linux-hardening - Re: [PATCH v2 04/17] module: ensure __cfi

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Message-ID: <CAKwvOdkbok-BoYGfvHH1HR=cMztaBYZKB-UHRDZ4g5YjSCuq2A@mail.gmail.com>
Date:   Thu, 18 Mar 2021 12:27:43 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        bpf <bpf@...r.kernel.org>, linux-hardening@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 04/17] module: ensure __cfi_check alignment

On Thu, Mar 18, 2021 at 10:11 AM Sami Tolvanen <samitolvanen@...gle.com> 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 | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/module.lds.S b/scripts/module.lds.S
> index 168cd27e6122..93518579cf5d 100644
> --- a/scripts/module.lds.S
> +++ b/scripts/module.lds.S
> @@ -3,10 +3,19 @@
>   * Archs are free to supply their own linker scripts.  ld will
>   * combine them automatically.
>   */
> +#include <asm/page.h>
> +
> +#ifdef CONFIG_CFI_CLANG
> +# define ALIGN_CFI ALIGN(PAGE_SIZE)
> +#else
> +# define ALIGN_CFI
> +#endif
> +
>  SECTIONS {
>         /DISCARD/ : {
>                 *(.discard)
>                 *(.discard.*)
> +               *(.eh_frame)

Do we want to unconditionally discard this section from modules for
all arches/configs?  I like how we conditionally do so on
SANITIZER_DISCARDS in include/asm-generic/vmlinux.lds.h for example.

>         }
>
>         __ksymtab               0 : { *(SORT(___ksymtab+*)) }
> @@ -40,7 +49,14 @@ SECTIONS {
>                 *(.rodata..L*)
>         }
>
> -       .text : { *(.text .text.[0-9a-zA-Z_]*) }
> +       /*
> +        * With CONFIG_CFI_CLANG, we assume __cfi_check is at the beginning
> +        * of the .text section, and is aligned to PAGE_SIZE.
> +        */
> +       .text : ALIGN_CFI {
> +               *(.text.__cfi_check)
> +               *(.text .text.[0-9a-zA-Z_]* .text..L.cfi*)
> +       }
>  }
>
>  /* bring in arch-specific sections */
> --
> 2.31.0.291.g576ba9dcdaf-goog
>


-- 
Thanks,
~Nick Desaulniers