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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/04/18 15:49, Konrad Rzeszutek Wilk wrote:
> 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?

The problem is the use of MSR_SPEC_CTRL/MSR_PRED_CMD before we've
reloaded the microcode which causes those MSRs to magic themselves into
existence.

~Andrew

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