[PATCH AUTOSEL 5.2 35/63] KVM: hyperv: Fix Direct Synthetic timers assert an interrupt w/o lapic_in_kernel

[Sasha Levin]

From: Wanpeng Li <wanpen...@tencent.com>

[ Upstream commit a073d7e3ad687a7ef32b65affe80faa7ce89bf92 ]

Reported by syzkaller:

        kasan: GPF could be caused by NULL-ptr deref or user memory access
        general protection fault: 0000 [#1] PREEMPT SMP KASAN
        RIP: 0010:__apic_accept_irq+0x46/0x740 arch/x86/kvm/lapic.c:1029
        Call Trace:
        kvm_apic_set_irq+0xb4/0x140 arch/x86/kvm/lapic.c:558
        stimer_notify_direct arch/x86/kvm/hyperv.c:648 [inline]
        stimer_expiration arch/x86/kvm/hyperv.c:659 [inline]
        kvm_hv_process_stimers+0x594/0x1650 arch/x86/kvm/hyperv.c:686
        vcpu_enter_guest+0x2b2a/0x54b0 arch/x86/kvm/x86.c:7896
        vcpu_run+0x393/0xd40 arch/x86/kvm/x86.c:8152
        kvm_arch_vcpu_ioctl_run+0x636/0x900 arch/x86/kvm/x86.c:8360
        kvm_vcpu_ioctl+0x6cf/0xaf0 
arch/x86/kvm/../../../virt/kvm/kvm_main.c:2765

The testcase programs HV_X64_MSR_STIMERn_CONFIG/HV_X64_MSR_STIMERn_COUNT,
in addition, there is no lapic in the kernel, the counters value are small
enough in order that kvm_hv_process_stimers() inject this already-expired
timer interrupt into the guest through lapic in the kernel which triggers
the NULL deferencing. This patch fixes it by don't advertise direct mode
synthetic timers and discarding the inject when lapic is not in kernel.

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=1752fe0a600000

Reported-by: syzbot+dff25ee91f0c7d5c1...@syzkaller.appspotmail.com
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Radim Krčmář <rkrc...@redhat.com>
Signed-off-by: Wanpeng Li <wanpen...@tencent.com>
Reviewed-by: Vitaly Kuznetsov <vkuzn...@redhat.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---
 arch/x86/kvm/hyperv.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 742ecf5b6c009..72200998687cd 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -645,7 +645,9 @@ static int stimer_notify_direct(struct kvm_vcpu_hv_stimer 
*stimer)
                .vector = stimer->config.apic_vector
        };
 
-       return !kvm_apic_set_irq(vcpu, &irq, NULL);
+       if (lapic_in_kernel(vcpu))
+               return !kvm_apic_set_irq(vcpu, &irq, NULL);
+       return 0;
 }
 
 static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)
@@ -1854,7 +1856,13 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, 
struct kvm_cpuid2 *cpuid,
 
                        ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
                        ent->edx |= HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
-                       ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
+
+                       /*
+                        * Direct Synthetic timers only make sense with 
in-kernel
+                        * LAPIC
+                        */
+                       if (lapic_in_kernel(vcpu))
+                               ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
 
                        break;
 
-- 
2.20.1