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

[Greg Kurz]

On Fri, 22 Apr 2016 17:21:03 +0800
Wei Yang <richard.weiyang@xxxxxxxxxx> wrote:

> Hi, Greg
> 

Hi Wei !

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

Yes and the reason is given in the changelog:
- ARM and s390 already have such a check
- PowerPC can live without this check
- this patch simply moves the check to MIPS and x86

Does it clarify ?

Cheers.

--
Greg

> 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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