Subject: Re: [PATCH 1/2] netlink: Bounds-check nlmsg_len()

From: Kees Cook <[email-masked]>
Date: Wed, 31 Aug 2022 23:27:08 -0700

On Wed, Aug 31, 2022 at 08:18:25PM -0700, Jakub Kicinski wrote:
> On Wed, 31 Aug 2022 20:06:09 -0700 Kees Cook wrote:
> >  static inline int nlmsg_len(const struct nlmsghdr *nlh)
> >  {
> > -	return nlh->nlmsg_len - NLMSG_HDRLEN;
> > +	u32 nlmsg_contents_len;
> > +
> > +	if (WARN_ON_ONCE(check_sub_overflow(nlh->nlmsg_len,
> > +					    (u32)NLMSG_HDRLEN,
> > +					    &nlmsg_contents_len)))
> > +		return 0;
> > +	if (WARN_ON_ONCE(nlmsg_contents_len > INT_MAX))
> > +		return INT_MAX;
> > +	return nlmsg_contents_len;
> 
> We check the messages on input, making sure the length is valid wrt
> skb->len, and sane, ie > NLMSG_HDRLEN. See netlink_rcv_skb().
> 
> Can we not, pretty please? :(

This would catch corrupted values...

Is the concern the growth in image size? The check_sub_overflow() isn't
large at all -- it's just adding a single overflow bit test. The WARNs
are heavier, but they're all out-of-line.

-- 
Kees Cook