KVM: VMX: Fix crash due to uninitialized current_vmcs

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