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

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

[Greg Kurz]

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@linux.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);

Re: [PATCH] KVM: remove buggy vcpu id check on vcpu creation

[James Hogan]

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Hi Greg,

On Wed, Apr 20, 2016 at 05:07:58PM +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.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.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;

This needs to go before the kzalloc above, otherwise you introduce a
memory leak.

Cheers
James

> +
>  	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);
> 

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Re: [PATCH] KVM: remove buggy vcpu id check on vcpu creation

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1698 bytes --]

Hi,

[auto build test WARNING on kvm/linux-next]
[also build test WARNING on v4.6-rc4 next-20160420]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Greg-Kurz/KVM-remove-buggy-vcpu-id-check-on-vcpu-creation/20160420-231233
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git linux-next
config: x86_64-randconfig-x017-201616 (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All warnings (new ones prefixed by >>):

   arch/x86/kvm/x86.c: In function 'kvm_arch_vcpu_create':
>> arch/x86/kvm/x86.c:7362:10: warning: return makes pointer from integer without a cast [-Wint-conversion]
      return -EINVAL;
             ^

vim +7362 arch/x86/kvm/x86.c

  7346	}
  7347	
  7348	void kvm_arch_vcpu_free(struct kvm_vcpu *vcpu)
  7349	{
  7350		kvmclock_reset(vcpu);
  7351	
  7352		free_cpumask_var(vcpu->arch.wbinvd_dirty_mask);
  7353		kvm_x86_ops->vcpu_free(vcpu);
  7354	}
  7355	
  7356	struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
  7357							unsigned int id)
  7358	{
  7359		struct kvm_vcpu *vcpu;
  7360	
  7361		if (id >= KVM_MAX_VCPUS)
> 7362			return -EINVAL;
  7363	
  7364		if (check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
  7365			printk_once(KERN_WARNING
  7366			"kvm: SMP vm created on host with unstable TSC; "
  7367			"guest TSC will not be reliable\n");
  7368	
  7369		vcpu = kvm_x86_ops->vcpu_create(kvm, id);
  7370	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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Re: [PATCH] KVM: remove buggy vcpu id check on vcpu creation

[Greg Kurz]

On Wed, 20 Apr 2016 16:12:46 +0100
James Hogan <james.hogan@imgtec.com> wrote:

> Hi Greg,
> 
> On Wed, Apr 20, 2016 at 05:07:58PM +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.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.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;  
> 
> This needs to go before the kzalloc above, otherwise you introduce a
> memory leak.
> 

Oops you're right.. my bad. I'll post a v2 right away.

Thanks !

--
Greg

> Cheers
> James
> 
> > +
> >  	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);
> >