KVM: x86: work around infinite loop in microcode when #AC is delivered

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>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/uapi/asm/svm.h b/arch/x86/include/uapi/asm/svm.h
index b5d7640abc5d6172838a99569d144f8acc96763a..8a4add8e463932445ba071903378927f76912cfa 100644 (file)
--- 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 f2ba91990b4e14593876ed3a13d41c1274155074..183926483c3ac86127c4dffe6ac10d9e2423f33e 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1019,6 +1019,7 @@ 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);
@@ -1707,6 +1708,12 @@ 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);
@@ -3270,6 +3277,7 @@ 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 b765b036a048926753ba286b3ea3b82b0d5e8ed8..89aaedd2a91d536e830ede4a131af2e679c325fe 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1639,7 +1639,7 @@ 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))
@@ -5261,6 +5261,9 @@ 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 &