Re: [PATCH 1/2] KVM: nVMX: fix CR4_READ_SHADOW when L0 updates CR4 during a signal

[Sean Christopherson]

On Tue, Apr 16, 2024, Thomas Prescher wrote:
> Hi Sean,
> 
> On Tue, 2024-04-16 at 07:35 -0700, Sean Christopherson wrote:
> > On Tue, Apr 16, 2024, Julian Stecklina wrote:
> > > From: Thomas Prescher <thomas.prescher@xxxxxxxxxxxxxxxxxxxxx>
> > > 
> > > This issue occurs when the kernel is interrupted by a signal while
> > > running a L2 guest. If the signal is meant to be delivered to the L0 VMM,
> > > and L0 updates CR4 for L1, i.e. when the VMM sets KVM_SYNC_X86_SREGS in
> > > kvm_run->kvm_dirty_regs, the kernel programs an incorrect read shadow
> > > value for L2's CR4.
> > > 
> > > The result is that the guest can read a value for CR4 where bits from L1
> > > have leaked into L2.
> > 
> > No, this is a userspace bug.  If L2 is active when userspace stuffs
> > register state, then from KVM's perspective the incoming value is L2's
> > value.  E.g.  if userspace *wants* to update L2 CR4 for whatever reason,
> > this patch would result in L2 getting a stale value, i.e. the value of CR4
> > at the time of VM-Enter.
> > 
> > And even if userspace wants to change L1, this patch is wrong, as KVM is
> > writing vmcs02.GUEST_CR4, i.e. is clobbering the L2 CR4 that was programmed
> > by L1, *and* is dropping the CR4 value that userspace wanted to stuff for
> > L1.
> > 
> > To fix this, your userspace needs to either wait until L2 isn't active, or
> > force the vCPU out of L2 (which isn't easy, but it's doable if absolutely
> > necessary).
> 
> What you say makes sense. Is there any way for
> userspace to detect whether L2 is currently active after
> returning from KVM_RUN? I couldn't find anything in the official
> documentation https://docs.kernel.org/virt/kvm/api.html
> 
> Can you point me into the right direction?

Hmm, the only way to query that information is via KVM_GET_NESTED_STATE, which is
a bit unfortunate as that is a fairly "heavy" ioctl().