linux-kernel - Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

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Message-ID: <070d9d78981f0aad2baf740233e8dfc32ecd29d7.camel@intel.com>
Date:   Fri, 06 Mar 2020 10:37:13 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, 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>,
        Borislav Petkov <bp@...en8.de>,
        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>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for
 user-mode Shadow Stack protection

On Wed, 2020-02-26 at 10:05 -0800, Dave Hansen wrote:
> On 2/5/20 10:19 AM, Yu-cheng Yu wrote:
> > +# Check assembler Shadow Stack suppot
> 
> 				  ^ support
> 
> > +ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> > +  ifeq ($(call as-instr, saveprevssp, y),)
> > +      $(error CONFIG_X86_INTEL_SHADOW_STACK_USER not supported by the assembler)
> > +  endif
> > +endif
> 
> Is this *just* looking for instruction support in the assembler?
> 
> We usually just .byte them, like this for pkeys:
> 
>         asm volatile(".byte 0x0f,0x01,0xee\n\t"
>                      : "=a" (pkru), "=d" (edx)
>                      : "c" (ecx));
> 
> That way everybody with old toolchains can still build the kernel (and
> run/test code with your config option on, btw...).

We used to do this for CET instructions, but after adding kernel-mode
instructions and inserting ENDBR's, the code becomes cluttered.  I also
found an earlier discussion on the ENDBR:

https://lore.kernel.org/lkml/CALCETrVRH8LeYoo7V1VBPqg4WS0Enxtizt=T7dPvgoeWfJrdzA@mail.gmail.com/

It makes sense to let the user know early on that the system cannot support
CET and cannot build a CET-enabled kernel.

One thing we can do is to disable CET in Kconfig and not in kernel
build, which I will do in the next version.

Yu-cheng