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

[Kees Cook <keescook@xxxxxxxxxxxx>]

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