linux-kernel - Re: [PATCH 04/17] module: cfi: ensure __cfi

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Date:   Thu, 11 Mar 2021 18:39:23 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        bpf@...r.kernel.org, linux-hardening@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kbuild@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/17] module: cfi: ensure __cfi_check alignment

On Thu, Mar 11, 2021 at 04:49:06PM -0800, Sami Tolvanen wrote:
> CONFIG_CFI_CLANG_SHADOW assumes the __cfi_check() function is page
> aligned and at the beginning of the .text section. While Clang would
> normally align the function correctly, it fails to do so for modules
> with no executable code.
> 
> This change ensures the correct __cfi_check() location and
> alignment. It also discards the .eh_frame section, which Clang can
> generate with certain sanitizers, such as CFI.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=46293
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  scripts/module.lds.S | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/module.lds.S b/scripts/module.lds.S
> index 168cd27e6122..552ddb084f76 100644
> --- a/scripts/module.lds.S
> +++ b/scripts/module.lds.S
> @@ -3,10 +3,13 @@
>   * Archs are free to supply their own linker scripts.  ld will
>   * combine them automatically.
>   */
> +#include <asm/page.h>
> +
>  SECTIONS {
>  	/DISCARD/ : {
>  		*(.discard)
>  		*(.discard.*)
> +		*(.eh_frame)
>  	}
>  
>  	__ksymtab		0 : { *(SORT(___ksymtab+*)) }
> @@ -40,7 +43,16 @@ SECTIONS {
>  		*(.rodata..L*)
>  	}
>  
> -	.text : { *(.text .text.[0-9a-zA-Z_]*) }
> +#ifdef CONFIG_CFI_CLANG
> +	/*
> +	 * With CFI_CLANG, ensure __cfi_check is at the beginning of the
> +	 * .text section, and that the section is aligned to page size.
> +	 */
> +	.text : ALIGN(PAGE_SIZE) {
> +		*(.text.__cfi_check)
> +		*(.text .text.[0-9a-zA-Z_]* .text..L.cfi*)
> +	}
> +#endif

Whoops, I think this reverts to the default .text declaration when
CONFIG_CFI_CLANG is unset.

I think the only thing that needs the ifdef is the ALIGN, yes? Perhaps
something like this?

#ifdef CONFIG_CFI_CLANG
# define ALIGN_CFI ALIGN(PAGE_SIZE)
#else
# define ALIGN_CFI
#endif

	.text : ALIGN_CFI {
		*(.text.__cfi_check)
		*(.text .text.[0-9a-zA-Z_]* .text..L.cfi*)
	}


-Kees

>  }
>  
>  /* bring in arch-specific sections */
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
> 

-- 
Kees Cook