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 18:11:59 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     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, 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.

> > @@ -85,3 +91,5 @@ ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT en
> >  #else
> >  ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> >  #endif
> > +
> > +ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected runtime relocations detected!")
>
> I think I should be doing this same ASSERT style for other explicit
> DISCARDS in my orphan series so we'll notice if they change, instead
> of being silently dropped if they grow.
>