Re: [PATCH 1/2] KVM: PPC: Book3S HV: check caller of H_SVM_* Hcalls

From: Laurent Dufour <ldufour@linux.ibm.com>
To: Greg Kurz <groug@kaod.org>, Paul Mackerras <paulus@ozlabs.org>
Cc: linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org,
	Bharata B Rao <bharata@linux.ibm.com>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 1/2] KVM: PPC: Book3S HV: check caller of H_SVM_* Hcalls
Date: Tue, 24 Mar 2020 14:13:11 +0100	[thread overview]
Message-ID: <4dc4bb94-9ddd-30f7-3b4c-0b8439119349@linux.ibm.com> (raw)
In-Reply-To: <20200324130052.373fdf89@bahia.lan>

Le 24/03/2020 à 13:00, Greg Kurz a écrit :
> On Tue, 24 Mar 2020 10:43:23 +1100
> Paul Mackerras <paulus@ozlabs.org> wrote:
> 
>> On Fri, Mar 20, 2020 at 01:22:48PM +0100, Greg Kurz wrote:
>>> On Fri, 20 Mar 2020 11:26:42 +0100
>>> Laurent Dufour <ldufour@linux.ibm.com> wrote:
>>>
>>>> The Hcall named H_SVM_* are reserved to the Ultravisor. However, nothing
>>>> prevent a malicious VM or SVM to call them. This could lead to weird result
>>>> and should be filtered out.
>>>>
>>>> Checking the Secure bit of the calling MSR ensure that the call is coming
>>>> from either the Ultravisor or a SVM. But any system call made from a SVM
>>>> are going through the Ultravisor, and the Ultravisor should filter out
>>>> these malicious call. This way, only the Ultravisor is able to make such a
>>>> Hcall.
>>>
>>> "Ultravisor should filter" ? And what if it doesn't (eg. because of a bug) ?
>>>
>>> Shouldn't we also check the HV bit of the calling MSR as well to
>>> disambiguate SVM and UV ?
>>
>> The trouble with doing that (checking the HV bit) is that KVM does not
>> expect to see the HV bit set on an interrupt that occurred while we
>> were in the guest, and if it is set, it indicates a serious problem,
>> i.e. that an interrupt occurred while we were in the code that
>> transitions from host context to guest context, or from guest context
>> to host context.  In those cases we don't know how much of the
>> transition has been completed and therefore whether we have guest
>> values or host values in the CPU registers (GPRs, FPRs/VSRs, SPRs).
>> If we do see HV set then KVM reports a severe error to userspace which
>> should cause userspace to terminate the guest.
>>
>> Therefore the UV should *always* have the HV bit clear in HSRR1/SRR1
>> when transitioning to KVM.
>>
> 
> Indeed... thanks for the clarification. So I guess we'll just assume
> that the UV doesn't reflect these SVM specific hcalls if they happened
> to be issued by the guest then.

As mentioned in the series's description:
"It is assumed that the UV will filtered out such Hcalls made by a malicious
SVM."

> Cheers,
> 
> --
> Greg
> 
>> Paul.
>