[PATCH 4.9 21/43] KVM: nSVM: avoid picking up unsupported bits from L2 in int_ctl (CVE-2021-3653)

Sasha Levin
2021-08-24T17:37:07Z

From: Maxim Levitsky <mlevitsk@redhat.com>

[ upstream commit 0f923e07124df069ba68d8bb12324398f4b6b709 ]

* 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/svm.h | 2 ++
arch/x86/kvm/svm.c | 6 +++++-
2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 14824fc78f7e..509b9f3307e4 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -113,6 +113,8 @@ 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.c b/arch/x86/kvm/svm.c
index cbc7f177bbd8..03fdeab057d2 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -3048,7 +3048,11 @@ static bool nested_svm_vmrun(struct vcpu_svm *svm)
svm->nested.intercept = nested_vmcb->control.intercept;

svm_flush_tlb(&svm->vcpu);
- svm->vmcb->control.int_ctl = nested_vmcb->control.int_ctl | V_INTR_MASKING_MASK;
+ svm->vmcb->control.int_ctl = nested_vmcb->control.int_ctl &
+ (V_TPR_MASK | V_IRQ_INJECTION_BITS_MASK);
+
+ svm->vmcb->control.int_ctl |= V_INTR_MASKING_MASK;
+
if (nested_vmcb->control.int_ctl & V_INTR_MASKING_MASK)
svm->vcpu.arch.hflags |= HF_VINTR_MASK;
else
--
2.30.2