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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Fri, 18 Sep 2020 14:06:02 -0700
Message-ID: <CAMe9rOrHCE51dKSz3fPXG-ORNim_Ok7rtwQxnudH9et4ecHBRA@mail.gmail.com>
Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <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>,
        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>
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On Fri, Sep 18, 2020 at 2:00 PM Pavel Machek <pavel@ucw.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.

-- 
H.J.