KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC

msr bitmap can be used to avoid a VM exit (interception) on guest MSR
accesses.  In some configurations of VMX controls, the guest can even
directly access host's x2APIC MSRs.  See SDM 29.5 VIRTUALIZING MSR-BASED
APIC ACCESSES.

L2 could read all L0's x2APIC MSRs and write TPR, EOI, and SELF_IPI.
To do so, L1 would first trick KVM to disable all possible interceptions
by enabling APICv features and then would turn those features off;
nested_vmx_merge_msr_bitmap() only disabled interceptions, so VMX would
not intercept previously enabled MSRs even though they were not safe
with the new configuration.

Correctly re-enabling interceptions is not enough as a second bug would
still allow L1+L2 to access host's MSRs: msr bitmap was shared for all
VMCSs, so L1 could trigger a race to get the desired combination of msr
bitmap and VMX controls.

This fix allocates a msr bitmap for every L1 VCPU, allows only safe
x2APIC MSRs from L1's msr bitmap, and disables msr bitmaps if they would
have to intercept everything anyway.

Fixes: 3af18d9c5fe9 ("KVM: nVMX: Prepare for using hardware MSR bitmap")
Reported-by: Jim Mattson <jmattson@google.com>
Suggested-by: Wincy Van <fanwenyi0529@gmail.com>
Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -435,6 +435,8 @@ struct nested_vmx {
 	bool pi_pending;
 	u16 posted_intr_nv;
 
+	unsigned long *msr_bitmap;
+
 	struct hrtimer preemption_timer;
 	bool preemption_timer_expired;
 
@@ -924,7 +926,6 @@ static unsigned long *vmx_msr_bitmap_legacy;
 static unsigned long *vmx_msr_bitmap_longmode;
 static unsigned long *vmx_msr_bitmap_legacy_x2apic;
 static unsigned long *vmx_msr_bitmap_longmode_x2apic;
-static unsigned long *vmx_msr_bitmap_nested;
 static unsigned long *vmx_vmread_bitmap;
 static unsigned long *vmx_vmwrite_bitmap;
 
@@ -2508,7 +2509,7 @@ static void vmx_set_msr_bitmap(struct kvm_vcpu *vcpu)
 	unsigned long *msr_bitmap;
 
 	if (is_guest_mode(vcpu))
-		msr_bitmap = vmx_msr_bitmap_nested;
+		msr_bitmap = to_vmx(vcpu)->nested.msr_bitmap;
 	else if (cpu_has_secondary_exec_ctrls() &&
 		 (vmcs_read32(SECONDARY_VM_EXEC_CONTROL) &
 		  SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE)) {
@@ -6363,13 +6364,6 @@ static __init int hardware_setup(void)
 	if (!vmx_msr_bitmap_longmode_x2apic)
 		goto out4;
 
-	if (nested) {
-		vmx_msr_bitmap_nested =
-			(unsigned long *)__get_free_page(GFP_KERNEL);
-		if (!vmx_msr_bitmap_nested)
-			goto out5;
-	}
-
 	vmx_vmread_bitmap = (unsigned long *)__get_free_page(GFP_KERNEL);
 	if (!vmx_vmread_bitmap)
 		goto out6;
@@ -6392,8 +6386,6 @@ static __init int hardware_setup(void)
 
 	memset(vmx_msr_bitmap_legacy, 0xff, PAGE_SIZE);
 	memset(vmx_msr_bitmap_longmode, 0xff, PAGE_SIZE);
-	if (nested)
-		memset(vmx_msr_bitmap_nested, 0xff, PAGE_SIZE);
 
 	if (setup_vmcs_config(&vmcs_config) < 0) {
 		r = -EIO;
@@ -6529,9 +6521,6 @@ out8:
 out7:
 	free_page((unsigned long)vmx_vmread_bitmap);
 out6:
-	if (nested)
-		free_page((unsigned long)vmx_msr_bitmap_nested);
-out5:
 	free_page((unsigned long)vmx_msr_bitmap_longmode_x2apic);
 out4:
 	free_page((unsigned long)vmx_msr_bitmap_longmode);
@@ -6557,8 +6546,6 @@ static __exit void hardware_unsetup(void)
 	free_page((unsigned long)vmx_io_bitmap_a);
 	free_page((unsigned long)vmx_vmwrite_bitmap);
 	free_page((unsigned long)vmx_vmread_bitmap);
-	if (nested)
-		free_page((unsigned long)vmx_msr_bitmap_nested);
 
 	free_kvm_area();
 }
@@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
 		return 1;
 	}
 
+	if (cpu_has_vmx_msr_bitmap()) {
+		vmx->nested.msr_bitmap =
+				(unsigned long *)__get_free_page(GFP_KERNEL);
+		if (!vmx->nested.msr_bitmap)
+			goto out_msr_bitmap;
+	}
+
 	vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
 	if (!vmx->nested.cached_vmcs12)
-		return -ENOMEM;
+		goto out_cached_vmcs12;
 
 	if (enable_shadow_vmcs) {
 		shadow_vmcs = alloc_vmcs();
-		if (!shadow_vmcs) {
-			kfree(vmx->nested.cached_vmcs12);
-			return -ENOMEM;
-		}
+		if (!shadow_vmcs)
+			goto out_shadow_vmcs;
 		/* mark vmcs as shadow */
 		shadow_vmcs->revision_id |= (1u << 31);
 		/* init shadow vmcs */
@@ -7024,6 +7016,15 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
 	skip_emulated_instruction(vcpu);
 	nested_vmx_succeed(vcpu);
 	return 1;
+
+out_shadow_vmcs:
+	kfree(vmx->nested.cached_vmcs12);
+
+out_cached_vmcs12:
+	free_page((unsigned long)vmx->nested.msr_bitmap);
+
+out_msr_bitmap:
+	return -ENOMEM;
 }
 
 /*
@@ -7098,6 +7099,10 @@ static void free_nested(struct vcpu_vmx *vmx)
 	vmx->nested.vmxon = false;
 	free_vpid(vmx->nested.vpid02);
 	nested_release_vmcs12(vmx);
+	if (vmx->nested.msr_bitmap) {
+		free_page((unsigned long)vmx->nested.msr_bitmap);
+		vmx->nested.msr_bitmap = NULL;
+	}
 	if (enable_shadow_vmcs)
 		free_vmcs(vmx->nested.current_shadow_vmcs);
 	kfree(vmx->nested.cached_vmcs12);
@@ -9472,8 +9477,10 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
 {
 	int msr;
 	struct page *page;
-	unsigned long *msr_bitmap;
+	unsigned long *msr_bitmap_l1;
+	unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.msr_bitmap;
 
+	/* This shortcut is ok because we support only x2APIC MSRs so far. */
 	if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
 		return false;
 
@@ -9482,63 +9489,37 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
 		WARN_ON(1);
 		return false;
 	}
-	msr_bitmap = (unsigned long *)kmap(page);
-	if (!msr_bitmap) {
+	msr_bitmap_l1 = (unsigned long *)kmap(page);
+	if (!msr_bitmap_l1) {
 		nested_release_page_clean(page);
 		WARN_ON(1);
 		return false;
 	}
 
+	memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
+
 	if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {
 		if (nested_cpu_has_apic_reg_virt(vmcs12))
 			for (msr = 0x800; msr <= 0x8ff; msr++)
 				nested_vmx_disable_intercept_for_msr(
-					msr_bitmap,
-					vmx_msr_bitmap_nested,
+					msr_bitmap_l1, msr_bitmap_l0,
 					msr, MSR_TYPE_R);
-		/* TPR is allowed */
-		nested_vmx_disable_intercept_for_msr(msr_bitmap,
-				vmx_msr_bitmap_nested,
+
+		nested_vmx_disable_intercept_for_msr(
+				msr_bitmap_l1, msr_bitmap_l0,
 				APIC_BASE_MSR + (APIC_TASKPRI >> 4),
 				MSR_TYPE_R | MSR_TYPE_W);
+
 		if (nested_cpu_has_vid(vmcs12)) {
-			/* EOI and self-IPI are allowed */
 			nested_vmx_disable_intercept_for_msr(
-				msr_bitmap,
-				vmx_msr_bitmap_nested,
+				msr_bitmap_l1, msr_bitmap_l0,
 				APIC_BASE_MSR + (APIC_EOI >> 4),
 				MSR_TYPE_W);
 			nested_vmx_disable_intercept_for_msr(
-				msr_bitmap,
-				vmx_msr_bitmap_nested,
+				msr_bitmap_l1, msr_bitmap_l0,
 				APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
 				MSR_TYPE_W);
 		}
-	} else {
-		/*
-		 * Enable reading intercept of all the x2apic
-		 * MSRs. We should not rely on vmcs12 to do any
-		 * optimizations here, it may have been modified
-		 * by L1.
-		 */
-		for (msr = 0x800; msr <= 0x8ff; msr++)
-			__vmx_enable_intercept_for_msr(
-				vmx_msr_bitmap_nested,
-				msr,
-				MSR_TYPE_R);
-
-		__vmx_enable_intercept_for_msr(
-				vmx_msr_bitmap_nested,
-				APIC_BASE_MSR + (APIC_TASKPRI >> 4),
-				MSR_TYPE_W);
-		__vmx_enable_intercept_for_msr(
-				vmx_msr_bitmap_nested,
-				APIC_BASE_MSR + (APIC_EOI >> 4),
-				MSR_TYPE_W);
-		__vmx_enable_intercept_for_msr(
-				vmx_msr_bitmap_nested,
-				APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
-				MSR_TYPE_W);
 	}
 	kunmap(page);
 	nested_release_page_clean(page);
@@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 	}
 
 	if (cpu_has_vmx_msr_bitmap() &&
-	    exec_control & CPU_BASED_USE_MSR_BITMAPS) {
-		nested_vmx_merge_msr_bitmap(vcpu, vmcs12);
-		/* MSR_BITMAP will be set by following vmx_set_efer. */
-	} else
+	    exec_control & CPU_BASED_USE_MSR_BITMAPS &&
+	    nested_vmx_merge_msr_bitmap(vcpu, vmcs12))
+		; /* MSR_BITMAP will be set by following vmx_set_efer. */
+	else
 		exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;
 
 	/*