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().
