On 14/03/2018 18:30, Christopherson, Sean J wrote:
> On Wed, 2018-03-14, Sean Christopherson wrote:
>> On Wed, 2018-03-14 at 14:14 +0100, Paolo Bonzini wrote:
>>> On 09/03/2018 19:37, Radim Krčmář wrote:
>>>>
>>>> 2018-03-05 09:33-0800, Sean Christopherson:
>>>>>
>>>>> Segment registers must be synchronized prior to any code that may
>>>>> trigger a call to emulation_required()/guest_state_valid(), e.g.
>>>>> vmx_set_cr0(). Because preparing vmcs02 writes segmentation fields
>>>>> directly, i.e. doesn't use vmx_set_segment(), emulation_required
>>>>> will not be re-evaluated when synchronizing the segment registers,
>>>>> which can result in L0 incorrectly starting emulation of L2.
>>>>>
>>>>> Fixes: 8665c3f97320 ("KVM: nVMX: initialize descriptor cache fields in prepare_vmcs02_full")
>>>>>
>>>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>>>>> ---
>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>>> @@ -10563,11 +10563,8 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>>>>> return 0;
>>>>> }
>>>>>
>>>>> -static void prepare_vmcs02_full(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>>>>> - bool from_vmentry)
>>>> Nice catch!
>>>>
>>>> Paolo, was there a reason to that prevented prepare_vmcs02_full at the
>>>> beginning of prepare_vmcs02?
>>> No, I placed it there because initially I wanted to move more work to
>>> prepare_vmcs02_full (such as entry/exit controls, CR0/CR4/EFER, etc.),
>>> but that didn't quite work out.
>>>
>>> Sean, do you have a test case for this bug? (And if it can be fixed by
>>> moving prepare_vmcs02_full at the beginning, as suggested by Radim, I'm
>>> all for it).
>>
>> I don't have a formal unit test, but I hit the bug 100% of the time
>> simply by booting L2 with stock OVMF and unrestricted_guest=0 in L0,
>> i.e. it's trivially easy for me to reproduce.
>>
>> I think I could create a unit test relatively easily. I hit the bug
>> early in BIOS (OVMF) in L2, e.g. less than 10 instructions after reset.
>> In theory it should be straightforward to extract/port OVMF's reset
>> code to a unit test.
>
> The existing kvm-unit-tests tests/realmode hits this when setting
> CR0.PM in exec_in_big_real_mode(), just need to load kvm_intel with
> unrestricted_guest=0 in L0. I thought I had tried tests/realmode before
> sending the previous email, but I think I forgot to load kvm_intel in L1
> and so it ran with accel=tcg, doh.
Cool, I'll try to reproduce both ways and use it to test the move of
prepare_vmcs02_full at the beginning.
Paolo
