linux-kernel - Re: rwx mapping between ex

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Message-ID: <alpine.DEB.2.11.1510011048010.4500@nanos>
Date:	Thu, 1 Oct 2015 11:03:21 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Kees Cook <keescook@...omium.org>
cc:	Stephen Smalley <sds@...ho.nsa.gov>,
	"x86@...nel.org" <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: rwx mapping between ex_table and rodata

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.

Thanks,

	tglx


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