[PATCH v2] KVM: VMX: Fix crash due to uninitialized current_vmcs

[Alexandru Matei <alexandru.matei@uipath.com>]

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]
 ? __vmalloc_node+0x4a/0x70
 kvm_arch_vcpu_create+0x1d1/0x2e0 [kvm]
 kvm_vm_ioctl_create_vcpu+0x178/0x430 [kvm]
 ? __handle_mm_fault+0x3cb/0x750
 kvm_vm_ioctl+0x53f/0x790 [kvm]
 ? syscall_exit_work+0x11a/0x150
 ? syscall_exit_to_user_mode+0x12/0x30
 ? do_syscall_64+0x69/0x90
 ? handle_mm_fault+0xc5/0x2a0
 __x64_sys_ioctl+0x8a/0xc0
 do_syscall_64+0x5c/0x90
 ? exc_page_fault+0x62/0x150
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Alexandru Matei <alexandru.matei@uipath.com>
---
v2:
  - pass (e)vmcs01 to evmcs_touch_msr_bitmap
  - use loaded_vmcs * instead of vcpu_vmx * to avoid
    including vmx.h which generates circular dependency

v1: https://lore.kernel.org/kvm/20230118141348.828-1-alexandru.matei@uipath.com/

 arch/x86/kvm/vmx/hyperv.h | 16 +++++++++++-----
 arch/x86/kvm/vmx/vmx.c    |  2 +-
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/hyperv.h b/arch/x86/kvm/vmx/hyperv.h
index 571e7929d14e..132e32e57d2d 100644
--- a/arch/x86/kvm/vmx/hyperv.h
+++ b/arch/x86/kvm/vmx/hyperv.h
@@ -190,13 +190,19 @@ static inline u16 evmcs_read16(unsigned long field)
 	return *(u16 *)((char *)current_evmcs + offset);
 }
 
-static inline void evmcs_touch_msr_bitmap(void)
+static inline void evmcs_touch_msr_bitmap(struct loaded_vmcs *vmcs01)
 {
-	if (unlikely(!current_evmcs))
+	/*
+	 * Enlightened MSR Bitmap feature is enabled only for L1, i.e.
+	 * always operates on (e)vmcs01
+	 */
+	struct hv_enlightened_vmcs* evmcs = (void*)vmcs01->vmcs;
+
+	if (WARN_ON_ONCE(!evmcs))
 		return;
 
-	if (current_evmcs->hv_enlightenments_control.msr_bitmap)
-		current_evmcs->hv_clean_fields &=
+	if (evmcs->hv_enlightenments_control.msr_bitmap)
+		evmcs->hv_clean_fields &=
 			~HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP;
 }
 
@@ -219,7 +225,7 @@ static inline u64 evmcs_read64(unsigned long field) { return 0; }
 static inline u32 evmcs_read32(unsigned long field) { return 0; }
 static inline u16 evmcs_read16(unsigned long field) { return 0; }
 static inline void evmcs_load(u64 phys_addr) {}
-static inline void evmcs_touch_msr_bitmap(void) {}
+static inline void evmcs_touch_msr_bitmap(struct loaded_vmcs *vmcs01) {}
 #endif /* IS_ENABLED(CONFIG_HYPERV) */
 
 #define EVMPTR_INVALID (-1ULL)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fe5615fd8295..2a3be8e8a1bf 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3869,7 +3869,7 @@ static void vmx_msr_bitmap_l01_changed(struct vcpu_vmx *vmx)
 	 * bitmap has changed.
 	 */
 	if (static_branch_unlikely(&enable_evmcs))
-		evmcs_touch_msr_bitmap();
+		evmcs_touch_msr_bitmap(&vmx->vmcs01);
 
 	vmx->nested.force_msr_bitmap_recalc = true;
 }
-- 
2.25.1