linux-kernel - Re: [PATCH] vfio: Limit group opens

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Date:	Fri, 21 Jun 2013 20:57:28 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] vfio: Limit group opens

On Sat, 2013-06-22 at 12:44 +1000, Alexey Kardashevskiy wrote:
> On 06/22/2013 11:26 AM, Alex Williamson wrote:
> > On Sat, 2013-06-22 at 11:16 +1000, Alexey Kardashevskiy wrote:
> >> Cool, thanks!
> >>
> >> So we will need only this (to be called from KVM), and that will be it, right?
> > 
> > For what?  This is not the external lock you're looking for.  As I've
> > mentioned, the file can only hold the group, but that doesn't give you
> > any guarantee that the group is protected by the IOMMU.  Thanks,
> 
> 
> I am confused, sorry :) With this patch, a group fd cannot be reopened if
> already opened, and this is the only way for user space to take control
> over a group. If it is not an external lock, then what is it? And all I
> have to do now is to verify that the group fd passed to KVM is correct and
> I am happy. Who and how can break anything (group? KVM?) now?

By that logic all a user needs to do is open() a group and they they're
free to pass the fd to KVM, right?  But the IOMMU protection isn't
enabled until the user calls SET_CONTAINER and SET_IOMMU, so you'd be
giving KVM access to the IOMMU that the user hasn't enabled.  The group
may still have devices attached to host drivers.  Likewise, a user need
only call UNSET_CONTAINER to teardown the IOMMU protection.  At that
point a device could be re-bound to host drivers, thus making it unsafe
for KVM to be directly poking the IOMMU.

This patch is just a bug fix for inconsistent behavior.  Thanks,

Alex
 
> >> int vfio_group_iommu_id_from_file(struct file *filep)
> >> ...
> >>
> >>
> >>
> >> On 06/22/2013 07:12 AM, Alex Williamson wrote:
> >>> vfio_group_fops_open attempts to limit concurrent sessions by
> >>> disallowing opens once group->container is set.  This really doesn't
> >>> do what we want and allow for inconsistent behavior, for instance a
> >>> group can be opened twice, then a container set giving the user two
> >>> file descriptors to the group.  But then it won't allow more to be
> >>> opened.  There's not much reason to have the group opened multiple
> >>> times since most access is through devices or the container, so
> >>> complete what the original code intended and only allow a single
> >>> instance.
> >>>
> >>> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> >>> ---
> >>>  drivers/vfio/vfio.c |   14 ++++++++++++++
> >>>  1 file changed, 14 insertions(+)
> >>>
> >>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> >>> index 6d78736..d30f44d 100644
> >>> --- a/drivers/vfio/vfio.c
> >>> +++ b/drivers/vfio/vfio.c
> >>> @@ -76,6 +76,7 @@ struct vfio_group {
> >>>  	struct notifier_block		nb;
> >>>  	struct list_head		vfio_next;
> >>>  	struct list_head		container_next;
> >>> +	atomic_t			opened;
> >>>  };
> >>>  
> >>>  struct vfio_device {
> >>> @@ -206,6 +207,7 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group)
> >>>  	INIT_LIST_HEAD(&group->device_list);
> >>>  	mutex_init(&group->device_lock);
> >>>  	atomic_set(&group->container_users, 0);
> >>> +	atomic_set(&group->opened, 0);
> >>>  	group->iommu_group = iommu_group;
> >>>  
> >>>  	group->nb.notifier_call = vfio_iommu_group_notifier;
> >>> @@ -1236,12 +1238,22 @@ static long vfio_group_fops_compat_ioctl(struct file *filep,
> >>>  static int vfio_group_fops_open(struct inode *inode, struct file *filep)
> >>>  {
> >>>  	struct vfio_group *group;
> >>> +	int opened;
> >>>  
> >>>  	group = vfio_group_get_from_minor(iminor(inode));
> >>>  	if (!group)
> >>>  		return -ENODEV;
> >>>  
> >>> +	/* Do we need multiple instances of the group open?  Seems not. */
> >>> +	opened = atomic_cmpxchg(&group->opened, 0, 1);
> >>> +	if (opened) {
> >>> +		vfio_group_put(group);
> >>> +		return -EBUSY;
> >>> +	}
> >>> +
> >>> +	/* Is something still in use from a previous open? */
> >>>  	if (group->container) {
> >>> +		atomic_dec(&group->opened);
> >>>  		vfio_group_put(group);
> >>>  		return -EBUSY;
> >>>  	}
> >>> @@ -1259,6 +1271,8 @@ static int vfio_group_fops_release(struct inode *inode, struct file *filep)
> >>>  
> >>>  	vfio_group_try_dissolve_container(group);
> >>>  
> >>> +	atomic_dec(&group->opened);
> >>> +
> >>>  	vfio_group_put(group);
> >>>  
> >>>  	return 0;
> >>>
> >>
> >>
> > 
> > 
> > 
> 
> 



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