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KVM: x86: nVMX: close leak of L0's x2APIC MSRs (CVE-2019-3887)

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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 15303ba5d1.

Signed-off-by: Marc Orr <marcorr@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Marc Orr 2019-04-01 23:55:59 -07:00 committed by Paolo Bonzini
parent b86bc2858b
commit acff78477b
1 changed files with 44 additions and 28 deletions
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72
arch/x86/kvm/vmx/nested.c
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@ -500,6 +500,17 @@ static void nested_vmx_disable_intercept_for_msr(unsigned long *msr_bitmap_l1,
} }
} }
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 * Merge L0's and L1's MSR bitmap, return false to indicate that
* we do not use the hardware. * we do not use the hardware.
@ -541,39 +552,44 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
return false; return false;
msr_bitmap_l1 = (unsigned long *)kmap(page); 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, * To keep the control flow simple, pay eight 8-byte writes (sixteen
X2APIC_MSR(APIC_TASKPRI), * 4-byte writes on 32-bit systems) up front to enable intercepts for
MSR_TYPE_W); * 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( nested_vmx_disable_intercept_for_msr(
msr_bitmap_l1, msr_bitmap_l0, msr_bitmap_l1, msr_bitmap_l0,
X2APIC_MSR(APIC_EOI), X2APIC_MSR(APIC_TASKPRI),
MSR_TYPE_W);
nested_vmx_disable_intercept_for_msr(
msr_bitmap_l1, msr_bitmap_l0,
X2APIC_MSR(APIC_SELF_IPI),
MSR_TYPE_W); 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) if (spec_ctrl)