CVE-2023-53756

In the Linux kernel, the following vulnerability has been resolved:

KVM: VMX: Fix crash due to uninitialized current_vmcs

KVM enables 'Enlightened VMCS' and 'Enlightened MSR Bitmap' when running as
a nested hypervisor on top of Hyper-V. When MSR bitmap is updated,
evmcs_touch_msr_bitmap function uses current_vmcs per-cpu variable to mark
that the msr bitmap was changed.

vmx_vcpu_create() modifies the msr bitmap via vmx_disable_intercept_for_msr
-> vmx_msr_bitmap_l01_changed which in the end calls this function. The
function checks for current_vmcs if it is null but the check is
insufficient because current_vmcs is not initialized. Because of this, the
code might incorrectly write to the structure pointed by current_vmcs value
left by another task. Preemption is not disabled, the current task can be
preempted and moved to another CPU while current_vmcs is accessed multiple
times from evmcs_touch_msr_bitmap() which leads to crash.

The manipulation of MSR bitmaps by callers happens only for vmcs01 so the
solution is to use vmx->vmcs01.vmcs instead of current_vmcs.

  BUG: kernel NULL pointer dereference, address: 0000000000000338
  PGD 4e1775067 P4D 0
  Oops: 0002 [#1] PREEMPT SMP NOPTI
  ...
  RIP: 0010:vmx_msr_bitmap_l01_changed+0x39/0x50 [kvm_intel]
  ...
  Call Trace:
   vmx_disable_intercept_for_msr+0x36/0x260 [kvm_intel]
   vmx_vcpu_create+0xe6/0x540 [kvm_intel]
   kvm_arch_vcpu_create+0x1d1/0x2e0 [kvm]
   kvm_vm_ioctl_create_vcpu+0x178/0x430 [kvm]
   kvm_vm_ioctl+0x53f/0x790 [kvm]
   __x64_sys_ioctl+0x8a/0xc0
   do_syscall_64+0x5c/0x90
   entry_SYSCALL_64_after_hwframe+0x63/0xcd