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

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Date:   Mon, 30 Nov 2020 14:48:09 -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 11/30/2020 10:15 AM, Borislav Petkov wrote:
> On Sat, Nov 28, 2020 at 08:23:59AM -0800, Yu, Yu-cheng wrote:
>> We have X86_BRANCH_TRACKING_USER too.  My thought was, X86_CET means any of
>> kernel/user shadow stack/ibt.
> 
> It is not about what it means - it is what you're going to use/need. You have
> ifdeffery both with X86_CET and X86_SHADOW_STACK_USER.
> 
> This one
> 
> +#ifdef CONFIG_X86_SHADOW_STACK_USER
> +#define DISABLE_SHSTK	0
> +#else
> +#define DISABLE_SHSTK	(1 << (X86_FEATURE_SHSTK & 31))
> +#endif
> 
> for example, is clearly wrong and wants to be #ifdef CONFIG_X86_CET, for
> example. Unless I'm missing something totally obvious.

Logically, enabling IBT without shadow stack does not make sense, but 
these features have different CPUIDs, and CONFIG_X86_SHADOW_STACK_USER 
and CONFIG_X86_BRANCH_TRACKING_USER can be selected separately.

Do we want to have only one selection for both features?  In other 
words, we turn on both or neither.

Thanks,
Yu-cheng

> 
> In any case, you need to analyze what Kconfig defines the code will
> need and to what they belong and add only the minimal subset needed.
> Our Kconfig symbols space is already nuts so adding more needs to be
> absolutely justified.
> 
> Thx.
>