Re: [PATCH] KVM: nVMX: mask unrestricted_guest if disabled on L0

[Bandan Das <bsd@redhat.com>]

Jan Kiszka <jan.kiszka@siemens.com> writes:

> On 2015-02-24 17:30, Radim Krčmář wrote:
>> 2015-02-23 19:05+0100, Kashyap Chamarthy:
>>> Tested with the _correct_ Kernel[1] (that has Radim's patch) now --
>>> applied it on both L0 and L1.
>>>
>>> Result: Same as before -- Booting L2 causes L1 to reboot. However, the
>>>         stack trace from `dmesg` on L0 is took slightly different path than
>>>         before -- it's using MSR handling:
>> 
>> Thanks, the problem was deeper ... L1 enabled unrestricted mode while L0
>> had it disabled.  L1 could then vmrun a L2 state that L0 would have to
>> emulate, but that doesn't work.  There are at least these solutions:
>> 
>>  1) don't expose unrestricted_guest when L0 doesn't have it
>
> Reminds me of a patch called "KVM: nVMX: Disable unrestricted mode if
> ept=0" by Bandan. I thought that would have caught it - apparently not.

Yeah... Unrestricted guest could be disabled even if ept=0,
and I incorrectly didn't take that into account.

>>  2) fix unrestricted mode emulation code
>>  3) handle the failure a without killing L1
>> 
>> I'd do just (1) -- emulating unrestricted mode is a loss.
>
> Agreed.
>
> Jan
>
>> 
>> I have done initial testing and at least qemu-sanity-check works now:
>> 
>> ---8<---
>> If EPT was enabled, unrestricted_guest was allowed in L1 regardless of
>> L0.  L1 triple faulted when running L2 guest that required emulation.
>> 
>> Another side effect was 'WARN_ON_ONCE(vmx->nested.nested_run_pending)'
>> in L0's dmesg:
>>   WARNING: CPU: 0 PID: 0 at arch/x86/kvm/vmx.c:9190 nested_vmx_vmexit+0x96e/0xb00 [kvm_intel] ()
>> 
>> Prevent this scenario by masking SECONDARY_EXEC_UNRESTRICTED_GUEST when
>> the host doesn't have it enabled.
>> 
>> Fixes: 78051e3b7e35 ("KVM: nVMX: Disable unrestricted mode if ept=0")
>> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

We should Cc stable on this patch.

Bandan
>> ---
>>  arch/x86/kvm/vmx.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index f7b20b417a3a..dbabea21357b 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2476,8 +2476,7 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
>>  	if (enable_ept) {
>>  		/* nested EPT: emulate EPT also to L1 */
>>  		vmx->nested.nested_vmx_secondary_ctls_high |=
>> -			SECONDARY_EXEC_ENABLE_EPT |
>> -			SECONDARY_EXEC_UNRESTRICTED_GUEST;
>> +			SECONDARY_EXEC_ENABLE_EPT;
>>  		vmx->nested.nested_vmx_ept_caps = VMX_EPT_PAGE_WALK_4_BIT |
>>  			 VMX_EPTP_WB_BIT | VMX_EPT_2MB_PAGE_BIT |
>>  			 VMX_EPT_INVEPT_BIT;
>> @@ -2491,6 +2490,10 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
>>  	} else
>>  		vmx->nested.nested_vmx_ept_caps = 0;
>>  
>> +	if (enable_unrestricted_guest)
>> +		vmx->nested.nested_vmx_secondary_ctls_high |=
>> +			SECONDARY_EXEC_UNRESTRICTED_GUEST;
>> +
>>  	/* miscellaneous data */
>>  	rdmsr(MSR_IA32_VMX_MISC,
>>  		vmx->nested.nested_vmx_misc_low,
>>