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Subject: Re: [PATCH v24 04/30] x86/cpufeatures: Introduce X86_FEATURE_CET and
 setup functions
To:     Borislav Petkov <bp@alien8.de>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>
References: <20210401221104.31584-1-yu-cheng.yu@intel.com>
 <20210401221104.31584-5-yu-cheng.yu@intel.com>
 <20210409101214.GC15567@zn.tnic>
 <c7cb0ed6-2725-ba0d-093e-393eab9918b2@intel.com>
 <20210409171408.GG15567@zn.tnic>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <f7a1299a-916f-70fe-6881-0951fe4fe38a@intel.com>
Date:   Fri, 9 Apr 2021 16:14:09 -0700
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On 4/9/2021 10:14 AM, Borislav Petkov wrote:
> On Fri, Apr 09, 2021 at 08:52:52AM -0700, Yu, Yu-cheng wrote:
>> Recall we had complicated code for the XSAVES features detection in
>> xstate.c.  Dave Hansen proposed the solution and then the whole thing
>> becomes simple.  Because of this flag, even when only the shadow stack is
>> available, the code handles it nicely.
> 
> Is that what you mean?
> 
> @@ -53,6 +55,8 @@ static short xsave_cpuid_features[] __initdata = {
>   	X86_FEATURE_INTEL_PT,
>   	X86_FEATURE_PKU,
>   	X86_FEATURE_ENQCMD,
> +	X86_FEATURE_CET, /* XFEATURE_CET_USER */
> +	X86_FEATURE_CET, /* XFEATURE_CET_KERNEL */
> 
> or what is the piece which becomes simpler?

Yes, this is it.

>> Would this equal to only CONFIG_X86_CET (one Kconfig option)?  In fact, when
>> you proposed only CONFIG_X86_CET, things became much simpler.
> 
> When you use CONFIG_X86_SHADOW_STACK instead, it should remain same
> simple no?
> 

Signals, arch_prctl, and ELF header are three places that need to depend 
on either shadow stack or IBT is configured.  To remain simple, we can 
make all three depend on CONFIG_X86_SHADOW_STACK, and in Kconfig, make 
CONFIG_X86_IBT depend on CONFIG_X86_SHADOW_STACK.  Without shadow stack, 
IBT itself is not as useful anyway.

>> Practically, IBT is not much in terms of code size.  Since we have already
>> separated the two, why don't we leave it as-is.  When people start using it
>> more, there will be more feedback, and we can decide if one Kconfig is
>> better?
> 
> Because when we add stuff to the kernel, we add the simplest and
> cleanest version possible and later, when we determine that additional
> functionality is needed, *then* we add it. Not the other way around.
> 
> Our Kconfig symbol space is already an abomination so we can't just add
> some more and decide later.
> 
> What happens in such situations usually is stuff gets added, it bitrots
> and some poor soul - very likely a maintainer who has to mop up after
> everybody - comes and cleans it up. I'd like to save myself that
> cleaning up.
> 
> Thx.
>