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Subject: Re: [PATCH v5 21/27] x86/cet/shstk: Introduce WRUSS instruction
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
Date: Tue, 06 Nov 2018 12:21:38 -0800
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To: Dave Hansen <dave.hansen@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>, Dave Hansen <dave.hansen@linux.intel.com>, Eugene Syromiatnikov <esyr@redhat.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@chromium.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>, Randy Dunlap <rdunlap@infradead.org>, "Ravi V. Shankar" <ravi.v.shankar@intel.com>, Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
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On Tue, 2018-11-06 at 10:43 -0800, Dave Hansen wrote:
> On 10/11/18 8:15 AM, Yu-cheng Yu wrote:
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long
> > error_code,
> >  		error_code |= X86_PF_USER;
> >  		flags |= FAULT_FLAG_USER;
> >  	} else {
> > +		/*
> > +		 * WRUSS is a kernel instruction and but writes
> > +		 * to user shadow stack.  When a fault occurs,
> > +		 * both X86_PF_USER and X86_PF_SHSTK are set.
> > +		 * Clear X86_PF_USER here.
> > +		 */
> > +		if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
> > +		    (X86_PF_USER | X86_PF_SHSTK))
> > +			error_code &= ~X86_PF_USER;
> 
> This hunk of code basically points out that the architecture of WRUSS is
> broken for Linux.  The setting of X86_PF_USER for a ring-0 instruction
> really is a mis-feature of the architecture for us and we *undo* it in
> software which is unfortunate.  Wish I would have caught this earlier.
> 
> Andy, note that this is another case where hw_error_code and
> sw_error_code will diverge, unfortunately.
> 
> Anyway, this is going to necessitate some comment updates in the page
> fault code.  Yu-cheng, you are going to collide with some recent changes
> I made to the page fault code.  Please be careful with the context when
> you do the merge and make sure that all the new comments stay correct.

Ok.  Thanks!

Yu-cheng