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

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Message-ID: <20210525150337.021aabd8.pasic@linux.ibm.com>
Date:   Tue, 25 May 2021 15:03:37 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com
Subject: Re: [PATCH v4 1/2] s390/vfio-ap: fix memory leak in mdev remove
 callback

On Fri, 21 May 2021 15:36:47 -0400
Tony Krowiak <akrowiak@...ux.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@...r.kernel.org
> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
> Reviewed-by: Cornelia Huck <cohuck@...hat.com>

AFAIU we all agree that, after patch there is a possibility for an 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? 

Regards,
Halil