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

[Andy Lutomirski]

On Tue, Nov 6, 2018 at 10:43 AM Dave Hansen <dave.hansen@xxxxxxxxx> 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.

I'm going to send a patch set in the next day or two that cleans it up
further and is probably good preparation for WRUSS.