Re: [PATCH] KVM: nVMX: VMX instructions: fix segment checks when L1 is in long mode.

[Paolo Bonzini]

On 24/06/2016 15:04, Quentin Casasnovas wrote:
> On Thu, Jun 23, 2016 at 06:03:01PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 18/06/2016 11:01, Quentin Casasnovas wrote:
>>> Cross-checking the KVM/VMX VMREAD emulation code with the Intel Software
>>> Developper Manual Volume 3C - "VMREAD - Read Field from Virtual-Machine
>>> Control Structure", I found that we're enforcing that the destination
>>> operand is NOT located in a read-only data segment or any code segment when
>>> the L1 is in long mode - BUT that check should only happen when it is in
>>> protected mode.
>>>
>>> Shuffling the code a bit to make our emulation follow the specification
>>> allows me to boot a Xen dom0 in a nested KVM and start HVM L2 guests
>>> without problems.
>>
>> That's great, and I'm applying the patch, but it's also pretty weird. :)
>>  Do you have a pointer to Xen source code that does a VMREAD into a
>> read-only data segment or a code segment?
> 
> It is indeed pretty weird.  Looking at the Xen stack trace, it looks like
> the vmread is writing to an on-stack buffer, and surely it must be writable
> so I wonder if Xen might not be using an executable stack for some reason?
> That would be a bit scary so I'm surely missing something.
> 
> Is there an easy way to know from my KVM host the different segment
> permission setup by the guest?

Remove your patch, call dump_vmcs() where the #GP is injected, and
you'll find the VMCS (including segment permissions, but not the
instruction info field---you probably should add it) in dmesg.

Thanks,

Paolo