linux-kernel - Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations

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Message-ID: <20200629184335.GA1102734@rani.riverdale.lan>
Date:   Mon, 29 Jun 2020 14:43:35 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "H . J . Lu" <hjl@...rceware.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime
 relocations

On Mon, Jun 29, 2020 at 09:20:31AM -0700, Kees Cook wrote:
> On Mon, Jun 29, 2020 at 06:11:59PM +0200, Ard Biesheuvel wrote:
> > On Mon, 29 Jun 2020 at 18:09, Kees Cook <keescook@...omium.org> wrote:
> > >
> > > Should these be marked (INFO) as well?
> > >
> > 
> > Given that sections marked as (INFO) will still be emitted into the
> > ELF image, it does not really make a difference to do this for zero
> > sized sections.
> 
> Oh, I misunderstood -- I though they were _not_ emitted; I see now what
> you said was not allocated. So, disk space used for the .got.plt case,
> but not memory space used. Sorry for the confusion!
> 
> -Kees
> 

To confuse the issue a bit more, there are subtleties around "disk space
used" :) The section will be present in the ELF format file, but at
least on x86, that file is then converted into binary format via
objcopy. At this point a non-allocated section at the end of the file
will be stripped off. So on 32-bit x86, moving .got.plt to the end and
marking it INFO will shave 16 bytes off the bzImage kernel.