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

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Date:   Mon, 29 Jun 2020 12:56:03 -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:
> > >
> > > On Mon, Jun 29, 2020 at 10:09:28AM -0400, Arvind Sankar wrote:
> > > > Add a linker script check that there are no runtime relocations, and
> > > > remove the old one that tries to check via looking for specially-named
> > > > sections in the object files.
> > > >
> > > > Drop the tests for -fPIE compiler option and -pie linker option, as they
> > > > are available in all supported gcc and binutils versions (as well as
> > > > clang and lld).
> > > >
> > > > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> > > > Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> > > > Reviewed-by: Fangrui Song <maskray@...gle.com>
> > > > ---
> > > >  arch/x86/boot/compressed/Makefile      | 28 +++-----------------------
> > > >  arch/x86/boot/compressed/vmlinux.lds.S |  8 ++++++++
> > > >  2 files changed, 11 insertions(+), 25 deletions(-)
> > >
> > > Reviewed-by: Kees Cook <keescook@...omium.org>
> > >
> > > question below ...
> > >
> > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > > > index a4a4a59a2628..a78510046eec 100644
> > > > --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > > > @@ -42,6 +42,12 @@ SECTIONS
> > > >               *(.rodata.*)
> > > >               _erodata = . ;
> > > >       }
> > > > +     .rel.dyn : {
> > > > +             *(.rel.*)
> > > > +     }
> > > > +     .rela.dyn : {
> > > > +             *(.rela.*)
> > > > +     }
> > > >       .got : {
> > > >               *(.got)
> > > >       }
> > >
> > > 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

In the case of the REL[A] and .got sections, they are actually already
not emitted at all into the ELF file now that they are zero size.

For .got.plt, it is only emitted for 32-bit (with the 3 reserved
entries), the 64-bit linker seems to get rid of it.