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SubjectRe: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction
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On Tue, 2018-07-10 at 16:48 -0700, Dave Hansen wrote:
> >
> > +/*
> > + * WRUSS is a kernel instrcution and but writes to user
> > + * shadow stack memory.  When a fault occurs, both
> > + * X86_PF_USER and X86_PF_SHSTK are set.
> > + */
> > +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));
> > +}
> I thought X86_PF_USER was set based on the mode in which the fault
> occurred.  Does this mean that the architecture of this bit is different
> now?

Yes.

> That seems like something we need to call out if so.  It also means we
> need to update the SDM because some of the text is wrong.

It needs to mention the WRUSS case.

>
> >
> >  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:
> >    */
> This needs commenting about why is_wruss() is special.

Ok.
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