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Date:   Mon, 29 Jun 2020 08:48:05 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Fangrui Song <maskray@google.com>,
        Dmitry Golovin <dima@golovin.in>,
        clang-built-linux@googlegroups.com,
        Ard Biesheuvel <ardb@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "H . J . Lu" <hjl@sourceware.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/7] x86/boot/compressed: Move .got.plt entries out of
 the .got section
Message-ID: <202006290846.5A5C76A4D7@keescook>
References: <20200629140928.858507-1-nivedita@alum.mit.edu>
 <20200629140928.858507-2-nivedita@alum.mit.edu>
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On Mon, Jun 29, 2020 at 10:09:22AM -0400, Arvind Sankar wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The .got.plt section contains the part of the GOT which is used by PLT
> entries, and which gets updated lazily by the dynamic loader when
> function calls are dispatched through those PLT entries.
> 
> On fully linked binaries such as the kernel proper or the decompressor,
> this never happens, and so in practice, the .got.plt section consists
> only of the first 3 magic entries that are meant to point at the _DYNAMIC
> section and at the fixup routine in the loader. However, since we don't
> use a dynamic loader, those entries are never populated or used.
> 
> This means that treating those entries like ordinary GOT entries, and
> updating their values based on the actual placement of the executable in
> memory is completely pointless, and we can just ignore the .got.plt
> section entirely, provided that it has no additional entries beyond
> the first 3 ones.
> 
> So add an assertion in the linker script to ensure that this assumption
> holds, and move the contents out of the [_got, _egot) memory range that
> is modified by the GOT fixup routines.
> 
> While at it, drop the KEEP(), since it has no effect on the contents
> of output sections that are created by the linker itself.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Acked-by: Arvind Sankar <nivedita@alum.mit.edu>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> From: Ard Biesheuvel <ardb@kernel.org>
> Link: https://lore.kernel.org/r/20200523120021.34996-2-ardb@kernel.org
> ---
>  arch/x86/boot/compressed/vmlinux.lds.S | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 8f1025d1f681..b17d218ccdf9 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -44,10 +44,13 @@ SECTIONS
>  	}
>  	.got : {
>  		_got = .;
> -		KEEP(*(.got.plt))
>  		KEEP(*(.got))
>  		_egot = .;
>  	}
> +	.got.plt : {
> +		*(.got.plt)
> +	}

This is also being done on arm64, and the section was specified slightly
differently (with INFO) which maybe should be done here too?

       .got.plt (INFO) : { *(.got.plt) }
       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, ".got.plt not empty")

Otherwise, yes, looks good.

Reviewed-by: Kees Cook <keescook@chromium.org>

> +
>  	.data :	{
>  		_data = . ;
>  		*(.data)
> @@ -77,3 +80,9 @@ SECTIONS
>  
>  	DISCARDS
>  }
> +
> +#ifdef CONFIG_X86_64
> +ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
> +#else
> +ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> +#endif
> -- 
> 2.26.2
> 

-- 
Kees Cook