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

Andy Lutomirski
2018-11-06T18:55:36Z

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