Re: [PATCH v5 28/34] x86/fred: fixup fault on ERETU by jumping to fred_entrypoint_user

Lai Jiangshan
2023-03-17T13:03:09Z

On Fri, Mar 17, 2023 at 5:56 PM <andrew.cooper3@citrix.com> wrote:
>
> On 17/03/2023 9:39 am, Lai Jiangshan wrote:
> >> +#ifdef CONFIG_X86_FRED
> >> +static bool ex_handler_eretu(const struct exception_table_entry *fixup,
> >> + struct pt_regs *regs, unsigned long error_code)
> >> +{
> >> + struct pt_regs *uregs = (struct pt_regs *)(regs->sp - offsetof(struct pt_regs, ip));
> >> + unsigned short ss = uregs->ss;
> >> + unsigned short cs = uregs->cs;
> >> +
> >> + fred_info(uregs)->edata = fred_event_data(regs);
> >> + uregs->ssx = regs->ssx;
> >> + uregs->ss = ss;
> >> + uregs->csx = regs->csx;
> >> + uregs->current_stack_level = 0;
> >> + uregs->cs = cs;
> > Hello
> >
> > If the ERETU instruction had tried to return from NMI to ring3 and just faulted,
> > is NMI still blocked?
> >
> > We know that IRET unconditionally enables NMI, but I can't find any clue in the
> > FRED's manual.
> >
> > In the pseudocode of ERETU in the manual, it seems that NMI is only enabled when
> > ERETU succeeds with bit28 in csx set. If so, this code will fail to reenable
> > NMI if bit28 is not explicitly re-set in csx.
>
> IRET clearing NMI blocking is the source of an immense amount of grief,
> and ultimately the reason why Linux and others can't use supervisor
> shadow stacks at the moment.
>
> Changing this property, so NMIs only get unblocked on successful
> execution of an ERET{S,U}, was a key demand of the FRED spec.
>
> i.e. until you have successfully ERET*'d, you're still logically in the
> NMI handler and NMIs need to remain blocked even when handling the #GP
> from a bad ERET.
>

Handling of the #GP for a bad ERETU can be rescheduled. It is not
OK to reschedule with NMI blocked.

I think "regs->nmi = 1;" (not uregs->nmi) can fix the problem.