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Subject[PATCH 3.16.y-ckt 078/126] KVM: x86: work around infinite loop in microcode when #AC is delivered
Date
3.16.7-ckt21 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Northup <digitaleric@google.com>

commit 54a20552e1eae07aa240fa370a0293e006b5faed upstream.

It was found that a guest can DoS a host by triggering an infinite
stream of "alignment check" (#AC) exceptions. This causes the
microcode to enter an infinite loop where the core never receives
another interrupt. The host kernel panics pretty quickly due to the
effects (CVE-2015-5307).

Signed-off-by: Eric Northup <digitaleric@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/x86/include/uapi/asm/svm.h | 1 +
arch/x86/kvm/svm.c | 8 ++++++++
arch/x86/kvm/vmx.c | 5 ++++-
3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/svm.h b/arch/x86/include/uapi/asm/svm.h
index b5d7640abc5d..8a4add8e4639 100644
--- a/arch/x86/include/uapi/asm/svm.h
+++ b/arch/x86/include/uapi/asm/svm.h
@@ -100,6 +100,7 @@
{ SVM_EXIT_EXCP_BASE + UD_VECTOR, "UD excp" }, \
{ SVM_EXIT_EXCP_BASE + PF_VECTOR, "PF excp" }, \
{ SVM_EXIT_EXCP_BASE + NM_VECTOR, "NM excp" }, \
+ { SVM_EXIT_EXCP_BASE + AC_VECTOR, "AC excp" }, \
{ SVM_EXIT_EXCP_BASE + MC_VECTOR, "MC excp" }, \
{ SVM_EXIT_INTR, "interrupt" }, \
{ SVM_EXIT_NMI, "nmi" }, \
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 539f233965f1..9e59e2d6ba7f 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1103,6 +1103,7 @@ static void init_vmcb(struct vcpu_svm *svm)
set_exception_intercept(svm, PF_VECTOR);
set_exception_intercept(svm, UD_VECTOR);
set_exception_intercept(svm, MC_VECTOR);
+ set_exception_intercept(svm, AC_VECTOR);

set_intercept(svm, INTERCEPT_INTR);
set_intercept(svm, INTERCEPT_NMI);
@@ -1787,6 +1788,12 @@ static int ud_interception(struct vcpu_svm *svm)
return 1;
}

+static int ac_interception(struct vcpu_svm *svm)
+{
+ kvm_queue_exception_e(&svm->vcpu, AC_VECTOR, 0);
+ return 1;
+}
+
static void svm_fpu_activate(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);
@@ -3327,6 +3334,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
[SVM_EXIT_EXCP_BASE + PF_VECTOR] = pf_interception,
[SVM_EXIT_EXCP_BASE + NM_VECTOR] = nm_interception,
[SVM_EXIT_EXCP_BASE + MC_VECTOR] = mc_interception,
+ [SVM_EXIT_EXCP_BASE + AC_VECTOR] = ac_interception,
[SVM_EXIT_INTR] = intr_interception,
[SVM_EXIT_NMI] = nmi_interception,
[SVM_EXIT_SMI] = nop_on_interception,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 0502753be5f0..d1d7086f0e9b 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1467,7 +1467,7 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
u32 eb;

eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR) |
- (1u << NM_VECTOR) | (1u << DB_VECTOR);
+ (1u << NM_VECTOR) | (1u << DB_VECTOR) | (1u << AC_VECTOR);
if ((vcpu->guest_debug &
(KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)) ==
(KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP))
@@ -4881,6 +4881,9 @@ static int handle_exception(struct kvm_vcpu *vcpu)
return handle_rmode_exception(vcpu, ex_no, error_code);

switch (ex_no) {
+ case AC_VECTOR:
+ kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+ return 1;
case DB_VECTOR:
dr6 = vmcs_readl(EXIT_QUALIFICATION);
if (!(vcpu->guest_debug &

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