Re: [PATCH] arm64: Trap WFI executed in userspace

[Dave Martin]

On Tue, Aug 07, 2018 at 11:24:34AM +0100, Marc Zyngier wrote:
> On 07/08/18 11:05, Dave Martin wrote:
> > On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote:
> >> It recently came to light that userspace can execute WFI, and that
> >> the arm64 kernel doesn trap this event. This sounds rather benign,
> >> but the kernel should decide when it wants to wait for an interrupt,
> >> and not userspace.
> >>
> >> Let's trap WFI and treat it as a way to yield the CPU to another
> >> process.
> > 
> > This doesn't amount to a justification.
> > 
> > If the power controller is unexpectedly left in a bad state so that
> > WFI will do something nasty to a cpu that may enter userspace, then we
> > probably have bigger problems.
> > 
> > So, maybe it really is pretty harmless to let userspace execute this.
> 
> Or not. It is also a very good way for userspace to find out when an
> interrupt gets delivered and start doing all kind of probing on the
> kernel. The least the userspace knows about that, the better I feel.

Possibly.  I suspect there are other ways to guess pretty accurately
when an interrupt occurs, but WFI allows greater precision.

> > I can't think of a legitimate reason for userspace to execute WFI
> > however.  Userspace doesn't have interrupts under Linux, so it makes
> > no sense to wait for one.
> > 
> > Have we seen anybody using WFI in userspace?  It may be cleaner to
> > map this to SIGILL rather than be permissive and regret it later.
> 
> I couldn't find any user, and I'm happy to just send userspace to hell
> in that case. But it could also been said that since it was never
> prevented, it is a de-facto ABI.

Agreed.  I wonder whether it's sufficient to have this mapping to SIGILL
in -next for a while and see whether anybody complains.

Cheers
---Dave