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

Quentin Casasnovas
2016-06-24T13:02:05Z

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?

Quentin