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From:   Florian Weimer <fweimer@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
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        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@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>,
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        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>
Subject: Re: [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled
References: <20200825002645.3658-1-yu-cheng.yu@intel.com>
        <20200825002645.3658-10-yu-cheng.yu@intel.com>
        <CALCETrVXwUDu2m-XEd-_J03L=sricM4cMxQYVkdGRWZDjmMB2g@mail.gmail.com>
Date:   Tue, 25 Aug 2020 11:14:37 +0200
In-Reply-To: <CALCETrVXwUDu2m-XEd-_J03L=sricM4cMxQYVkdGRWZDjmMB2g@mail.gmail.com>
        (Andy Lutomirski's message of "Mon, 24 Aug 2020 17:32:35 -0700")
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* Andy Lutomirski:

> On Mon, Aug 24, 2020 at 5:30 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>
>> From: "H.J. Lu" <hjl.tools@gmail.com>
>>
>> Emulation of the legacy vsyscall page is required by some programs built
>> before 2013.  Newer programs after 2013 don't use it.  Disallow vsyscall
>> emulation when Control-flow Enforcement (CET) is enabled to enhance
>> security.
>
> NAK.
>
> By all means disable execute emulation if CET-IBT is enabled at the
> time emulation is attempted, and maybe even disable the vsyscall page
> entirely if you can magically tell that CET-IBT will be enabled when a
> process starts, but you don't get to just disable it outright on a
> CET-enabled kernel.

Yeah, we definitely would have to revert/avoid this downstream.  People
definitely want to run glibc-2.12-era workloads on current kernels.
Thanks for catching it.

Florian