink.h | 1 +
 net/netlink/af_netlink.c     | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 855dffb4c1c3..47f9342d51bc 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h

@@ -47,6 +47,7 @@ struct nlmsghdr {
 	__u16		nlmsg_flags;	/* Additional flags */
 	__u32		nlmsg_seq;	/* Sequence number */
 	__u32		nlmsg_pid;	/* Sending process port ID */
+	__u8		nlmsg_payload[];/* Contents of message */
 };
 
 /* Flags values */

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 1b5a9c2e1c29..09346aee1022 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c

@@ -2445,7 +2445,10 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 			  NLMSG_ERROR, payload, flags);
 	errmsg = nlmsg_data(rep);
 	errmsg->error = err;
-	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
+	errmsg->msg = *nlh;
+	if (payload > sizeof(*errmsg))
+		memcpy(errmsg->msg.nlmsg_payload, nlh->nlmsg_payload,
+		       nlh->nlmsg_len - sizeof(*nlh));
 
 	if (nlk_has_extack && extack) {
 		if (extack->_msg) {

-- 
2.32.0