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Re: [Xen-devel] Resume from suspend to RAM broken when using early microcode updates

On Wed, Apr 11, 2018 at 01:12:51PM +0100, Andrew Cooper wrote:
> On 11/04/18 13:01, Simon Gaiser wrote:
> > Andrew Cooper:
> >> On 11/04/18 12:48, Simon Gaiser wrote:
> >>> Hi,
> >>>
> >>> when I use early microcode loading with the microcode update with the
> >>> BTI mitigations, resuming from suspend to RAM is broken.
> >>>
> >>> Based on added logging to enter_state() (from power.c) it doesn't
> >>> survive the local_irq_restore(flags) call (at least a printk() after the
> >>> call doesn't output anything on the serial console).
> >>>
> >>> I guess that some irq handler tries to use IBRS/IBPB. But the microcode
> >>> is only loaded later.
> >>>
> >>> If I simply move the microcode_resume_cpu(0) directly before the
> >>> local_irq_restore(flags) everything seems to work fine. But I'm not sure
> >>> if this has unintended consequences.
> >>>
> >>> I tested the above with Xen 4.8.3 from Qubes which includes the BTI and
> >>> microcode patches from staging-4.8. AFAICS there are no commits which
> >>> changes the affected code or other commits which sound relevant so this
> >>> probably affected also all the newer branches.
> >> S3 support is a very unloved area of the hypervisor.
> >>
> >> Yes - we definitely need to get microcode reloaded before interrupts are
> >> enabled.
> > Do you see any problems with simply moving microcode_resume_cpu(0)
> > directly before the local_irq_restore(flags) call? (I'm not familiar
> > with the code at all and (early) resume handling sounds like something
> > which is easy to break in non obvious ways)
> Judging by what is going on, it wants to be between tboot_s3_error() and
> the done label.
> We only need to restore microcode if we successfully went into S3.  The
> done and enable_cpu labels are only used by paths which don't need to
> restore microcode.
> OTOH, you should check the return value and panic if restoration
> failed.  As you've seen, the system won't survive trying to blindly
> continue resuming.
> >
> >> That said, I would have expected a backtrace complaining about
> >> a GP fault if we had hit the use of IBRS/IBPB before the microcode was
> >> reloaded.
> > Yeah, not sure what's happening here. I don't get any output from after
> > local_irq_restore(flags). If you have some ideas for more debug output I
> > can easily test it.
> In hindsight, I am.  We take a #GP fault because of a bad MSR, and at
> the head of the exception handler try to use the same bad MSR.  It will
> repeatedly fault until hitting a guard page (or other read-only page),
> at which point we take a double fault, and suffer a #GP yet again. 
> Taking a #DF will reset the stack to a moderately sane value, and the
> system will livelock taking faults.
> This is an unfortunate consequence of having $MAGIC in the exception
> handlers.

We can just disable IBRS before going to sleep?

> ~Andrew
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