linux-kernel - [PATCH v2] KVM: VMX: Fix invalid guest state detection after task-switch emulation

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Message-ID: <20170719161952.GC17303@potion>
Date:   Wed, 19 Jul 2017 18:19:52 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH v2] KVM: VMX: Fix invalid guest state detection after
 task-switch emulation

2017-07-19 08:14-0700, Nadav Amit:
> Radim Krčmář <rkrcmar@...hat.com> wrote:
> > @@ -2363,6 +2368,8 @@ static unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
> > 
> > static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
> > {
> > +	unsigned long old_rflags = to_vmx(vcpu)->rflags;
> 
> It assumes rflags was decached from the VMCS before. Probably it is true, but…

Right, it's better to use accessors everywhere, thanks.
The line should read:

+	unsigned long old_rflags = vmx_get_rflags(vcpu);

---8<---
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.

Suggested-by: Nadav Amit <nadav.amit@...il.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
[Wanpeng wrote the commit message with initial patch and Radim moved the
 update to vmx_set_rflags and added Paolo's suggestion for the check.]
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
---
 arch/x86/kvm/vmx.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 84e62acf2dd8..a776aea0043a 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2326,6 +2326,11 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
 	__vmx_load_host_state(to_vmx(vcpu));
 }
 
+static bool emulation_required(struct kvm_vcpu *vcpu)
+{
+	return emulate_invalid_guest_state && !guest_state_valid(vcpu);
+}
+
 static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu);
 
 /*
@@ -2363,6 +2368,8 @@ static unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
 
 static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 {
+	unsigned long old_rflags = vmx_get_rflags(vcpu);
+
 	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
 	to_vmx(vcpu)->rflags = rflags;
 	if (to_vmx(vcpu)->rmode.vm86_active) {
@@ -2370,6 +2377,9 @@ static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 		rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
 	}
 	vmcs_writel(GUEST_RFLAGS, rflags);
+
+	if ((old_rflags ^ rflags) & X86_EFLAGS_VM)
+		to_vmx(vcpu)->emulation_required = emulation_required(vcpu);
 }
 
 static u32 vmx_get_pkru(struct kvm_vcpu *vcpu)
@@ -3857,11 +3867,6 @@ static __init int alloc_kvm_area(void)
 	return 0;
 }
 
-static bool emulation_required(struct kvm_vcpu *vcpu)
-{
-	return emulate_invalid_guest_state && !guest_state_valid(vcpu);
-}
-
 static void fix_pmode_seg(struct kvm_vcpu *vcpu, int seg,
 		struct kvm_segment *save)
 {
-- 
2.13.2