Re: [PATCH v4 2/2] KVM: move vcpu id checking to archs

[Wei Yang]

Hi, Greg

One confusion.

There are 5 kvm_arch_vcpu_create() while in this patch you changed 2 of them.
Some particular reason?

On Thu, Apr 21, 2016 at 04:20:53PM +0200, Greg Kurz wrote:
>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. While here,
>we also update the documentation to dissociate vcpu ids from the maximum
>number of vcpus per virtual machine.
>
>Acked-by: James Hogan <james.hogan@xxxxxxxxxx>
>Acked-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
>Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
>---
>v4: - updated subject for more clarity on what the patch does
>    - added James's and Connie's A-b tags
>    - updated documentation
>
> Documentation/virtual/kvm/api.txt |    7 +++----
> arch/mips/kvm/mips.c              |    7 ++++++-
> arch/x86/kvm/x86.c                |    3 +++
> virt/kvm/kvm_main.c               |    3 ---
> 4 files changed, 12 insertions(+), 8 deletions(-)
>
>diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>index 4d0542c5206b..486a1d783b82 100644
>--- a/Documentation/virtual/kvm/api.txt
>+++ b/Documentation/virtual/kvm/api.txt
>@@ -199,11 +199,10 @@ Type: vm ioctl
> Parameters: vcpu id (apic id on x86)
> Returns: vcpu fd on success, -1 on error
> 
>-This API adds a vcpu to a virtual machine.  The vcpu id is a small integer
>-in the range [0, max_vcpus).
>+This API adds a vcpu to a virtual machine.  The vcpu id is a positive integer.
> 
>-The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of
>-the KVM_CHECK_EXTENSION ioctl() at run-time.
>+The recommended maximum number of vcpus (max_vcpus) can be retrieved using the
>+KVM_CAP_NR_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.
> The maximum possible value for max_vcpus can be retrieved using the
> KVM_CAP_MAX_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.
> 
>diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
>index 70ef1a43c114..0278ea146db5 100644
>--- a/arch/mips/kvm/mips.c
>+++ b/arch/mips/kvm/mips.c
>@@ -248,9 +248,14 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
> 	int err, size, offset;
> 	void *gebase;
> 	int i;
>+	struct kvm_vcpu *vcpu;
> 
>-	struct kvm_vcpu *vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);
>+	if (id >= KVM_MAX_VCPUS) {
>+		err = -EINVAL;
>+		goto out;
>+	}
> 
>+	vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);
> 	if (!vcpu) {
> 		err = -ENOMEM;
> 		goto out;
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 9b7798c7b210..7738202edcce 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 ERR_PTR(-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);
>
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-- 
Richard Yang\nHelp you, Help me