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Date:   Tue, 16 Mar 2021 13:03:45 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Kees Cook <keescook@...omium.org>
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 <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/17] module: cfi: ensure __cfi_check alignment

On Thu, Mar 11, 2021 at 6:39 PM Kees Cook <keescook@...omium.org> wrote:
>
> 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.

Oops, thanks for pointing this out. I'll fix it in v2.

> 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*)
>         }

Agreed, that looks better.

Sami

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