Re: [PATCH] module: Harden STRICT_MODULE_RWX

Peter Zijlstra
2020-04-06T11:27:42Z

On Mon, Apr 06, 2020 at 12:46:17PM +0200, Jessica Yu wrote:
> +++ Miroslav Benes [06/04/20 11:55 +0200]:
> > On Fri, 3 Apr 2020, Josh Poimboeuf wrote:
> >
> > > On Fri, Apr 03, 2020 at 06:37:16PM +0200, Peter Zijlstra wrote:
> > > > +{
> > > > + int i;
> > > > +
> > > > + for (i = 0; i < hdr->e_shnum; i++) {
> > > > + if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE))
> > > > + return -ENOEXEC;
> > >
> > > I think you only want the error when both are set?
> > >
> > > if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE) == (SHF_EXECINSTR|SHF_WRITE))
> >
> > A section with SHF_EXECINSTR and SHF_WRITE but without SHF_ALLOC would be
> > strange though, no? It wouldn't be copied to the final module later
> > anyway.
>
> That's right - move_module() ignores !SHF_ALLOC sections and does not
> copy them over to their final location. So I think we want to look for
> SHF_EXECINSTR|SHF_WRITE|SHF_ALLOC here..

So I did notice that !SHF_ALLOC sections get ignored, but since this
check is about W^X we don't strictly care about SHF_ALLOC. What we care
about it never allowing a writable and executable map.

Adding ALLOC to the test only allows for future mistakes and doesn't
make the check any better.