linux-kernel - [PATCH] KVM: VMX: Fix invalid guest state detection after task-switch emulation

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Date:   Tue, 11 Jul 2017 00:13:00 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH] KVM: VMX: Fix invalid guest state detection after task-switch emulation

From: Wanpeng Li <wanpeng.li@...mail.com>

This can be reproduced by EPT=1, unrestricted_guest=N, emulate_invalid_state=Y 
or EPT=0, the trace of kvm-unit-tests/taskswitch2.flat is like below, it tries 
to emulate invalid guest state task-switch:

kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
......................

It appears that the task-switch emulation updates rflags (and vm86 
flag) only after the segments are loaded, causing vmx->emulation_required 
to be set, when in fact invalid guest state emulation is not needed.

This patch fixes it by updating vmx->emulation_required after the 
rflags (and vm86 flag) is updated in task-switch emulation.

Suggested-by: Nadav Amit <nadav.amit@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Nadav Amit <nadav.amit@...il.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
 arch/x86/kvm/vmx.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f50cbfd..70270a2 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6255,6 +6255,7 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 	 * TODO: What about debug traps on tss switch?
 	 *       Are we supposed to inject them and update dr6?
 	 */
+	vmx->emulation_required = emulation_required(vcpu);
 
 	return 1;
 }
-- 
2.7.4