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Subject[PATCH 5.0 115/117] KVM: x86: nVMX: close leak of L0s x2APIC MSRs (CVE-2019-3887)
Date
From: Marc Orr <marcorr@google.com>

commit acff78477b9b4f26ecdf65733a4ed77fe837e9dc upstream.

The nested_vmx_prepare_msr_bitmap() function doesn't directly guard the
x2APIC MSR intercepts with the "virtualize x2APIC mode" MSR. As a
result, we discovered the potential for a buggy or malicious L1 to get
access to L0's x2APIC MSRs, via an L2, as follows.

1. L1 executes WRMSR(IA32_SPEC_CTRL, 1). This causes the spec_ctrl
variable, in nested_vmx_prepare_msr_bitmap() to become true.
2. L1 disables "virtualize x2APIC mode" in VMCS12.
3. L1 enables "APIC-register virtualization" in VMCS12.

Now, KVM will set VMCS02's x2APIC MSR intercepts from VMCS12, and then
set "virtualize x2APIC mode" to 0 in VMCS02. Oops.

This patch closes the leak by explicitly guarding VMCS02's x2APIC MSR
intercepts with VMCS12's "virtualize x2APIC mode" control.

The scenario outlined above and fix prescribed here, were verified with
a related patch in kvm-unit-tests titled "Add leak scenario to
virt_x2apic_mode_test".

Note, it looks like this issue may have been introduced inadvertently
during a merge---see 15303ba5d1cd.

Signed-off-by: Marc Orr <marcorr@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/vmx/nested.c | 72 ++++++++++++++++++++++++++++------------------
1 file changed, 44 insertions(+), 28 deletions(-)

--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -500,6 +500,17 @@ static void nested_vmx_disable_intercept
}
}

+static inline void enable_x2apic_msr_intercepts(unsigned long *msr_bitmap) {
+ int msr;
+
+ for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
+ unsigned word = msr / BITS_PER_LONG;
+
+ msr_bitmap[word] = ~0;
+ msr_bitmap[word + (0x800 / sizeof(long))] = ~0;
+ }
+}
+
/*
* Merge L0's and L1's MSR bitmap, return false to indicate that
* we do not use the hardware.
@@ -541,39 +552,44 @@ static inline bool nested_vmx_prepare_ms
return false;

msr_bitmap_l1 = (unsigned long *)kmap(page);
- if (nested_cpu_has_apic_reg_virt(vmcs12)) {
- /*
- * L0 need not intercept reads for MSRs between 0x800 and 0x8ff, it
- * just lets the processor take the value from the virtual-APIC page;
- * take those 256 bits directly from the L1 bitmap.
- */
- for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
- unsigned word = msr / BITS_PER_LONG;
- msr_bitmap_l0[word] = msr_bitmap_l1[word];
- msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
- }
- } else {
- for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
- unsigned word = msr / BITS_PER_LONG;
- msr_bitmap_l0[word] = ~0;
- msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
- }
- }

- nested_vmx_disable_intercept_for_msr(
- msr_bitmap_l1, msr_bitmap_l0,
- X2APIC_MSR(APIC_TASKPRI),
- MSR_TYPE_W);
+ /*
+ * To keep the control flow simple, pay eight 8-byte writes (sixteen
+ * 4-byte writes on 32-bit systems) up front to enable intercepts for
+ * the x2APIC MSR range and selectively disable them below.
+ */
+ enable_x2apic_msr_intercepts(msr_bitmap_l0);
+
+ if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {
+ if (nested_cpu_has_apic_reg_virt(vmcs12)) {
+ /*
+ * L0 need not intercept reads for MSRs between 0x800
+ * and 0x8ff, it just lets the processor take the value
+ * from the virtual-APIC page; take those 256 bits
+ * directly from the L1 bitmap.
+ */
+ for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
+ unsigned word = msr / BITS_PER_LONG;
+
+ msr_bitmap_l0[word] = msr_bitmap_l1[word];
+ }
+ }

- if (nested_cpu_has_vid(vmcs12)) {
- nested_vmx_disable_intercept_for_msr(
- msr_bitmap_l1, msr_bitmap_l0,
- X2APIC_MSR(APIC_EOI),
- MSR_TYPE_W);
nested_vmx_disable_intercept_for_msr(
msr_bitmap_l1, msr_bitmap_l0,
- X2APIC_MSR(APIC_SELF_IPI),
+ X2APIC_MSR(APIC_TASKPRI),
MSR_TYPE_W);
+
+ if (nested_cpu_has_vid(vmcs12)) {
+ nested_vmx_disable_intercept_for_msr(
+ msr_bitmap_l1, msr_bitmap_l0,
+ X2APIC_MSR(APIC_EOI),
+ MSR_TYPE_W);
+ nested_vmx_disable_intercept_for_msr(
+ msr_bitmap_l1, msr_bitmap_l0,
+ X2APIC_MSR(APIC_SELF_IPI),
+ MSR_TYPE_W);
+ }
}

if (spec_ctrl)

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