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

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Date:   Wed, 26 Feb 2020 17:02:28 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <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>,
        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, Feb 26, 2020 at 10:05 AM Dave Hansen <dave.hansen@...el.com> 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...).

CET requires a complete new OS image from kernel, toolchain, run-time.
CET enabled kernel without the rest of updated OS won't give you CET
at all.

-- 
H.J.