linux-kernel - Re: [PATCH 07/29] x86: Restore "text" Program Header with dummy section

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Date:   Thu, 10 Oct 2019 09:46:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-c6x-dev@...ux-c6x.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Michal Simek <monstr@...str.eu>, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/29] x86: Restore "text" Program Header with dummy
 section

On Thu, Oct 10, 2019 at 12:33:05PM +0200, Borislav Petkov wrote:
> On Thu, Sep 26, 2019 at 10:55:40AM -0700, Kees Cook wrote:
> > Instead of depending on markings in the section following NOTES to
> > restore the associated Program Header, use a dummy section, as done
> > in other architectures.
> 
> This is very laconic and after some staring at ld.info, I think you mean
> this:
> 
> "   If you place a section in one or more segments using ':PHDR', then
> the linker will place all subsequent allocatable sections which do not
> specify ':PHDR' in the same segments."
> 
> but I could be way off. Yes, no?
> 
> IOW, please write in the commit messages first what the problem is
> you're addressing.

Yeah, that's much improved. I really struggled to describe this, given
how weird the linker script logic is here.

> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -147,8 +147,9 @@ SECTIONS
> >  	} :text = 0x9090
> >  
> >  	NOTES :text :note
> > +	.dummy : { *(.dummy) } :text
> >  
> > -	EXCEPTION_TABLE(16) :text = 0x9090
> > +	EXCEPTION_TABLE(16)
> 
> This is killing the filler byte but I have a suspicion that'll change
> eventually to INT3... :)

Yes, though since the exception table isn't executable, filling with
0x90 has no meaning, and I think when I looked at the alignment there
were actually no fill bytes from here on. But it doesn't matter; it all
goes away in the end.

-- 
Kees Cook