[PATCH v4] netlink: Bounds-check struct nlmsgerr creation

[Kees Cook]

In preparation for FORTIFY_SOURCE doing bounds-check on memcpy(),
switch from __nlmsg_put to nlmsg_put(), and explain the bounds check
for dealing with the memcpy() across a composite flexible array struct.
Avoids this future run-time warning:

  memcpy: detected field-spanning write (size 32) of single field "&errmsg->msg" at net/netlink/af_netlink.c:2447 (size 16)

Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx>
Cc: Florian Westphal <fw@xxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Cc: netfilter-devel@xxxxxxxxxxxxxxx
Cc: coreteam@xxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220901071336.1418572-1-keescook@xxxxxxxxxxxx
---
v4:
 - switch to nlmsg_put() instead of __nlmsg_put() (kuba)
v3: https://lore.kernel.org/lkml/20220901071336.1418572-1-keescook@xxxxxxxxxxxx
v2: https://lore.kernel.org/lkml/20220901064858.1417126-1-keescook@xxxxxxxxxxxx
v1: https://lore.kernel.org/lkml/20220901030610.1121299-3-keescook@xxxxxxxxxxxx
---
 net/netfilter/ipset/ip_set_core.c | 8 +++++---
 net/netlink/af_netlink.c          | 8 +++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
index 16ae92054baa..6b31746f9be3 100644
--- a/net/netfilter/ipset/ip_set_core.c
+++ b/net/netfilter/ipset/ip_set_core.c
@@ -1719,11 +1719,13 @@ call_ad(struct net *net, struct sock *ctnl, struct sk_buff *skb,
 		skb2 = nlmsg_new(payload, GFP_KERNEL);
 		if (!skb2)
 			return -ENOMEM;
-		rep = __nlmsg_put(skb2, NETLINK_CB(skb).portid,
-				  nlh->nlmsg_seq, NLMSG_ERROR, payload, 0);
+		rep = nlmsg_put(skb2, NETLINK_CB(skb).portid,
+				nlh->nlmsg_seq, NLMSG_ERROR, payload, 0);
 		errmsg = nlmsg_data(rep);
 		errmsg->error = ret;
-		memcpy(&errmsg->msg, nlh, nlh->nlmsg_len);
+		unsafe_memcpy(&errmsg->msg, nlh, nlh->nlmsg_len,
+			      /* Bounds checked by the skb layer. */);
+
 		cmdattr = (void *)&errmsg->msg + min_len;
 
 		ret = nla_parse(cda, IPSET_ATTR_CMD_MAX, cmdattr,
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index f89ba302ac6e..a662e8a5ff84 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2494,11 +2494,13 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		return;
 	}
 
-	rep = __nlmsg_put(skb, NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
-			  NLMSG_ERROR, payload, flags);
+	rep = nlmsg_put(skb, NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
+			NLMSG_ERROR, payload, flags);
 	errmsg = nlmsg_data(rep);
 	errmsg->error = err;
-	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
+	unsafe_memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg)
+					 ? nlh->nlmsg_len : sizeof(*nlh),
+		      /* Bounds checked by the skb layer. */);
 
 	if (tlvlen)
 		netlink_ack_tlv_fill(in_skb, skb, nlh, err, extack);
-- 
2.34.1