Re: [PATCH 01/32] netlink: Avoid memcpy() across flexible array boundary

Kees Cook
2022-05-04T03:37:49Z

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:
> [...]
> > 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;
}

> Also, it seems there is at least one more instance of this same issue:
>
> diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
> index 16ae92054baa..d06184b94af5 100644
> --- a/net/netfilter/ipset/ip_set_core.c
> +++ b/net/netfilter/ipset/ip_set_core.c
> @@ -1723,7 +1723,8 @@ call_ad(struct net *net, struct sock *ctnl, struct sk_buff *skb,
> nlh->nlmsg_seq, NLMSG_ERROR, payload, 0);
> errmsg = nlmsg_data(rep);
> errmsg->error = ret;
> - memcpy(&errmsg->msg, nlh, nlh->nlmsg_len);
> + errmsg->msg = *nlh;
> + memcpy(errmsg->msg.nlmsg_payload, nlh->nlmsg_payload, nlmsg_len(nlh));

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