On Tue, May 03, 2022 at 10:31:05PM -0500, Gustavo A. R. Silva wrote:

On Tue, May 03, 2022 at 06:44:10PM -0700, Kees Cook wrote:
[...]

quoted

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));

They have nlmsg_len()[1] for the length of the payload without the header:

/**
 * nlmsg_len - length of message payload
 * @nlh: netlink message header
 */
static inline int nlmsg_len(const struct nlmsghdr *nlh)
{
	return nlh->nlmsg_len - NLMSG_HDRLEN;
}

Oh, hm, yeah, that would be much cleaner. The relationship between
"payload" and nlmsg_len is confusing in here. :)

So, this should be simpler:

-	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
+	errmsg->msg = *nlh;
+	memcpy(errmsg->msg.nlmsg_payload, nlh->nlmsg_payload, nlmsg_len(nlh));

It's actually this case that triggered my investigation in __bos(1)'s
misbehavior around sub-structs, since this case wasn't getting silenced:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832

It still feels like it should be possible to get this right without
splitting the memcpy, though. Hmpf.

(would that function use some sanitization, though? what if nlmsg_len is
somehow manipulated to be less than NLMSG_HDRLEN?...)

Maybe something like:

static inline int nlmsg_len(const struct nlmsghdr *nlh)
{
	if (WARN_ON(nlh->nlmsg_len < NLMSG_HDRLEN))
		return 0;
	return nlh->nlmsg_len - NLMSG_HDRLEN;
}

Ah, yes, nice catch!

                cmdattr = (void *)&errmsg->msg + min_len;

                ret = nla_parse(cda, IPSET_ATTR_CMD_MAX, cmdattr,

--
Gustavo

[1] https://elixir.bootlin.com/linux/v5.18-rc5/source/include/net/netlink.h#L577

-- 
Kees Cook