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

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Date:   Fri, 07 Jun 2019 12:56:40 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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>,
        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>,
        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

On Fri, 2019-06-07 at 11:58 -0700, Dave Hansen wrote:
> On 6/7/19 11:29 AM, Andy Lutomirski wrote:
> ...
> > > I think this new MSR probably needs to get included in oops output when
> > > CET is enabled.
> > 
> > This shouldn’t be able to OOPS because it only happens at CPL 3,
> > right?  We should put it into core dumps, though.
> 
> Good point.
> 
> Yu-cheng, can you just confirm that the bitmap can't be referenced in
> ring-0, no matter what?  We should also make sure that no funny business
> happens if we put an address in the bitmap that faults, or is
> non-canonical.  Do we have any self-tests for that?

Yes, the bitmap is user memory, but the kernel can still get to it (e.g.
copy_from_user()).  We can do more check on the address.

> 
> Let's say userspace gets a fault on this.  Do they have the
> introspection capability to figure out why they faulted, say in their
> signal handler?

The bitmap address is kept by the application; the kernel won't provide it again
to user-space.  In the signal handler, the app can find out from its own record.

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