Re: [PATCH] [PATCH v2] AARCH64: Add gcc Shadow Call Stack support

[Dan Li]

On 2/25/22 12:58, Kees Cook wrote:
> On Thu, Feb 24, 2022 at 07:24:10PM -0800, Dan Li wrote:
> > Signed-off-by: Dan Li <ashimida@xxxxxxxxxxxxxxxxx>
>
> Thanks for the tweaks!
>
> > ---
> > FYI:
> > This function can be used to test if the shadow call stack works:
> > //noinline void __noscs scs_test(void)
> > noinline void scs_test(void)
> > {
> >      unsigned long * lr = (unsigned long *)__builtin_frame_address(0) + 1;
> >
> >      asm volatile("str xzr, [%0]\n\t": : "r"(lr) : "x30");
> > }
>
> Not a big deal, but just FYI, there's a lot of whitespace trailing the
> "}" above...

Ah, sorry for the mistake.
> > If SCS protection is enabled, this function will return normally.
> > If the function has __noscs attribute (scs disabled), it will crash due to 0
> > address access.
>
> It would be cool to turn this into an LKDTM test... (see things like the
> CFI_FORWARD_PROTO test). I imagine this should be CFI_BACKWARD_SHADOW or
> something...

OK, I'll add it in the next version.

> Also, I assume you're using real hardware to test this? It'd be nice to
> see if qemu can be convinced to run with the needed features. Whenever
> I've tried this it becomes impossibly slow. :)

I also use qemu to test the patch (qemu 6.1.0 with command "-cpu max"),
and can feel the performance drop.

Maybe because my test environment only has simple busybox and ltp,
the feeling of a slow system running is not that strong for me :)

For comparison, I simply tested the difference in kernel boot time
in my test environment:
//run qemu with "-cpu cortex-a57",
[    1.254481] Run /linuxrc as init process
//run qemu with "-cpu max"
[    3.566091] Run /linuxrc as init process

> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index ccbbd31b3aae..deff5b308470 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -97,6 +97,10 @@
> >   #define KASAN_ABI_VERSION 4
> >   #endif
> >   
> > +#ifdef CONFIG_SHADOW_CALL_STACK
> > +#define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
> > +#endif
>
> I initially wondered if we need a separate __no_sanitize(STUFF) patch to
> make the compiler-clang.h macros easier, but I see there are places
> where we do multiple ("address", "hwaddress") and have specialized
> macros, so I think this is fine. And since GCC doesn't support
> "__has_feature", I think this is the correct location for this.

As in:
https://lore.kernel.org/all/26a0a816-bc3e-2ac0-d773-0819d9f225af@xxxxxxxxxxxxxxxxx/

I think maybe we could use "#ifdef CONFIG_SHADOW_CALL_STACK"
instead of "#if __has_attribute(__no_sanitize_address__)" here,
then move it to `compiler_types.h`.

From my current test results, __noscs seems to work fine in
clang compilation.

Thanks,
Dan.