Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack

[Yu, Yu-cheng]

On 11/27/2020 9:10 AM, Borislav Petkov wrote:
> On Tue, Nov 10, 2020 at 08:21:50AM -0800, Yu-cheng Yu wrote:
> > +config X86_CET
> > +	def_bool n
> > +
> > +config ARCH_HAS_SHADOW_STACK
> > +	def_bool n
> > +
> > +config X86_SHADOW_STACK_USER
>
> Is X86_SHADOW_STACK_KERNEL coming too?
>
> Regardless, you can add it when it comes and you can use only X86_CET
> for now and drop this one and simplify this pile of Kconfig symbols.

We have X86_BRANCH_TRACKING_USER too.  My thought was, X86_CET means any 
of kernel/user shadow stack/ibt.

> > +	prompt "Intel Shadow Stacks for user-mode"
> > +	def_bool n
> > +	depends on CPU_SUP_INTEL && X86_64
> > +	depends on AS_HAS_SHADOW_STACK
> > +	select ARCH_USES_HIGH_VMA_FLAGS
> > +	select X86_CET
> > +	select ARCH_HAS_SHADOW_STACK
> > +	help
> > +	  Shadow Stacks provides protection against program stack
> > +	  corruption.  It's a hardware feature.  This only matters
> > +	  if you have the right hardware.  It's a security hardening
> > +	  feature and apps must be enabled to use it.  You get no
> > +	  protection "for free" on old userspace.  The hardware can
> > +	  support user and kernel, but this option is for user space
> > +	  only.
> > +	  Support for this feature is only known to be present on
> > +	  processors released in 2020 or later.  CET features are also
> > +	  known to increase kernel text size by 3.7 KB.
>
> This help text needs some rewriting. You can find an inspiration about
> more adequate style in that same Kconfig file.

I will work on it.

> > +
> > +	  If unsure, say N.
> > +
> >   config EFI
> >   	bool "EFI runtime service support"
> >   	depends on ACPI
> > diff --git a/scripts/as-x86_64-has-shadow-stack.sh b/scripts/as-x86_64-has-shadow-stack.sh
> > new file mode 100755
> > index 000000000000..fac1d363a1b8
> > --- /dev/null
> > +++ b/scripts/as-x86_64-has-shadow-stack.sh
> > @@ -0,0 +1,4 @@
> > +#!/bin/sh
> > +# SPDX-License-Identifier: GPL-2.0
> > +
> > +echo "wrussq %rax, (%rbx)" | $* -x assembler -c -
>
> 						      2> /dev/null
>
> otherwise you get
>
> {standard input}: Assembler messages:
> {standard input}:1: Error: no such instruction: `wrussq %rax,(%rbx)
>
> on non-enlightened toolchains during build.

Yes, I will fix this in the next revision.

Yu-cheng

> Thx.