linux-kernel - Re: [RFC PATCH v2 12/27] x86/mm: Shadow stack page fault error checking

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Message-ID: <1531330096.15351.10.camel@intel.com>
Date:   Wed, 11 Jul 2018 10:28:16 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.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>,
        Cyrill Gorcunov <gorcunov@...il.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@...omiun.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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 12/27] x86/mm: Shadow stack page fault error
 checking

On Tue, 2018-07-10 at 15:52 -0700, Dave Hansen wrote:
> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > 
> > +++ b/arch/x86/include/asm/traps.h
> > @@ -157,6 +157,7 @@ enum {
> >   *   bit 3 ==				1: use of reserved
> > bit detected
> >   *   bit 4 ==				1: fault was an
> > instruction fetch
> >   *   bit 5 ==				1: protection keys
> > block access
> > + *   bit 6 ==				1: shadow stack
> > access fault
> >   */
> Could we document this bit better?
> 
> Is this a fault where the *processor* thought it should be a shadow
> stack fault?  Or is it also set on faults to valid shadow stack PTEs
> that just happen to fault for other reasons, say protection keys?

Thanks Vedvyas for explaining this to me.
I will add this to comments:

This flag is 1 if (1) CR4.CET = 1; and (2) the access causing the page-
fault exception was a shadow-stack data access.

So this bit does not report the reason for the fault. It reports the
type of access; i.e. it was a shadow-stack-load or a shadow-stack-store 
that took the page fault. The fault could have been caused by any
variety of reasons including protection keys.