Re: [PATCH v4 1/2] s390/vfio-ap: fix memory leak in mdev remove callback

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 5/25/21 9:03 AM, Halil Pasic wrote:
> On Fri, 21 May 2021 15:36:47 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> The mdev remove callback for the vfio_ap device driver bails out with
>> -EBUSY if the mdev is in use by a KVM guest. The intended purpose was
>> to prevent the mdev from being removed while in use; however, returning a
>> non-zero rc does not prevent removal. This could result in a memory leak
>> of the resources allocated when the mdev was created. In addition, the
>> KVM guest will still have access to the AP devices assigned to the mdev
>> even though the mdev no longer exists.
>>
>> To prevent this scenario, cleanup will be done - including unplugging the
>> AP adapters, domains and control domains - regardless of whether the mdev
>> is in use by a KVM guest or not.
>>
>> Fixes: 258287c994de ("s390: vfio-ap: implement mediated device open callback")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> AFAIU we all agree that, after patch there is a possibility for an use
> after free error.

I am assuming here that you meant to say that after applying
patch 1/2, there is a possibility for a use after free error.

> I'm a little confused by the fact that we want this
> one for stable, but the patch that fixes the use after free as no
> Cc stable (it can't have a proper fixes tag, because this one is not yet
> merged). Actually I'm not a big fan of splitting up patches to the
> extent that when not all patches of the series are applied we get bugous
> behavior (e.g. patch n breaks something that is live at patch n level,
> but it is supposed to be OK, because patch n+m is going to fix it (where
> n,m \in \Z^{+}).
>
> Do we want to squash these? Is the use after free possible prior to this
> patch?

I am fine with squashing these if that is the consensus here. Prior
to this patch, the remove callback function returned -EBUSY
if a guest is still using the matrix_mdev (i.e., matrix_mdev->kvm
not NULL), so the matrix_mdev was not freed and hence the
memory leak for this this patch was designed to fix.

>
> Regards,
> Halil