linux-kernel - Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

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Message-ID: <4f93b106-53fc-40df-4c9b-c26490be24c8@intel.com>
Date:   Fri, 18 Sep 2020 14:17:03 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "H.J. Lu" <hjl.tools@...il.com>, Pavel Machek <pavel@....cz>
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>,
        "open list:DOCUMENTATION" <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>,
        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>
Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is
 enabled

On 9/18/20 2:06 PM, H.J. Lu wrote:
> On Fri, Sep 18, 2020 at 2:00 PM Pavel Machek <pavel@....cz> wrote:
>> On Fri 2020-09-18 12:32:57, Dave Hansen wrote:
>>> On 9/18/20 12:23 PM, Yu-cheng Yu wrote:
>>>> Emulation of the legacy vsyscall page is required by some programs
>>>> built before 2013.  Newer programs after 2013 don't use it.
>>>> Disable vsyscall emulation when Control-flow Enforcement (CET) is
>>>> enabled to enhance security.
>>> How does this "enhance security"?
>>>
>>> What is the connection between vsyscall emulation and CET?
>> Boom.
>>
>> We don't break compatibility by default, and you should not tell
>> people to enable CET by default if you plan to do this.
>>
> Nothing will be broken.   CET enabled applications don't use/need
> vsyscall emulation.

Hi H.J.,

Could you explain your logic a bit more thoroughly, please?

I also suspect that Pavel was confused by your changelog where you said
that you do this when "CET is enabled".  Does enabled in this context mean:
1. Just CET support compiled in, or
2. Compiled in and on CET hardware, or
3. Compiled in to the kernel enabled in the app and running on CET
   hardware?