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

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Message-ID: <20201130181500.GH6019@zn.tnic>
Date:   Mon, 30 Nov 2020 19:15:00 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
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 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.

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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette