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Re: [Xen-devel] [PATCH for-4.12] x86/svm: Fix handling of ICEBP intercepts
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 4 Feb 2019 19:44:06 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 04 Feb 2019 19:44:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 04/02/2019 09:16, Jan Beulich wrote:
>>>> On 01.02.19 at 18:09, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 01/02/2019 16:55, Jan Beulich wrote:
>>>>>> On 01.02.19 at 17:25, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> If it were just getting insn_len incorrectly as 0, then the guest would
>>>> livelock as we wouldn't inject the #DB with trap semantics it requires,
>>> I'm confused again: Why trap semantics? The ICEBP has fault
>>> semantics as you confirmed above.
>> The ICEBP intercept has fault semantics. An ICEBP instruction executing
>> in the guest has trap semantics.
> Oh, okay - I was mis-remembering this aspect.
>
>>>> but as the #GP is already raised, this will combine to #DF.
>>> How that? #DB is a benign exception, so according to the table on the
>>> #DF page in the SDM, with #GP it shouldn't combine to #DF.
>> #GP is raised first. It is contributory.
>>
>> A subsequent #DB getting raised causes #GP to turn into #DF.
> That's based on what?
Based on actually trying this error scenario.
(d1) --- Xen Test Framework ---
(d1) Environment: HVM 64bit (Long mode 4 levels)
(d1) Hello World
(XEN) ** Got ICEBP intercept
(d1) ******************************
(d1) PANIC: Unhandled exception at 0046:0000000000000008
(d1) Vec 8 #DF[4740]
(d1) ******************************
Clearly something is off-by-one in the eventual stack frame, which
probably means we've got a a bug in svm_inject_event(). I suspect the
escalation to #DF doesn't overwrite the #DB's "no error code"
information, but I've not investigated yet.
> The table on the #DF page clearly says
> otherwise, at least according to my reading.
Hmm - so it does. Looks like we've got a 3rd bug, in the general
exception combining logic.
> But in the end there shouldn't be any attempt to inject #DB anyway
> when #GP is already pending, irrespective of the fact that this #GP
> is non-architectural.
Correct. That bug is what this patch is fixing.
~Andrew
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