Re: [PATCH v9 01/26] arm64: Fix HCR.TGE status for NMI contexts

[Marc Zyngier]

On 31/01/2019 09:40, Julien Thierry wrote:
> 
> 
> On 31/01/2019 09:27, Christoffer Dall wrote:
>> On Thu, Jan 31, 2019 at 08:56:04AM +0000, Julien Thierry wrote:
>>>
>>>
>>> On 31/01/2019 08:19, Christoffer Dall wrote:
>>>> On Mon, Jan 28, 2019 at 03:42:42PM +0000, Julien Thierry wrote:
>>>>> Hi James,
>>>>>
>>>>> On 28/01/2019 11:48, James Morse wrote:
>>>>>> Hi Julien,
>>>>>>
>>>>>> On 21/01/2019 15:33, Julien Thierry wrote:
>>>>>>> When using VHE, the host needs to clear HCR_EL2.TGE bit in order
>>>>>>> to interract with guest TLBs, switching from EL2&0 translation regime
>>>>>>
>>>>>> (interact)
>>>>>>
>>>>>>
>>>>>>> to EL1&0.
>>>>>>>
>>>>>>> However, some non-maskable asynchronous event could happen while TGE is
>>>>>>> cleared like SDEI. Because of this address translation operations
>>>>>>> relying on EL2&0 translation regime could fail (tlb invalidation,
>>>>>>> userspace access, ...).
>>>>>>>
>>>>>>> Fix this by properly setting HCR_EL2.TGE when entering NMI context and
>>>>>>> clear it if necessary when returning to the interrupted context.
>>>>>>
>>>>>> Yes please. This would not have been fun to debug!
>>>>>>
>>>>>> Reviewed-by: James Morse <james.morse@xxxxxxx>
>>>>>>
>>>>>>
>>>>>
>>>>> Thanks.
>>>>>
>>>>>>
>>>>>> I was looking for why we need core code to do this, instead of updating the
>>>>>> arch's call sites. Your 'irqdesc: Add domain handlers for NMIs' patch (pointed
>>>>>> to from the cover letter) is the reason: core-code calls nmi_enter()/nmi_exit()
>>>>>> itself.
>>>>>>
>>>>>
>>>>> Yes, that's the main reason.
>>>>>
>>>> I wondered the same thing, but I don't understand the explanation :(
>>>>
>>>> Why can't we do a local_daif_mask() around the (very small) calls that
>>>> clear TGE instead?
>>>>
>>>
>>> That would protect against the pseudo-NMIs, but you can still get an
>>> SDEI at that point even with all daif bits set. Or did I misunderstand
>>> how SDEI works?
>>>
>>
>> I don't know the details of SDEI.  From looking at this patch, the
>> logical conclusion would be that SDEIs can then only be delivered once
>> we've called nmi_enter, but since we don't call this directly from the
>> code that clears TGE for doing guest TLB invalidation (or do we?) then
>> masking interrupts at the PSTATE level should be sufficient.
>>
>> Surely I'm missing some part of the bigger picture here.
>>
> 
> I'm not sure I understand. SDEI uses the NMI context and AFAIU, it is an
> interrupt that the firmware sends to the OS, and it is sent regardless
> of the PSTATE at the OS EL.

I don't think we can describe SDEI as an interrupt. It is not even an
exception. It is just EL3 ERET-ing to a pre-defined location. And yes,
it will completely ignore any form of mask bit.

> 
> So, the worrying part is:
> - Hyp clears TGE
> - Exception/interrupt taken to EL3
> - Firmware decides it's a good time to send an SDEI to the OS
> - SDEI handler (at EL2 for VHE) does nmi_enter()
> - SDEI handler needs to do cache invalidation or something with the
> EL2&0 translation regime but TGE is cleared
> 
> We don't expect the code that clears TGE to call nmi_enter().

Indeed. Without this patch, SDEI is already broken. Pseudo-NMIs only
make the bug easier to trigger.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...