Re: [PATCH v5 09/15] x86: Use an opaque type for functions not callable from C

Kees Cook
2021-10-14T16:07:59Z

On Thu, Oct 14, 2021 at 01:21:38PM +0200, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 11:16:52AM -0700, Sami Tolvanen wrote:
> > The kernel has several assembly functions that are not directly callable
> > from C. Use an opaque type for these function prototypes to make misuse
> > harder, and to avoid the need to annotate references to these functions
> > for Clang's Control-Flow Integrity (CFI).
> >
> > Suggested-by: Andy Lutomirski <luto@amacapital.net>
> > Suggested-by: Alexander Lobakin <alobakin@pm.me>
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> > ---
> > arch/x86/include/asm/ftrace.h | 2 +-
> > arch/x86/include/asm/idtentry.h | 10 +++++-----
> > arch/x86/include/asm/page_64.h | 7 ++++---
> > arch/x86/include/asm/paravirt_types.h | 3 ++-
> > arch/x86/include/asm/processor.h | 2 +-
> > arch/x86/include/asm/proto.h | 25 +++++++++++++------------
> > arch/x86/include/asm/uaccess_64.h | 9 +++------
> > arch/x86/kernel/alternative.c | 2 +-
> > arch/x86/kernel/ftrace.c | 2 +-
> > arch/x86/kernel/paravirt.c | 4 ++--
> > arch/x86/kvm/emulate.c | 4 ++--
> > arch/x86/kvm/kvm_emulate.h | 9 ++-------
> > arch/x86/xen/enlighten_pv.c | 6 +++---
> > arch/x86/xen/xen-ops.h | 10 +++++-----
> > 14 files changed, 45 insertions(+), 50 deletions(-)
>
> No matter from which direction I look at it, wrapping some functions
> which a crazy macro doesn't look good.
>
> So what's the plan here?
>
> Everytime someone adds an asm function which is not callable from C but
> forgets to use that magic macro, someone from team CFI will send a patch
> fixing that?
>
> I.e., a whack-a-mole game?

I don't think it's a super common thing to add, so in this case, yes,
I think doing it on a case-by-case basis will be fine. This is common
practice in the kernel; not everyone tests all CONFIGs, so stuff gets
missed, patches are sent, life goes on. :)

> If we're going to do that keep-CFI-working game, we might just as well
> not do the macro but use the C code it evaluates to, so that at least it
> looks ok-ish:
>
> DECLARE_NOT_CALLED_FROM_C(int3_magic);
>
> vs
>
> extern const u8 int3_magic[];

I'd _much_ prefer keeping the macro, as it explains what's going on,
which doesn't require a comment at every "extern const u8 foo[]" usage.
It serves as an annotation, etc.

And, there's been a lot of discussion on the best way to do this, what
to name it, etc. This looks to be the best option currently.

-Kees

--
Kees Cook