Re: [PATCH v5 21/27] x86/cet/shstk: Introduce WRUSS instruction

[Yu-cheng Yu]

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