linux-kernel - Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

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Date:   Sat, 8 Jun 2019 22:52:18 +0200
From:   Pavel Machek <pavel@....cz>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Yu-cheng Yu <yu-cheng.yu@...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@...capital.net>,
        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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function

Hi!

> > I've no idea what the kernel should do; since you failed to answer the
> > question what happens when you point this to garbage.
> > 
> > Does it then fault or what?
> 
> Yeah, I think you'll fault with a rather mysterious CR2 value since
> you'll go look at the instruction that faulted and not see any
> references to the CR2 value.
> 
> I think this new MSR probably needs to get included in oops output when
> CET is enabled.
> 
> Why don't we require that a VMA be in place for the entire bitmap?
> Don't we need a "get" prctl function too in case something like a JIT is
> running and needs to find the location of this bitmap to set bits itself?
> 
> Or, do we just go whole-hog and have the kernel manage the bitmap
> itself. Our interface here could be:
> 
> 	prctl(PR_MARK_CODE_AS_LEGACY, start, size);
> 
> and then have the kernel allocate and set the bitmap for those code
> locations.

For the record, that sounds like a better interface than userspace knowing
about the bitmap formats...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html