Re: [Xen-devel] [PATCH] x86/HVM: don't give the wrong impression of WRMSR succeeding

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/27/2018 03:39 AM, Jan Beulich wrote:
>>>> On 23.02.18 at 08:55, <JBeulich@xxxxxxxx> wrote:
>>>>> On 22.02.18 at 23:16, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 02/22/2018 10:44 AM, Jan Beulich wrote:
>>>>>>> On 22.02.18 at 15:53, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> On 22/02/18 13:44, Jan Beulich wrote:
>>>>>> ... for unknown MSRs: wrmsr_hypervisor_regs()'s comment clearly says
>>>>>> that the function returns 0 for unrecognized MSRs, so
>>>>>> {svm,vmx}_msr_write_intercept() should not convert this into success.
>>>>>>
>>>>>> At the time it went in, commit 013e34f5a6 ("x86: handle paged gfn in
>>>>>> wrmsr_hypervisor_regs") was probably okay, since prior to that the
>>>>>> return value wasn't checked at all. But that's not how we want things
>>>>>> to be handled nowadays.
>>>>>>
>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> I agree in principle, but this does have a large potential risk for
>>>>> guests.  Any unknown MSR which guests don't check for #GP faults from
>>>>> will now cause the guests to crash.
>>>>>
>>>>> That said, it is the correct direction to go long-term, and we've got to
>>>>> throw the switch some time, but I expect this will cause problems in the
>>>>> short term, especially for migrated-in guests.
>>>> Thinking about this again, the RDMSR side of things already raises
>>>> #GP for inaccessible MSRs. We obviously can't do a probing WRMSR
>>>> in {svm,vmx}_msr_write_intercept(), but couldn't we rdmsr_safe()
>>>> in the "case 0:" block, treating the result as the verdict whether to
>>>> raise #GP to the guest? As the read path does this anyway, we're
>>>> not exposing ourselves to new risks.
>>> What about write-only MSRs?
>> Bad luck (I'm sorry to say so, but we have an actual bug to fix here).
>> If we find any such is used, we'll have to add individual case labels.
> Since it wasn't clear with your question above and you earlier
> given R-b, I had dropped the latter from v2. Could you clarify
> whether I may reinstate it?

Yes, please.

-boris

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