Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then NMI-safe

[Andy Lutomirski <luto@amacapital.net>]

On Sat, Jul 25, 2015 at 10:56 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Jul 24, 2015 at 9:59 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> And people will give me five new heads if I ignore Linus and do RET
>> even with IF=1, saving 300 cycles?
>
> So I'm still nervous about that "sti; ret" when we're back on the
> original kernel stack that took the original fault or interrupt.  But
> it's probably ok.
>
> Yes, it's irq-safe. But it's not NMI-safe, so if an NMI happens there,
> when the NMI returns, an interrupt might occur there too. But since
> we're back on the original stack where the original fault happened,
> and since interrupts were enabled, I don't see why that would be
> horrible. In theory, we might have a growing stack if this keeps
> happening, but since the only way to get that is to get the NMI in
> that one-instruction window (and apparently on at least _some_
> microarchitectures the sti shadow stops even NMI's), I don't see how
> any kind of unbounded growth would happen.
>
> So.
>
> I think it would work, and it might even be good for "coverage" (ie
> the whole "iret-to-ret-conversion" will not have a lot of testing if
> it only happens for faults with interrupts disabled).
>
> But it still worries me a bit.
>

What if we added something like:

if (regs->ip == ret_after_sti && !user_mode(regs) && (regs->flags &
X86_EFLAGS_IF)) {
  regs->ip--;
  regs->flags &= ~X86_EFLAGS_IF;
}

to do_nmi, do_machine_check, and do_debug (the latter because kernel
breakpoints, damnit)?

--Andy