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Subject[FYI PATCH] KVM: nSVM: avoid picking up unsupported bits from L2 in int_ctl (CVE-2021-3653)
Date
From: Maxim Levitsky <mlevitsk@redhat.com>

* Invert the mask of bits that we pick from L2 in
nested_vmcb02_prepare_control

* Invert and explicitly use VIRQ related bits bitmask in svm_clear_vintr

This fixes a security issue that allowed a malicious L1 to run L2 with
AVIC enabled, which allowed the L2 to exploit the uninitialized and enabled
AVIC to read/write the host physical memory at some offsets.

Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/include/asm/svm.h | 2 ++
arch/x86/kvm/svm/nested.c | 10 +++++++---
arch/x86/kvm/svm/svm.c | 9 +++++----
3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index e322676039f4..b00dbc5fac2b 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -184,6 +184,8 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
#define V_IGN_TPR_SHIFT 20
#define V_IGN_TPR_MASK (1 << V_IGN_TPR_SHIFT)

+#define V_IRQ_INJECTION_BITS_MASK (V_IRQ_MASK | V_INTR_PRIO_MASK | V_IGN_TPR_MASK)
+
#define V_INTR_MASKING_SHIFT 24
#define V_INTR_MASKING_MASK (1 << V_INTR_MASKING_SHIFT)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 61738ff8ef33..28381ca5221c 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -503,7 +503,11 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12

static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
{
- const u32 mask = V_INTR_MASKING_MASK | V_GIF_ENABLE_MASK | V_GIF_MASK;
+ const u32 int_ctl_vmcb01_bits =
+ V_INTR_MASKING_MASK | V_GIF_MASK | V_GIF_ENABLE_MASK;
+
+ const u32 int_ctl_vmcb12_bits = V_TPR_MASK | V_IRQ_INJECTION_BITS_MASK;
+
struct kvm_vcpu *vcpu = &svm->vcpu;

/*
@@ -535,8 +539,8 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
vcpu->arch.l1_tsc_offset + svm->nested.ctl.tsc_offset;

svm->vmcb->control.int_ctl =
- (svm->nested.ctl.int_ctl & ~mask) |
- (svm->vmcb01.ptr->control.int_ctl & mask);
+ (svm->nested.ctl.int_ctl & int_ctl_vmcb12_bits) |
+ (svm->vmcb01.ptr->control.int_ctl & int_ctl_vmcb01_bits);

svm->vmcb->control.virt_ext = svm->nested.ctl.virt_ext;
svm->vmcb->control.int_vector = svm->nested.ctl.int_vector;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e8ccab50ebf6..69639f9624f5 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1589,17 +1589,18 @@ static void svm_set_vintr(struct vcpu_svm *svm)

static void svm_clear_vintr(struct vcpu_svm *svm)
{
- const u32 mask = V_TPR_MASK | V_GIF_ENABLE_MASK | V_GIF_MASK | V_INTR_MASKING_MASK;
svm_clr_intercept(svm, INTERCEPT_VINTR);

/* Drop int_ctl fields related to VINTR injection. */
- svm->vmcb->control.int_ctl &= mask;
+ svm->vmcb->control.int_ctl &= ~V_IRQ_INJECTION_BITS_MASK;
if (is_guest_mode(&svm->vcpu)) {
- svm->vmcb01.ptr->control.int_ctl &= mask;
+ svm->vmcb01.ptr->control.int_ctl &= ~V_IRQ_INJECTION_BITS_MASK;

WARN_ON((svm->vmcb->control.int_ctl & V_TPR_MASK) !=
(svm->nested.ctl.int_ctl & V_TPR_MASK));
- svm->vmcb->control.int_ctl |= svm->nested.ctl.int_ctl & ~mask;
+
+ svm->vmcb->control.int_ctl |= svm->nested.ctl.int_ctl &
+ V_IRQ_INJECTION_BITS_MASK;
}

vmcb_mark_dirty(svm->vmcb, VMCB_INTR);
--
2.27.0
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