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

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Date:   Sat, 28 Nov 2020 08:23:59 -0800
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode
 Shadow Stack

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.
>