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

[Dan Li]

On 2/22/22 10:47, Mark Rutland wrote:
> Hi,
>
> On Tue, Feb 22, 2022 at 01:57:36AM -0800, Dan Li wrote:
> > Shadow call stack is available in GCC > 11.2.0, this patch makes
> > the corresponding kernel configuration available when compiling
> > the kernel with gcc.
>
> Neat!
>
> My local GCC devs told me that means GCC 12.x.x rather than 11.2.x or
> 11.3.x, so as others have said it'd be clearer to say `GCC >= 12.0.0`.

Ah, yes, I just saw this flag in gcc/BASE-VER.

> I'd like to try this with a GCC binary before I provide an Ack or R-b;
> but in the mean time I have a few comments below.

Thanks for your test, Mark.
Please let me know if there is any issues :)

> > ---
> > 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)
> > {
> >      register unsigned long *sp asm("sp");
> >      unsigned long * lr = sp + 1;
> >
> >      asm volatile("":::"x30");
> >      *lr = 0;
> > }
>
> It's probably be better to use __builtin_frame_address(0) to get the
> address of the frame record rather than assuming that fp==sp in the
> middle of the function.

Got it.

> >   config SHADOW_CALL_STACK
> > -	bool "Clang Shadow Call Stack"
> > -	depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
> > +	bool "Shadow Call Stack"
> > +	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> >   	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
> >   	help
> > -	  This option enables Clang's Shadow Call Stack, which uses a
> > +	  This option enables Clang/GCC's Shadow Call Stack, which uses a
> >   	  shadow stack to protect function return addresses from being
> >   	  overwritten by an attacker. More information can be found in
> >   	  Clang's documentation:
>
> Is there any additional GCC documentation that we can refer to?

Currently there is only a brief description of
-fsanitize=shadow-call-stack in the user manual.

Since the description of the clang documentation is more detailed, should
I add this gcc link to the configuration description?

Link: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#Instrumentation-Options

> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 09b885cc4db5..a48a604301aa 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -1255,7 +1255,7 @@ config HW_PERF_EVENTS
> >   config ARCH_HAS_FILTER_PGPROT
> >   	def_bool y
> >   
> > -# Supported by clang >= 7.0
> > +# Supported by clang >= 7.0 or GCC > 11.2.0
>
> As above, I beleive that should be `GCC >= 12.0.0`.

Yeah.

Thanks,
Dan.