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

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Date:   Wed, 23 Feb 2022 10:39:35 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Dan Li <ashimida@...ux.alibaba.com>
Cc:     catalin.marinas@....com, will@...nel.org, ndesaulniers@...gle.com,
        keescook@...omium.org, masahiroy@...nel.org, tglx@...utronix.de,
        akpm@...ux-foundation.org, mark.rutland@....com,
        samitolvanen@...gle.com, npiggin@...il.com, linux@...ck-us.net,
        mhiramat@...nel.org, ojeda@...nel.org, luc.vanoostenryck@...il.com,
        elver@...gle.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [PATCH] AARCH64: Add gcc Shadow Call Stack support

On Wed, Feb 23, 2022 at 12:50:21AM -0800, Dan Li wrote:
> 
> 
> On 2/22/22 08:16, Nathan Chancellor wrote:
> > 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.
> > >   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
> > 
> > I wonder if we want to just ditch the mention of the compiler if both
> > support it?
> > 
> 
> My intention is to remind users that this is a compiler feature.
> But since there is also a hint in CC_HAVE_SHADOW_CALL_STACK:
> +# Supported by clang >= 7.0 or GCC ...
> 
> Removing the specific compiler here also looks fine to me.
> Would this look better?
> 
> "This option enables Shadow Call Stack, which uses a ..."
> 
> or maybe:
> 
> "This option enables compiler's Shadow Call Stack, which uses a ..."

I do not honestly have a strong opinion around removing mention of the
compiler so either looks fine to me (might be better to say "the
compiler's Shadow ..." in the second one).

> > >   	  shadow stack to protect function return addresses from being
> > >   	  overwritten by an attacker. More information can be found in
> > >   	  Clang's documentation:
> > > 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
> > 
> > Same thing here, although eventually there may be a minimum GCC version
> > bump to something newer than 11.2.0, which would allow us to just drop
> > CONFIG_CC_HAVE_SHADOW_CALL_STACK altogether. No strong opinion.
> > 
> 
> As Guenter said, I thought maybe we could mark the minimum available
> version for users :)

Yes, that is what I was getting at with the "minimum version" comment.
It should remain around.

Cheers,
Nathan