linux-kernel - Re: [PATCH 1/2] Add RUNTIME_DISCARD

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Date:   Thu, 26 Mar 2020 13:39:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "H.J. Lu" <hjl.tools@...il.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 1/2] Add RUNTIME_DISCARD_EXIT to generic DISCARDS

On Thu, Mar 26, 2020 at 09:11:42PM +0100, Borislav Petkov wrote:
> On Thu, Mar 26, 2020 at 12:30:20PM -0700, H.J. Lu wrote:
> > In x86 kernel, .exit.text and .exit.data sections are discarded at
> > runtime, not by linker.  Add RUNTIME_DISCARD_EXIT to generic DISCARDS
> > and define it in x86 kernel linker script to keep them.
> > 
> > Signed-off-by: H.J. Lu <hjl.tools@...il.com>
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/x86/kernel/vmlinux.lds.S     |  1 +
> >  include/asm-generic/vmlinux.lds.h | 10 ++++++++--
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index e3296aa028fe..7206e1ac23dd 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -21,6 +21,7 @@
> >  #define LOAD_OFFSET __START_KERNEL_map
> >  #endif
> >  
> > +#define RUNTIME_DISCARD_EXIT
> >  #define EMITS_PT_NOTE
> >  #define RO_EXCEPTION_TABLE_ALIGN	16
> >  
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index e00f41aa8ec4..6b943fb8c5fd 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -894,10 +894,16 @@
> >   * section definitions so that such archs put those in earlier section
> >   * definitions.
> >   */
> > +#ifdef RUNTIME_DISCARD_EXIT
> > +#define EXIT_DISCARDS
> > +#else
> > +#define EXIT_DISCARDS							\
> > +	EXIT_TEXT							\
> > +	EXIT_DATA
> > +#endif
> 
> /me goes back and reads the old thread on this...
> 
> Kees, do you expect other arches to actually need this
> RUNTIME_DISCARD_EXIT thing or was that a hypothetical thing?
> 
> /me searches more...
> 
> oh, there's a patchset from you
> 
> https://lkml.kernel.org/r/20200228002244.15240-1-keescook@chromium.org
> 
> which already contains this patch *and* an ARM64 patch which defines
> RUNTIME_DISCARD_EXIT so I'm guessing ARM64 wants to discard at runtime
> too.

Correct.

> Which leaves the question why is H.J. sending that patch separate and
> you carry it in a patchset about orphan section warning? Seems like it
> wants to be in your patchset?

I had needed the same clean up for the orphan section handling, and
since it hadn't been picked up yet, I included it in my series. I'm
still stuck addressing several review comments, so there's no reason to
wait for me: I can easily rebase once these patches land somewhere. I'd
be happy to see them in -tip.

Thanks!

-Kees

-- 
Kees Cook