linux-kernel - Re: [PATCH v10 01/26] Documentation/x86: Add CET description

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Date:   Tue, 12 May 2020 16:20:32 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, 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>,
        "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 v10 01/26] Documentation/x86: Add CET description

On Wed, 2020-04-29 at 16:02 -0700, Yu-cheng Yu wrote:
> On Wed, 2020-04-29 at 15:53 -0700, Dave Hansen wrote:
> > On 4/29/20 3:07 PM, Yu-cheng Yu wrote:
> > > +Note:
> > > +  There is no CET-enabling arch_prctl function.  By design, CET is enabled
> > > +  automatically if the binary and the system can support it.
> > 
> > I think Andy and I danced around this last time.  Let me try to say it
> > more explicitly.
> > 
> > I want CET kernel enabling to able to be disconnected from the on-disk
> > binary.  I want a binary compiled with CET to be able to disable it, and
> > I want a binary not compiled with CET to be able to enable it.  I want
> > different threads in a process to be able to each have different CET status.
> 
> The kernel patches we have now can be modified to support this model.  If after
> discussion this is favorable, I will modify code accordingly.

To turn on/off and to lock CET are application-level decisions.  The kernel does
not prevent any of those.  Should there be a need to provide an arch_prctl() to
turn on CET, it can be added without any conflict to this series.

> > Which JITs was this tested with?  I think as a bare minimum we need to
> > know that this design can accommodate _a_ modern JIT.  It would be
> > horrible if the browser javascript engines couldn't use this design, for
> > instance.
> 
> JIT work is still in progress.  When that is available I will test it.

I found CET has been enabled in LLVM JIT, Mesa JIT as well as sljit which is
used by jit.  So the current model works with JIT.

Yu-cheng