linux-kernel - [PATCH] KVM: remove buggy vcpu id check on vcpu creation

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Date:	Wed, 20 Apr 2016 17:07:58 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com
Cc:	linux-mips@...ux-mips.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, qemu-ppc@...gnu.org,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: [PATCH] KVM: remove buggy vcpu id check on vcpu creation

Commit 338c7dbadd26 ("KVM: Improve create VCPU parameter (CVE-2013-4587)")
introduced a check to prevent potential kernel memory corruption in case
the vcpu id is too great.

Unfortunately this check assumes vcpu ids grow in sequence with a common
difference of 1, which is wrong: archs are free to use vcpu id as they fit.
For example, QEMU originated vcpu ids for PowerPC cpus running in boot3s_hv
mode, can grow with a common difference of 2, 4 or 8: if KVM_MAX_VCPUS is
1024, guests may be limited down to 128 vcpus on POWER8.

This means the check does not belong here and should be moved to some arch
specific function: kvm_arch_vcpu_create() looks like a good candidate.

ARM and s390 already have such a check.

I could not spot any path in the PowerPC or common KVM code where a vcpu
id is used as described in the above commit: I believe PowerPC can live
without this check.

In the end, this patch simply moves the check to MIPS and x86.

Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>
---
 arch/mips/kvm/mips.c |    3 +++
 arch/x86/kvm/x86.c   |    3 +++
 virt/kvm/kvm_main.c  |    3 ---
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index 70ef1a43c114..ce3f1e8a8b3f 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -251,6 +251,9 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)

 	struct kvm_vcpu *vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);

+	if (id >= KVM_MAX_VCPUS)
+		return -EINVAL;
+
 	if (!vcpu) {
 		err = -ENOMEM;
 		goto out;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9b7798c7b210..f705d57b12ed 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7358,6 +7358,9 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
 {
 	struct kvm_vcpu *vcpu;

+	if (id >= KVM_MAX_VCPUS)
+		return -EINVAL;
+
 	if (check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
 		printk_once(KERN_WARNING
 		"kvm: SMP vm created on host with unstable TSC; "
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4fd482fb9260..6b6cca3cb488 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2272,9 +2272,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 	int r;
 	struct kvm_vcpu *vcpu;

-	if (id >= KVM_MAX_VCPUS)
-		return -EINVAL;
-
 	vcpu = kvm_arch_vcpu_create(kvm, id);
 	if (IS_ERR(vcpu))
 		return PTR_ERR(vcpu);