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

From: Radim KrÄmÃÅ
Date: Thu Apr 21 2016 - 12:00:28 EST

2016-04-21 16:20+0200, 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. 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.

Userspace won't be able to tell if KVM_CREATE_VCPU failed because it
provided too high vcpu_id to an old KVM or because new KVM failed in
other areas. Not a huge problem (because I expect that userspace will
die on both), but a new KVM_CAP would be able to disambiguate it.

Toggleable capability doesn't seem necessary and only PowerPC changes,
so the capability could be arch specific ... I think that a generic one
makes more sense, though.

Userspace also doesn't know the vcpu id limit anymore, and it might
care. What do you think about returning the arch-specific limit (or the
highest positive integer) as int in KVM_CAP_MAX_VCPU_ID?

I think this would also clarify the connection between VCPU limit and
VCPU_ID limit. Or is a boolean cap better?

> -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.