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Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is
 enabled
To:     Pavel Machek <pavel@ucw.cz>, Dave Hansen <dave.hansen@intel.com>
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>,
        Borislav Petkov <bp@alien8.de>,
        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>,
        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>
References: <20200918192312.25978-1-yu-cheng.yu@intel.com>
 <20200918192312.25978-9-yu-cheng.yu@intel.com>
 <f02b511d-1d48-6dea-d2e6-84d58e21e6cd@intel.com>
 <20200918210026.GC4304@duo.ucw.cz>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <862eef02-eba2-e13f-ed67-f915f749ebca@intel.com>
Date:   Fri, 18 Sep 2020 14:21:10 -0700
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On 9/18/2020 2:00 PM, Pavel Machek 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.

I would revise the wording if there is another version.  What this patch 
does is:

If an application is compiled for CET and the system supports it, then 
the application cannot do vsyscall emulation.  Earlier we allow the 
emulation, and had a patch that fixes the shadow stack and endbr for the 
emulation code.  Since newer programs mostly do no do the emulation, we 
changed the patch do block it when attempted.

This patch would not block any legacy applications or any applications 
on older machines.

Yu-cheng