Re: [PATCH 04/17] module: cfi: ensure __cfi_check alignment

Kees Cook
2021-03-12T02:40:10Z

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@google.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