Subject: Re: [PATCH net-next v2] netlink: split up copies in the ack construction

From: Kees Cook <[email-masked]>
Date: Wed, 16 Nov 2022 16:27:51 -0800

On Wed, Nov 16, 2022 at 02:56:25PM -0800, Kees Cook wrote:
> On Mon, Nov 14, 2022 at 09:06:14AM -0800, Jakub Kicinski wrote:
> > On Sun, 13 Nov 2022 19:39:27 -0700 David Ahern wrote:
> > > On Thu, Oct 27, 2022 at 02:25:53PM -0700, Jakub Kicinski wrote:
> > > > diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> > > > index e2ae82e3f9f7..5da0da59bf01 100644
> > > > --- a/include/uapi/linux/netlink.h
> > > > +++ b/include/uapi/linux/netlink.h
> > > > @@ -48,6 +48,7 @@ struct sockaddr_nl {
> > > >   * @nlmsg_flags: Additional flags
> > > >   * @nlmsg_seq:   Sequence number
> > > >   * @nlmsg_pid:   Sending process port ID
> > > > + * @nlmsg_data:  Message payload
> > > >   */
> > > >  struct nlmsghdr {
> > > >  	__u32		nlmsg_len;
> > > > @@ -55,6 +56,7 @@ struct nlmsghdr {
> > > >  	__u16		nlmsg_flags;
> > > >  	__u32		nlmsg_seq;
> > > >  	__u32		nlmsg_pid;
> > > > +	__u8		nlmsg_data[];  
> > > 
> > > This breaks compile of iproute2 with clang. It does not like the
> > > variable length array in the middle of a struct. While I could re-do the
> > > structs in iproute2, I doubt it is alone in being affected by this
> > > change.
> 
> Eww.
> 
> > 
> > Kees, would you mind lending your expertise?

Perhaps this would be better? We could leave the _header_ struct alone,
but add the data to the nlmsgerr struct instead?

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 5da0da59bf01..d0629cb343b2 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -48,7 +48,6 @@ struct sockaddr_nl {
  * @nlmsg_flags: Additional flags
  * @nlmsg_seq:   Sequence number
  * @nlmsg_pid:   Sending process port ID
- * @nlmsg_data:  Message payload
  */
 struct nlmsghdr {
 	__u32		nlmsg_len;
@@ -56,7 +55,6 @@ struct nlmsghdr {
 	__u16		nlmsg_flags;
 	__u32		nlmsg_seq;
 	__u32		nlmsg_pid;
-	__u8		nlmsg_data[];
 };
 
 /* Flags values */
@@ -121,6 +119,7 @@ struct nlmsghdr {
 struct nlmsgerr {
 	int		error;
 	struct nlmsghdr msg;
+	__u8		data[];
 	/*
 	 * followed by the message contents unless NETLINK_CAP_ACK was set
 	 * or the ACK indicates success (error == 0)
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index b8afec32cff6..fe8493d3ae56 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2514,8 +2514,7 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		if (!nlmsg_append(skb, nlmsg_len(nlh)))
 			goto err_bad_put;
 
-		memcpy(errmsg->msg.nlmsg_data, nlh->nlmsg_data,
-		       nlmsg_len(nlh));
+		memcpy(errmsg->data, nlmsg_data(nlh), nlmsg_len(nlh));
 	}
 
 	if (tlvlen)

-- 
Kees Cook