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Message-ID: <1531503430.11680.2.camel@intel.com>
Subject: Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromiun.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Date:   Fri, 13 Jul 2018 10:37:10 -0700
In-Reply-To: <166536e2-b296-7be5-d1b7-982cf56f1f9b@linux.intel.com>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
         <20180710222639.8241-19-yu-cheng.yu@intel.com>
         <166536e2-b296-7be5-d1b7-982cf56f1f9b@linux.intel.com>
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On Fri, 2018-07-13 at 05:12 -0700, Dave Hansen wrote:
> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > 
> > +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));
> > +}
> > +
> >  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:
> >  		 */
> Please don't do it this way.
> 
> We have two styles of page fault:
> 1. User page faults: find a VMA, try to handle (allocate memory et al.),
>    kill process if we can't handle.
> 2. Kernel page faults: search for a *discrete* set of conditions that
>    can be handled, including faults in instructions marked in exception
>    tables.
> 
> X86_PF_USER *means*: do user page fault handling.  In the places where
> the hardware doesn't set it, but we still want user page fault handling,
> we manually set it, like this where we "downgrade" an implicit
> supervisor access to a user access:
> 
>         if (user_mode(regs)) {
>                 local_irq_enable();
>                 error_code |= X86_PF_USER;
>                 flags |= FAULT_FLAG_USER;
> 
> So, just please *clear* X86_PF_USER if !user_mode(regs) and X86_PF_SS.
> We do not want user page fault handling, thus we should not keep the bit
> set.

Agree.  I will change that.

Yu-cheng