linux-kernel - Re: [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled

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Message-ID: <87pn7f9jeq.fsf@oldenburg2.str.redhat.com>
Date:   Tue, 25 Aug 2020 11:14:37 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <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>,
        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>,
        "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>
Subject: Re: [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled

* Andy Lutomirski:

> On Mon, Aug 24, 2020 at 5:30 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>>
>> From: "H.J. Lu" <hjl.tools@...il.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