[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <db22392e-fb88-44f6-8592-7429c606cf10@intel.com>
Date: Thu, 7 Mar 2024 12:20:10 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Paolo Bonzini
<pbonzini@...hat.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
Yan Zhao <yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>,
Michael Roth <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>,
Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>,
"David Matlack" <dmatlack@...gle.com>
Subject: Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses
to emulated MMIO
On 7/03/2024 12:01 pm, Sean Christopherson wrote:
> On Thu, Mar 07, 2024, Kai Huang wrote:
>>
>>
>> On 7/03/2024 11:43 am, Sean Christopherson wrote:
>>> On Thu, Mar 07, 2024, Kai Huang wrote:
>>>>
>>>>
>>>> On 28/02/2024 3:41 pm, Sean Christopherson wrote:
>>>>> Explicitly detect and disallow private accesses to emulated MMIO in
>>>>> kvm_handle_noslot_fault() instead of relying on kvm_faultin_pfn_private()
>>>>> to perform the check. This will allow the page fault path to go straight
>>>>> to kvm_handle_noslot_fault() without bouncing through __kvm_faultin_pfn().
>>>>>
>>>>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>>>>> ---
>>>>> arch/x86/kvm/mmu/mmu.c | 5 +++++
>>>>> 1 file changed, 5 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>>>> index 5c8caab64ba2..ebdb3fcce3dc 100644
>>>>> --- a/arch/x86/kvm/mmu/mmu.c
>>>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>>>> @@ -3314,6 +3314,11 @@ static int kvm_handle_noslot_fault(struct kvm_vcpu *vcpu,
>>>>> {
>>>>> gva_t gva = fault->is_tdp ? 0 : fault->addr;
>>>>> + if (fault->is_private) {
>>>>> + kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
>>>>> + return -EFAULT;
>>>>> + }
>>>>> +
>>>>
>>>> As mentioned in another reply in this series, unless I am mistaken, for TDX
>>>> guest the _first_ MMIO access would still cause EPT violation with MMIO GFN
>>>> being private.
>>>>
>>>> Returning to userspace cannot really help here because the MMIO mapping is
>>>> inside the guest.
>>>
>>> That's a guest bug. The guest *knows* it's a TDX VM, it *has* to know. Accessing
>>> emulated MMIO and thus taking a #VE before enabling paging is nonsensical. Either
>>> enable paging and setup MMIO regions as shared, or go straight to TDCALL.
>>
>> +Kirill,
>>
>> I kinda forgot the detail, but what I am afraid is there might be bunch of
>> existing TDX guests (since TDX guest code is upstream-ed) using unmodified
>> drivers, which doesn't map MMIO regions as shared I suppose.
>>
>> Kirill,
>>
>> Could you clarify whether TDX guest code maps MMIO regions as shared since
>> beginning?
>
> Y'all get the same answer we gave the SNP folks: KVM does not yet support TDX,
> so as far is KVM is concerned, there is no existing functionality to support.
>
> s/firmware/Linux if this is a Linux kernel problem.
>
> On Thu, Feb 08, 2024, Paolo Bonzini wrote:
> > On Thu, Feb 8, 2024 at 6:27 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > No. KVM does not yet support SNP, so as far as KVM's ABI goes, there are no
> > > existing guests. Yes, I realize that I am burying my head in the sand to some
> > > extent, but it is simply not sustainable for KVM to keep trying to pick up the
> > > pieces of poorly defined hardware specs and broken guest firmware.
> >
> > 101% agreed. There are cases in which we have to and should bend
> > together backwards for guests (e.g. older Linux kernels), but not for
> > code that---according to current practices---is chosen by the host
> > admin.
> >
> > (I am of the opinion that "bring your own firmware" is the only sane
> > way to handle attestation/measurement, but that's not how things are
> > done currently).
Fair enough, and good to know. :-)
(Still better to hear from Kirill, though.)
Powered by blists - more mailing lists