KVM: nVMX: unconditionally cancel preemption timer in free_nested (CVE-2019-7221)

Bugzilla: 1671904

There are multiple code paths where an hrtimer may have been started to
emulate an L1 VMX preemption timer that can result in a call to free_nested
without an intervening L2 exit where the hrtimer is normally
cancelled. Unconditionally cancel in free_nested to cover all cases.

Embargoed until Feb 7th 2019.
Signed-off-by: Peter Shier <pshier@google.com>
Reported-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reported-by: Felix Wilhelm <fwilhelm@google.com>
Cc: stable@kernel.org
Message-Id: <20181011184646.154065-1-pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

CVE-2019-7221

(backported from commit ecec7688)
[tyhicks: Backport to 4.4:
 - free_nested() is in arch/x86/kvm/vmx.c
 - Minor contextual changes]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/kvm/vmx.c

@@ -7158,6 +7158,7 @@ static void free_nested(struct vcpu_vmx *vmx)
 	if (!vmx->nested.vmxon)
 		return;
 
+	hrtimer_cancel(&vmx->nested.preemption_timer);
 	vmx->nested.vmxon = false;
 	free_vpid(vmx->nested.vpid02);
 	nested_release_vmcs12(vmx);