KVM: VMX: Reject KVM_RUN if userspace forces emulation during nested VM-Enter

KVM: VMX: Reject KVM_RUN if userspace forces emulation during nested VM-Enter

Extend KVM's restrictions on userspace forcing "emulation required" at odd
times to cover stuffing invalid guest state while a nested run is pending.
Clobbering guest state while KVM is in the middle of emulating VM-Enter is
nonsensical, as it puts the vCPU into an architecturally impossible state,
and will trip KVM's sanity check that guards against KVM bugs, e.g. helps
detect missed VMX consistency checks.

  WARNING: CPU: 3 PID: 6336 at arch/x86/kvm/vmx/vmx.c:6480 __vmx_handle_exit arch/x86/kvm/vmx/vmx.c:6480 [inline]
  WARNING: CPU: 3 PID: 6336 at arch/x86/kvm/vmx/vmx.c:6480 vmx_handle_exit+0x40f/0x1f70 arch/x86/kvm/vmx/vmx.c:6637
  Modules linked in:
  CPU: 3 UID: 0 PID: 6336 Comm: syz.0.73 Not tainted 6.13.0-rc1-syzkaller-00316-gb5f217084ab3 #0
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
  RIP: 0010:__vmx_handle_exit arch/x86/kvm/vmx/vmx.c:6480 [inline]
  RIP: 0010:vmx_handle_exit+0x40f/0x1f70 arch/x86/kvm/vmx/vmx.c:6637
   <TASK>
   vcpu_enter_guest arch/x86/kvm/x86.c:11081 [inline]
   vcpu_run+0x3047/0x4f50 arch/x86/kvm/x86.c:11242
   kvm_arch_vcpu_ioctl_run+0x44a/0x1740 arch/x86/kvm/x86.c:11560
   kvm_vcpu_ioctl+0x6ce/0x1520 virt/kvm/kvm_main.c:4340
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:906 [inline]
   __se_sys_ioctl fs/ioctl.c:892 [inline]
   __x64_sys_ioctl+0x190/0x200 fs/ioctl.c:892
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

Reported-by: syzbot+ac0bc3a70282b4d586cc@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67598fb9.050a0220.17f54a.003b.GAE@google.com
Debugged-by: James Houghton <jthoughton@google.com>
Tested-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250224171409.2348647-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>