linux-kernel - Re: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

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Date:   Mon, 27 Jan 2020 15:34:38 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

On Fri, Jan 24, 2020 at 10:18:19AM -0800, H.J. Lu wrote:
> With the command-line option, -mx86-used-note=yes, the x86 assembler
> in binutils 2.32 and above generates a program property note in a note
> section, .note.gnu.property, to encode used x86 ISAs and features.
> But x86 kernel linker script only contains a signle NOTE segment:
> 
> PHDRS {
>  text PT_LOAD FLAGS(5);
>  data PT_LOAD FLAGS(6);
>  percpu PT_LOAD FLAGS(6);
>  init PT_LOAD FLAGS(7);
>  note PT_NOTE FLAGS(0);
> }
> SECTIONS
> {
> ...
>  .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
> e.*)) __stop_notes = .; } :text :note
> ...
> }
> 
> which may not be incompatible with note.gnu.property sections.  Since
> note.gnu.property section in kernel image is unused, this patch discards
> .note.gnu.property sections in kernel linker script by adding
> 
>  /DISCARD/ : {
>   *(.note.gnu.property)
>  }

I think this is happening in the wrong place? Shouldn't this be in the
DISCARDS macro in include/asm-generic/vmlinux.lds.h instead?

> before .notes sections.  Since .exit.text and .exit.data sections are
> discarded at runtime, it undefines EXIT_TEXT and EXIT_DATA to exclude
> .exit.text and .exit.data sections from default discarded sections.

This looks like a separate issue (though maybe related to DISCARDS)?

-Kees

> 
> Signed-off-by: H.J. Lu <hjl.tools@...il.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/kernel/vmlinux.lds.S | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 3a1a819da137..6c6cc26b0177 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -150,6 +150,11 @@ SECTIONS
>  	_etext = .;
>  	. = ALIGN(PAGE_SIZE);
>  
> +	/* .note.gnu.property sections should be discarded */
> +	/DISCARD/ : {
> +		*(.note.gnu.property)
> +	}
> +
>  	X86_ALIGN_RODATA_BEGIN
>  	RO_DATA(PAGE_SIZE)
>  	X86_ALIGN_RODATA_END
> @@ -413,6 +418,12 @@ SECTIONS
>  	STABS_DEBUG
>  	DWARF_DEBUG
>  
> +	/* Sections to be discarded.  EXIT_TEXT and EXIT_DATA discard at runtime.
> +	 * not link time.  */
> +#undef EXIT_TEXT
> +#define EXIT_TEXT
> +#undef EXIT_DATA
> +#define EXIT_DATA
>  	DISCARDS
>  	/DISCARD/ : {
>  		*(.eh_frame)
> -- 
> 2.24.1
> 

-- 
Kees Cook