Re: rwx mapping between ex_table and rodata

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> wrote:

> On Thu, Oct 1, 2015 at 2:12 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> >> On Mon, 28 Sep 2015, Kees Cook wrote:
> >> > > --- a/arch/x86/mm/init_64.c
> >> > > +++ b/arch/x86/mm/init_64.c
> >> > > @@ -1132,7 +1132,7 @@ void mark_rodata_ro(void)
> >> > >          * has been zapped already via cleanup_highmem().
> >> > >          */
> >> > >         all_end = roundup((unsigned long)_brk_end, PMD_SIZE);
> >> > > -       set_memory_nx(rodata_start, (all_end - rodata_start) >> PAGE_SHIFT);
> >> > > +       set_memory_nx(text_end, (all_end - text_end) >> PAGE_SHIFT);
> >> > >
> >> > >         rodata_test();
> >> > >
> >> > >
> >> >
> >> > That should work, yeah. I'd still like to find the default permissions and
> >> > make them W+nx, though. Regardless, let's get the above added.
> >>
> >> The default permissions are set at boot time when setting up the early page
> >> tables. When we split them up later on we inherit the PTE bits and then we do
> >> that _ro/nx cleanup after the overall layout has been settled.
> >>
> >> We can't make them W+nx in the early setup without shooting ourself in the foot,
> >> because we only set up at the pud/pmd level.
> >
> > So I think at minimum we should do a (debug) scan in late init, of the whole
> > range, for any leftover WX permissions? That would have caught this bug. (and
> > might catch other existing bugs that might occur with various configs/hw-layouts.)
> 
> I think this would be great. I'd like to disassociate it from PTDUMP,
> though, since that exposes kernel address to userspace. It'd be nice
> to have the check without also the debugfs entry.

Yeah, so it could still use pretty much the same code, except no registry in 
/debug?

Thanks,

	Ingo