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

[Eric Dumazet]

On Thu, Sep 1, 2022 at 12:49 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 31 Aug 2022 23:27:08 -0700 Kees Cook wrote:
> > 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.
>
> It turns the most obvious function into a noodle bar :(
>
> Looking at this function in particular is quite useful, because
> it clearly indicates that the nlmsg_len includes the header.
>
> How about we throw in a
>
>         WARN_ON_ONCE(nlh->nlmsg_len < NLMSG_HDRLEN ||
>                      nlh->nlmsg_len > INT_MAX);
>
> but leave the actual calculation human readable C?

This is inlined, and will add a lot of extra code. We are making the
kernel slower at each release.

What about letting fuzzers like syzbot find the potential issues ?

DEBUG_NET_WARN_ON_ONCE(...);