linux-kernel - Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction

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Date:   Thu, 12 Jul 2018 15:59:58 -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 18/27] x86/cet/shstk: Introduce WRUSS instruction

On Tue, 2018-07-10 at 16:48 -0700, Dave Hansen wrote:
> > 
> > +/*
> > + * WRUSS is a kernel instrcution and but writes to user
> > + * shadow stack memory.  When a fault occurs, both
> > + * X86_PF_USER and X86_PF_SHSTK are set.
> > + */
> > +static int is_wruss(struct pt_regs *regs, unsigned long error_code)
> > +{
> > +	return (((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
> > +		(X86_PF_USER | X86_PF_SHSTK)) && !user_mode(regs));
> > +}
> I thought X86_PF_USER was set based on the mode in which the fault
> occurred.  Does this mean that the architecture of this bit is different
> now?

Yes.

> That seems like something we need to call out if so.  It also means we
> need to update the SDM because some of the text is wrong.

It needs to mention the WRUSS case.

> 
> > 
> >  static void
> >  show_fault_oops(struct pt_regs *regs, unsigned long error_code,
> >  		unsigned long address)
> > @@ -848,7 +859,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
> >  	struct task_struct *tsk = current;
> >  
> >  	/* User mode accesses just cause a SIGSEGV */
> > -	if (error_code & X86_PF_USER) {
> > +	if ((error_code & X86_PF_USER) && !is_wruss(regs, error_code)) {
> >  		/*
> >  		 * It's possible to have interrupts off here:
> >  		 */
> This needs commenting about why is_wruss() is special.

Ok.