linux-kernel - Re: [PATCH 18/29] arm64: Move EXCEPTION_TABLE to RO

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Message-ID: <201910010846.D0712C1@keescook>
Date:   Tue, 1 Oct 2019 08:48:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "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 18/29] arm64: Move EXCEPTION_TABLE to RO_DATA segment

On Tue, Oct 01, 2019 at 10:03:56AM +0100, Will Deacon wrote:
> Hi Kees,
> 
> On Thu, Sep 26, 2019 at 10:55:51AM -0700, Kees Cook wrote:
> > The EXCEPTION_TABLE is read-only, so collapse it into RO_DATA.
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/arm64/kernel/vmlinux.lds.S | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > index 81d94e371c95..c6ba2eee0ee8 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -5,6 +5,8 @@
> >   * Written by Martin Mares <mj@...ey.karlin.mff.cuni.cz>
> >   */
> >  
> > +#define RO_DATA_EXCEPTION_TABLE_ALIGN	8
> > +
> >  #include <asm-generic/vmlinux.lds.h>
> >  #include <asm/cache.h>
> >  #include <asm/kernel-pgtable.h>
> > @@ -135,8 +137,8 @@ SECTIONS
> >  	. = ALIGN(SEGMENT_ALIGN);
> >  	_etext = .;			/* End of text section */
> >  
> > -	RO_DATA(PAGE_SIZE)		/* everything from this point to     */
> > -	EXCEPTION_TABLE(8)		/* __init_begin will be marked RO NX */
> > +	/* everything from this point to __init_begin will be marked RO NX */
> > +	RO_DATA(PAGE_SIZE)
> >  
> >  	. = ALIGN(PAGE_SIZE);
> 
> Do you reckon it would be worth merging this last ALIGN directive into the
> RO_DATA definition too? Given that we want to map the thing read-only, it
> really has to be aligned either side.

Actually, taking a closer look, this appears to be redundant: RO_DATA()
ends with:

	. = ALIGN(align)

(where "align" is the "PAGE_SIZE" argument to RO_DATA())

> Anyway, that's only a nit, so:
> 
> Acked-by: Will Deacon <will@...nel.org>

Thanks!

> P.S. Please CC the arm64 maintainers on arm64 patches -- I nearly missed
> this one!

Okay, I can re-expand my list. I originally had done this but it was
getting to be a rather large set of people. :)

-- 
Kees Cook