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

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Date:   Thu, 18 Mar 2021 14:43:09 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...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 12:27 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> 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.

I think nothing should generate this section unless one of the
sanitizers is enabled, but I'm fine with using the same logic as
vmlinux.lds.h here. I'll change this in v3.

Sami