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

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Message-ID: <202006290907.E5EF18A@keescook>
Date:   Mon, 29 Jun 2020 09:09:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux@...glegroups.com,
        Ard Biesheuvel <ardb@...nel.org>,
        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@...r.kernel.org
Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime
 relocations

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?

> @@ -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.

-- 
Kees Cook