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KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

PATCH In review Like Xu 3 participants 5 messages Jul 8, 2020

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#0 lore raw

PATCH 1/1 KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

In guest protected mode, if the current privilege level is not 0 and the PCE flag in the CR4 register is cleared, we will inject a #GP for RDPMC usage.

Signed-off-by Like Xu <[email protected]>
---
 arch/x86/kvm/pmu.c | 5 +++++
 1 file changed, 5 insertions(+)
arch/x86/kvm/pmu.c +5 -0
index b86346903f2e..d080d475c808 100644
@@ -372,6 +372,11 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
 	if (!pmc)
 		return 1;
 
+	if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
+	    !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
+	    (kvm_read_cr4(vcpu) & X86_CR0_PE))
+		return 1;
+
 	*data = pmc_read_counter(pmc) & mask;
 	return 0;
 }
Signature · 1 line
2.21.3
#1 lore raw
Quoted 3 lines · Like Xu

in guest protected mode, if the current privilege level is not 0 and the pce flag in the cr4 register is cleared, we will inject a #gp for rdpmc usage.

Wrapping at ~58 characters is a bit aggressive. checkpatch enforces 75 chars, something near that would be prefereable.

Quoted 10 lines
Signed-off-by Like Xu <[email protected]>
---
 arch/x86/kvm/pmu.c | 5 +++++
 1 file changed, 5 insertions(+)
arch/x86/kvm/pmu.c +0 -0
index b86346903f2e..d080d475c808 100644

arch/x86/kvm/pmu.c int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data) · line 377

Quoted 6 lines
arch/x86/kvm/pmu.c +3 -0
@@ -372,6 +372,11 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
 	if (!pmc)
 		return 1;
 
+	if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
+	    !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
+	    (kvm_read_cr4(vcpu) & X86_CR0_PE))

This reads CR4 but checks CR0.PE.

And maybe put the X86_CR4_PCE check first so that it's the focus of the statement?

Quoted 8 lines
arch/x86/kvm/pmu.c +2 -0
+		return 1;
+
 	*data = pmc_read_counter(pmc) & mask;
 	return 0;
 }
Signature · 1 line
2.21.3
maintainer
#2 lore raw
Quoted 21 lines · Sean Christopherson
Quoted 3 lines · Like Xu

in guest protected mode, if the current privilege level is not 0 and the pce flag in the cr4 register is cleared, we will inject a #gp for rdpmc usage.

Wrapping at ~58 characters is a bit aggressive. checkpatch enforces 75 chars, something near that would be prefereable.

Quoted 13 lines
Signed-off-by Like Xu <[email protected]>
---
 arch/x86/kvm/pmu.c | 5 +++++
 1 file changed, 5 insertions(+)
arch/x86/kvm/pmu.c +0 -0
index b86346903f2e..d080d475c808 100644
@@ -372,6 +372,11 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
 	if (!pmc)
 		return 1;
Quoted 8 lines

arch/x86/kvm/pmu.c int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data) · line 377

Quoted 3 lines
arch/x86/kvm/pmu.c +3 -0
+	if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
+	    !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
+	    (kvm_read_cr4(vcpu) & X86_CR0_PE))

This reads CR4 but checks CR0.PE.

And maybe put the X86_CR4_PCE check first so that it's the focus of the statement?

I'll squash this to fix it (I'm OOO next week and would like to get kvm/queue sorted out these few days that I've left).

arch/x86/kvm/pmu.c +3 -3
index d080d475c808..67741d2a0308 100644
@@ -372,9 +372,9 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
 	if (!pmc)
 		return 1;
 
-	if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
-	    !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
-	    (kvm_read_cr4(vcpu) & X86_CR0_PE))
+	if (!(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
+	    (kvm_x86_ops.get_cpl(vcpu) != 0) &&
+	    (kvm_read_cr0(vcpu) & X86_CR0_PE))
 		return 1;
 
 	*data = pmc_read_counter(pmc) & mask;

The order follows the SDM. I'm tempted to remove the CR0 check altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's keep it close to what the manual says.

Paolo

#3 lore raw
Quoted 3 lines · Paolo Bonzini

The order follows the SDM. I'm tempted to remove the CR0 check altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's keep it close to what the manual says.

Heh, it wouldn't surprise me in the least if there's a way to get the SS arbyte to hold a non-zero DPL in real mode :-).

maintainer
#4 lore raw
Quoted 7 lines · Sean Christopherson
Quoted 3 lines · Paolo Bonzini

The order follows the SDM. I'm tempted to remove the CR0 check altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's keep it close to what the manual says.

Heh, it wouldn't surprise me in the least if there's a way to get the SS arbyte to hold a non-zero DPL in real mode :-).

I'm not sure if SMM lets you set non-zero SS.DPL in real mode. It's one of the few things that are checked with unrestricted guest mode so there's hope; on the other hand I know for sure that in the past RSM could get you to VM86 mode with CPL=0, while in VMX it causes vmentry to fail.

It would be an interesting testcase to write for KVM, to see if you get a vmentry failure after you set the hidden AR bytes that way and RSM...

Paolo