linux-kernel - Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation

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Date:	Wed, 20 Apr 2016 18:48:07 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com, linux-mips@...ux-mips.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, qemu-ppc@...gnu.org,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation

On Wed, 20 Apr 2016 17:44:54 +0200
Greg Kurz <gkurz@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com>
> ---
> v3: use ERR_PTR()
> 
>  arch/mips/kvm/mips.c |    7 ++++++-
>  arch/x86/kvm/x86.c   |    3 +++
>  virt/kvm/kvm_main.c  |    3 ---
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 

Acked-by: Cornelia Huck <cornelia.huck@...ibm.com>