[v4,1/2] vfio: fix deadlock between group lock and kvm lock

  After 51cdc8bc120e, we have another deadlock scenario between the
kvm->lock and the vfio group_lock with two different codepaths acquiring
the locks in different order.  Specifically in vfio_open_device, vfio
holds the vfio group_lock when issuing device->ops->open_device but some
drivers (like vfio-ap) need to acquire kvm->lock during their open_device
routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
before calling vfio_file_set_kvm which will acquire the vfio group_lock.

To resolve this, let's remove the need for the vfio group_lock from the
kvm_vfio_release codepath.  This is done by introducing a new spinlock to
protect modifications to the vfio group kvm pointer, and acquiring a kvm
ref from within vfio while holding this spinlock, with the reference held
until the last close for the device in question.

Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
 drivers/vfio/group.c     | 44 +++++++++++++++++++++++-----
 drivers/vfio/vfio.h      | 15 ++++++++++
 drivers/vfio/vfio_main.c | 63 +++++++++++++++++++++++++++++++++++-----
 include/linux/vfio.h     |  2 +-
 4 files changed, 109 insertions(+), 15 deletions(-)
  

> From: Matthew Rosato <mjrosato@linux.ibm.com>
> Sent: Saturday, February 4, 2023 5:50 AM
> To: alex.williamson@redhat.com; pbonzini@redhat.com; Liu, Yi L
> 
> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order.  Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but
> some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> 
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
> 
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  drivers/vfio/group.c     | 44 +++++++++++++++++++++++-----
>  drivers/vfio/vfio.h      | 15 ++++++++++
>  drivers/vfio/vfio_main.c | 63
> +++++++++++++++++++++++++++++++++++-----
>  include/linux/vfio.h     |  2 +-
>  4 files changed, 109 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..98621ac082f0 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -154,6 +154,18 @@ static int vfio_group_ioctl_set_container(struct
> vfio_group *group,
>  	return ret;
>  }
> 
> +static void vfio_device_group_get_kvm_safe(struct vfio_device *device)
> +{
> +	spin_lock(&device->group->kvm_ref_lock);
> +	if (!device->group->kvm)
> +		goto unlock;
> +
> +	_vfio_device_get_kvm_safe(device, device->group->kvm);
> +
> +unlock:
> +	spin_unlock(&device->group->kvm_ref_lock);
> +}
> +
>  static int vfio_device_group_open(struct vfio_device *device)
>  {
>  	int ret;
> @@ -164,13 +176,23 @@ static int vfio_device_group_open(struct
> vfio_device *device)
>  		goto out_unlock;
>  	}
> 
> +	mutex_lock(&device->dev_set->lock);
> +
>  	/*
> -	 * Here we pass the KVM pointer with the group under the lock.  If
> the
> -	 * device driver will use it, it must obtain a reference and release it
> -	 * during close_device.
> +	 * Before the first device open, get the KVM pointer currently
> +	 * associated with the group (if there is one) and obtain a reference
> +	 * now that will be held until the open_count reaches 0 again.  Save

Nit: a redundant space before "Save". Other part looks good to me.

Reviewed-by: Yi Liu <yi.l.liu@intel.com>

Regards,
Yi Liu

> +	 * the pointer in the device for use by drivers.
>  	 */
> -	ret = vfio_device_open(device, device->group->iommufd,
> -			       device->group->kvm);
> +	if (device->open_count == 0)
> +		vfio_device_group_get_kvm_safe(device);
> +
> +	ret = vfio_device_open(device, device->group->iommufd, device->kvm);
> +
> +	if (device->open_count == 0)
> +		vfio_device_put_kvm(device);
> +
> +	mutex_unlock(&device->dev_set->lock);
> 
>  out_unlock:
>  	mutex_unlock(&device->group->group_lock);
> @@ -180,7 +202,14 @@ static int vfio_device_group_open(struct
> vfio_device *device)
>  void vfio_device_group_close(struct vfio_device *device)
>  {
>  	mutex_lock(&device->group->group_lock);
> +	mutex_lock(&device->dev_set->lock);
> +
>  	vfio_device_close(device, device->group->iommufd);
> +
> +	if (device->open_count == 0)
> +		vfio_device_put_kvm(device);
> +
> +	mutex_unlock(&device->dev_set->lock);
>  	mutex_unlock(&device->group->group_lock);
>  }
> 
> @@ -450,6 +479,7 @@ static struct vfio_group *vfio_group_alloc(struct
> iommu_group *iommu_group,
> 
>  	refcount_set(&group->drivers, 1);
>  	mutex_init(&group->group_lock);
> +	spin_lock_init(&group->kvm_ref_lock);
>  	INIT_LIST_HEAD(&group->device_list);
>  	mutex_init(&group->device_lock);
>  	group->iommu_group = iommu_group;
> @@ -803,9 +833,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm
> *kvm)
>  	if (!vfio_file_is_group(file))
>  		return;
> 
> -	mutex_lock(&group->group_lock);
> +	spin_lock(&group->kvm_ref_lock);
>  	group->kvm = kvm;
> -	mutex_unlock(&group->group_lock);
> +	spin_unlock(&group->kvm_ref_lock);
>  }
>  EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
> 
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index f8219a438bfb..24d6cd285945 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -74,6 +74,7 @@ struct vfio_group {
>  	struct file			*opened_file;
>  	struct blocking_notifier_head	notifier;
>  	struct iommufd_ctx		*iommufd;
> +	spinlock_t			kvm_ref_lock;
>  };
> 
>  int vfio_device_set_group(struct vfio_device *device,
> @@ -251,4 +252,18 @@ extern bool vfio_noiommu __read_mostly;
>  enum { vfio_noiommu = false };
>  #endif
> 
> +#ifdef CONFIG_HAVE_KVM
> +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm);
> +void vfio_device_put_kvm(struct vfio_device *device);
> +#else
> +static inline void _vfio_device_get_kvm_safe(struct vfio_device *device,
> +					     struct kvm *kvm)
> +{
> +}
> +
> +static inline void vfio_device_put_kvm(struct vfio_device *device)
> +{
> +}
> +#endif
> +
>  #endif
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..28c47cd6a6b5 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,9 @@
>  #include <linux/fs.h>
>  #include <linux/idr.h>
>  #include <linux/iommu.h>
> +#ifdef CONFIG_HAVE_KVM
> +#include <linux/kvm_host.h>
> +#endif
>  #include <linux/list.h>
>  #include <linux/miscdevice.h>
>  #include <linux/module.h>
> @@ -338,6 +341,55 @@ void vfio_unregister_group_dev(struct vfio_device
> *device)
>  }
>  EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
> 
> +#ifdef CONFIG_HAVE_KVM
> +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm)
> +{
> +	void (*pfn)(struct kvm *kvm);
> +	bool (*fn)(struct kvm *kvm);
> +	bool ret;
> +
> +	lockdep_assert_held(&device->dev_set->lock);
> +
> +	pfn = symbol_get(kvm_put_kvm);
> +	if (WARN_ON(!pfn))
> +		return;
> +
> +	fn = symbol_get(kvm_get_kvm_safe);
> +	if (WARN_ON(!fn)) {
> +		symbol_put(kvm_put_kvm);
> +		return;
> +	}
> +
> +	ret = fn(kvm);
> +	symbol_put(kvm_get_kvm_safe);
> +	if (!ret) {
> +		symbol_put(kvm_put_kvm);
> +		return;
> +	}
> +
> +	device->put_kvm = pfn;
> +	device->kvm = kvm;
> +}
> +
> +void vfio_device_put_kvm(struct vfio_device *device)
> +{
> +	lockdep_assert_held(&device->dev_set->lock);
> +
> +	if (!device->kvm)
> +		return;
> +
> +	if (WARN_ON(!device->put_kvm))
> +		goto clear;
> +
> +	device->put_kvm(device->kvm);
> +	device->put_kvm = NULL;
> +	symbol_put(kvm_put_kvm);
> +
> +clear:
> +	device->kvm = NULL;
> +}
> +#endif
> +
>  /* true if the vfio_device has open_device() called but not close_device()
> */
>  static bool vfio_assert_device_open(struct vfio_device *device)
>  {
> @@ -361,7 +413,6 @@ static int vfio_device_first_open(struct vfio_device
> *device,
>  	if (ret)
>  		goto err_module_put;
> 
> -	device->kvm = kvm;
>  	if (device->ops->open_device) {
>  		ret = device->ops->open_device(device);
>  		if (ret)
> @@ -370,7 +421,6 @@ static int vfio_device_first_open(struct vfio_device
> *device,
>  	return 0;
> 
>  err_unuse_iommu:
> -	device->kvm = NULL;
>  	if (iommufd)
>  		vfio_iommufd_unbind(device);
>  	else
> @@ -387,7 +437,6 @@ static void vfio_device_last_close(struct vfio_device
> *device,
> 
>  	if (device->ops->close_device)
>  		device->ops->close_device(device);
> -	device->kvm = NULL;
>  	if (iommufd)
>  		vfio_iommufd_unbind(device);
>  	else
> @@ -400,14 +449,14 @@ int vfio_device_open(struct vfio_device *device,
>  {
>  	int ret = 0;
> 
> -	mutex_lock(&device->dev_set->lock);
> +	lockdep_assert_held(&device->dev_set->lock);
> +
>  	device->open_count++;
>  	if (device->open_count == 1) {
>  		ret = vfio_device_first_open(device, iommufd, kvm);
>  		if (ret)
>  			device->open_count--;
>  	}
> -	mutex_unlock(&device->dev_set->lock);
> 
>  	return ret;
>  }
> @@ -415,12 +464,12 @@ int vfio_device_open(struct vfio_device *device,
>  void vfio_device_close(struct vfio_device *device,
>  		       struct iommufd_ctx *iommufd)
>  {
> -	mutex_lock(&device->dev_set->lock);
> +	lockdep_assert_held(&device->dev_set->lock);
> +
>  	vfio_assert_device_open(device);
>  	if (device->open_count == 1)
>  		vfio_device_last_close(device, iommufd);
>  	device->open_count--;
> -	mutex_unlock(&device->dev_set->lock);
>  }
> 
>  /*
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 35be78e9ae57..87ff862ff555 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -46,7 +46,6 @@ struct vfio_device {
>  	struct vfio_device_set *dev_set;
>  	struct list_head dev_set_list;
>  	unsigned int migration_flags;
> -	/* Driver must reference the kvm during open_device or never
> touch it */
>  	struct kvm *kvm;
> 
>  	/* Members below here are private, not for driver use */
> @@ -58,6 +57,7 @@ struct vfio_device {
>  	struct list_head group_next;
>  	struct list_head iommu_entry;
>  	struct iommufd_access *iommufd_access;
> +	void (*put_kvm)(struct kvm *kvm);
>  #if IS_ENABLED(CONFIG_IOMMUFD)
>  	struct iommufd_device *iommufd_device;
>  	struct iommufd_ctx *iommufd_ictx;
> --
> 2.39.1
  

On Sat, 4 Feb 2023 06:21:48 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> > From: Matthew Rosato <mjrosato@linux.ibm.com>
> > Sent: Saturday, February 4, 2023 5:50 AM
> > To: alex.williamson@redhat.com; pbonzini@redhat.com; Liu, Yi L
> > 
> > After 51cdc8bc120e, we have another deadlock scenario between the
> > kvm->lock and the vfio group_lock with two different codepaths acquiring
> > the locks in different order.  Specifically in vfio_open_device, vfio
> > holds the vfio group_lock when issuing device->ops->open_device but
> > some
> > drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> > routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> > before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> > 
> > To resolve this, let's remove the need for the vfio group_lock from the
> > kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> > protect modifications to the vfio group kvm pointer, and acquiring a kvm
> > ref from within vfio while holding this spinlock, with the reference held
> > until the last close for the device in question.
> > 
> > Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> > Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > ---
> >  drivers/vfio/group.c     | 44 +++++++++++++++++++++++-----
> >  drivers/vfio/vfio.h      | 15 ++++++++++
> >  drivers/vfio/vfio_main.c | 63
> > +++++++++++++++++++++++++++++++++++-----
> >  include/linux/vfio.h     |  2 +-
> >  4 files changed, 109 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index bb24b2f0271e..98621ac082f0 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -154,6 +154,18 @@ static int vfio_group_ioctl_set_container(struct
> > vfio_group *group,
> >  	return ret;
> >  }
> > 
> > +static void vfio_device_group_get_kvm_safe(struct vfio_device *device)
> > +{
> > +	spin_lock(&device->group->kvm_ref_lock);
> > +	if (!device->group->kvm)
> > +		goto unlock;
> > +
> > +	_vfio_device_get_kvm_safe(device, device->group->kvm);
> > +
> > +unlock:
> > +	spin_unlock(&device->group->kvm_ref_lock);
> > +}
> > +
> >  static int vfio_device_group_open(struct vfio_device *device)
> >  {
> >  	int ret;
> > @@ -164,13 +176,23 @@ static int vfio_device_group_open(struct
> > vfio_device *device)
> >  		goto out_unlock;
> >  	}
> > 
> > +	mutex_lock(&device->dev_set->lock);
> > +
> >  	/*
> > -	 * Here we pass the KVM pointer with the group under the lock.  If
> > the
> > -	 * device driver will use it, it must obtain a reference and release it
> > -	 * during close_device.
> > +	 * Before the first device open, get the KVM pointer currently
> > +	 * associated with the group (if there is one) and obtain a reference
> > +	 * now that will be held until the open_count reaches 0 again.  Save  
> 
> Nit: a redundant space before "Save". Other part looks good to me.

Two spaces between sentences is a common standard.  Not everyone
prefers this, but I do.  Thanks,

Alex

> Reviewed-by: Yi Liu <yi.l.liu@intel.com>
> 
> Regards,
> Yi Liu
> 
> > +	 * the pointer in the device for use by drivers.
> >  	 */
> > -	ret = vfio_device_open(device, device->group->iommufd,
> > -			       device->group->kvm);
> > +	if (device->open_count == 0)
> > +		vfio_device_group_get_kvm_safe(device);
> > +
> > +	ret = vfio_device_open(device, device->group->iommufd, device->kvm);
> > +
> > +	if (device->open_count == 0)
> > +		vfio_device_put_kvm(device);
> > +
> > +	mutex_unlock(&device->dev_set->lock);
> > 
> >  out_unlock:
> >  	mutex_unlock(&device->group->group_lock);
> > @@ -180,7 +202,14 @@ static int vfio_device_group_open(struct
> > vfio_device *device)
> >  void vfio_device_group_close(struct vfio_device *device)
> >  {
> >  	mutex_lock(&device->group->group_lock);
> > +	mutex_lock(&device->dev_set->lock);
> > +
> >  	vfio_device_close(device, device->group->iommufd);
> > +
> > +	if (device->open_count == 0)
> > +		vfio_device_put_kvm(device);
> > +
> > +	mutex_unlock(&device->dev_set->lock);
> >  	mutex_unlock(&device->group->group_lock);
> >  }
> > 
> > @@ -450,6 +479,7 @@ static struct vfio_group *vfio_group_alloc(struct
> > iommu_group *iommu_group,
> > 
> >  	refcount_set(&group->drivers, 1);
> >  	mutex_init(&group->group_lock);
> > +	spin_lock_init(&group->kvm_ref_lock);
> >  	INIT_LIST_HEAD(&group->device_list);
> >  	mutex_init(&group->device_lock);
> >  	group->iommu_group = iommu_group;
> > @@ -803,9 +833,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm
> > *kvm)
> >  	if (!vfio_file_is_group(file))
> >  		return;
> > 
> > -	mutex_lock(&group->group_lock);
> > +	spin_lock(&group->kvm_ref_lock);
> >  	group->kvm = kvm;
> > -	mutex_unlock(&group->group_lock);
> > +	spin_unlock(&group->kvm_ref_lock);
> >  }
> >  EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
> > 
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index f8219a438bfb..24d6cd285945 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -74,6 +74,7 @@ struct vfio_group {
> >  	struct file			*opened_file;
> >  	struct blocking_notifier_head	notifier;
> >  	struct iommufd_ctx		*iommufd;
> > +	spinlock_t			kvm_ref_lock;
> >  };
> > 
> >  int vfio_device_set_group(struct vfio_device *device,
> > @@ -251,4 +252,18 @@ extern bool vfio_noiommu __read_mostly;
> >  enum { vfio_noiommu = false };
> >  #endif
> > 
> > +#ifdef CONFIG_HAVE_KVM
> > +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm
> > *kvm);
> > +void vfio_device_put_kvm(struct vfio_device *device);
> > +#else
> > +static inline void _vfio_device_get_kvm_safe(struct vfio_device *device,
> > +					     struct kvm *kvm)
> > +{
> > +}
> > +
> > +static inline void vfio_device_put_kvm(struct vfio_device *device)
> > +{
> > +}
> > +#endif
> > +
> >  #endif
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index 5177bb061b17..28c47cd6a6b5 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -16,6 +16,9 @@
> >  #include <linux/fs.h>
> >  #include <linux/idr.h>
> >  #include <linux/iommu.h>
> > +#ifdef CONFIG_HAVE_KVM
> > +#include <linux/kvm_host.h>
> > +#endif
> >  #include <linux/list.h>
> >  #include <linux/miscdevice.h>
> >  #include <linux/module.h>
> > @@ -338,6 +341,55 @@ void vfio_unregister_group_dev(struct vfio_device
> > *device)
> >  }
> >  EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
> > 
> > +#ifdef CONFIG_HAVE_KVM
> > +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm
> > *kvm)
> > +{
> > +	void (*pfn)(struct kvm *kvm);
> > +	bool (*fn)(struct kvm *kvm);
> > +	bool ret;
> > +
> > +	lockdep_assert_held(&device->dev_set->lock);
> > +
> > +	pfn = symbol_get(kvm_put_kvm);
> > +	if (WARN_ON(!pfn))
> > +		return;
> > +
> > +	fn = symbol_get(kvm_get_kvm_safe);
> > +	if (WARN_ON(!fn)) {
> > +		symbol_put(kvm_put_kvm);
> > +		return;
> > +	}
> > +
> > +	ret = fn(kvm);
> > +	symbol_put(kvm_get_kvm_safe);
> > +	if (!ret) {
> > +		symbol_put(kvm_put_kvm);
> > +		return;
> > +	}
> > +
> > +	device->put_kvm = pfn;
> > +	device->kvm = kvm;
> > +}
> > +
> > +void vfio_device_put_kvm(struct vfio_device *device)
> > +{
> > +	lockdep_assert_held(&device->dev_set->lock);
> > +
> > +	if (!device->kvm)
> > +		return;
> > +
> > +	if (WARN_ON(!device->put_kvm))
> > +		goto clear;
> > +
> > +	device->put_kvm(device->kvm);
> > +	device->put_kvm = NULL;
> > +	symbol_put(kvm_put_kvm);
> > +
> > +clear:
> > +	device->kvm = NULL;
> > +}
> > +#endif
> > +
> >  /* true if the vfio_device has open_device() called but not close_device()
> > */
> >  static bool vfio_assert_device_open(struct vfio_device *device)
> >  {
> > @@ -361,7 +413,6 @@ static int vfio_device_first_open(struct vfio_device
> > *device,
> >  	if (ret)
> >  		goto err_module_put;
> > 
> > -	device->kvm = kvm;
> >  	if (device->ops->open_device) {
> >  		ret = device->ops->open_device(device);
> >  		if (ret)
> > @@ -370,7 +421,6 @@ static int vfio_device_first_open(struct vfio_device
> > *device,
> >  	return 0;
> > 
> >  err_unuse_iommu:
> > -	device->kvm = NULL;
> >  	if (iommufd)
> >  		vfio_iommufd_unbind(device);
> >  	else
> > @@ -387,7 +437,6 @@ static void vfio_device_last_close(struct vfio_device
> > *device,
> > 
> >  	if (device->ops->close_device)
> >  		device->ops->close_device(device);
> > -	device->kvm = NULL;
> >  	if (iommufd)
> >  		vfio_iommufd_unbind(device);
> >  	else
> > @@ -400,14 +449,14 @@ int vfio_device_open(struct vfio_device *device,
> >  {
> >  	int ret = 0;
> > 
> > -	mutex_lock(&device->dev_set->lock);
> > +	lockdep_assert_held(&device->dev_set->lock);
> > +
> >  	device->open_count++;
> >  	if (device->open_count == 1) {
> >  		ret = vfio_device_first_open(device, iommufd, kvm);
> >  		if (ret)
> >  			device->open_count--;
> >  	}
> > -	mutex_unlock(&device->dev_set->lock);
> > 
> >  	return ret;
> >  }
> > @@ -415,12 +464,12 @@ int vfio_device_open(struct vfio_device *device,
> >  void vfio_device_close(struct vfio_device *device,
> >  		       struct iommufd_ctx *iommufd)
> >  {
> > -	mutex_lock(&device->dev_set->lock);
> > +	lockdep_assert_held(&device->dev_set->lock);
> > +
> >  	vfio_assert_device_open(device);
> >  	if (device->open_count == 1)
> >  		vfio_device_last_close(device, iommufd);
> >  	device->open_count--;
> > -	mutex_unlock(&device->dev_set->lock);
> >  }
> > 
> >  /*
> > diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> > index 35be78e9ae57..87ff862ff555 100644
> > --- a/include/linux/vfio.h
> > +++ b/include/linux/vfio.h
> > @@ -46,7 +46,6 @@ struct vfio_device {
> >  	struct vfio_device_set *dev_set;
> >  	struct list_head dev_set_list;
> >  	unsigned int migration_flags;
> > -	/* Driver must reference the kvm during open_device or never
> > touch it */
> >  	struct kvm *kvm;
> > 
> >  	/* Members below here are private, not for driver use */
> > @@ -58,6 +57,7 @@ struct vfio_device {
> >  	struct list_head group_next;
> >  	struct list_head iommu_entry;
> >  	struct iommufd_access *iommufd_access;
> > +	void (*put_kvm)(struct kvm *kvm);
> >  #if IS_ENABLED(CONFIG_IOMMUFD)
> >  	struct iommufd_device *iommufd_device;
> >  	struct iommufd_ctx *iommufd_ictx;
> > --
> > 2.39.1  
>
  

> From: Matthew Rosato <mjrosato@linux.ibm.com>
> Sent: Saturday, February 4, 2023 5:50 AM
> 
> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order.  Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> 
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
> 
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
  

Tested-by: Tony Krowiak <akrowiak@linux.ibm.com>

On 2/3/23 4:50 PM, Matthew Rosato wrote:
> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order.  Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
>
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
>
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>   drivers/vfio/group.c     | 44 +++++++++++++++++++++++-----
>   drivers/vfio/vfio.h      | 15 ++++++++++
>   drivers/vfio/vfio_main.c | 63 +++++++++++++++++++++++++++++++++++-----
>   include/linux/vfio.h     |  2 +-
>   4 files changed, 109 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..98621ac082f0 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -154,6 +154,18 @@ static int vfio_group_ioctl_set_container(struct vfio_group *group,
>   	return ret;
>   }
>   
> +static void vfio_device_group_get_kvm_safe(struct vfio_device *device)
> +{
> +	spin_lock(&device->group->kvm_ref_lock);
> +	if (!device->group->kvm)
> +		goto unlock;
> +
> +	_vfio_device_get_kvm_safe(device, device->group->kvm);
> +
> +unlock:
> +	spin_unlock(&device->group->kvm_ref_lock);
> +}
> +
>   static int vfio_device_group_open(struct vfio_device *device)
>   {
>   	int ret;
> @@ -164,13 +176,23 @@ static int vfio_device_group_open(struct vfio_device *device)
>   		goto out_unlock;
>   	}
>   
> +	mutex_lock(&device->dev_set->lock);
> +
>   	/*
> -	 * Here we pass the KVM pointer with the group under the lock.  If the
> -	 * device driver will use it, it must obtain a reference and release it
> -	 * during close_device.
> +	 * Before the first device open, get the KVM pointer currently
> +	 * associated with the group (if there is one) and obtain a reference
> +	 * now that will be held until the open_count reaches 0 again.  Save
> +	 * the pointer in the device for use by drivers.
>   	 */
> -	ret = vfio_device_open(device, device->group->iommufd,
> -			       device->group->kvm);
> +	if (device->open_count == 0)
> +		vfio_device_group_get_kvm_safe(device);
> +
> +	ret = vfio_device_open(device, device->group->iommufd, device->kvm);
> +
> +	if (device->open_count == 0)
> +		vfio_device_put_kvm(device);
> +
> +	mutex_unlock(&device->dev_set->lock);
>   
>   out_unlock:
>   	mutex_unlock(&device->group->group_lock);
> @@ -180,7 +202,14 @@ static int vfio_device_group_open(struct vfio_device *device)
>   void vfio_device_group_close(struct vfio_device *device)
>   {
>   	mutex_lock(&device->group->group_lock);
> +	mutex_lock(&device->dev_set->lock);
> +
>   	vfio_device_close(device, device->group->iommufd);
> +
> +	if (device->open_count == 0)
> +		vfio_device_put_kvm(device);
> +
> +	mutex_unlock(&device->dev_set->lock);
>   	mutex_unlock(&device->group->group_lock);
>   }
>   
> @@ -450,6 +479,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
>   
>   	refcount_set(&group->drivers, 1);
>   	mutex_init(&group->group_lock);
> +	spin_lock_init(&group->kvm_ref_lock);
>   	INIT_LIST_HEAD(&group->device_list);
>   	mutex_init(&group->device_lock);
>   	group->iommu_group = iommu_group;
> @@ -803,9 +833,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
>   	if (!vfio_file_is_group(file))
>   		return;
>   
> -	mutex_lock(&group->group_lock);
> +	spin_lock(&group->kvm_ref_lock);
>   	group->kvm = kvm;
> -	mutex_unlock(&group->group_lock);
> +	spin_unlock(&group->kvm_ref_lock);
>   }
>   EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
>   
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index f8219a438bfb..24d6cd285945 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -74,6 +74,7 @@ struct vfio_group {
>   	struct file			*opened_file;
>   	struct blocking_notifier_head	notifier;
>   	struct iommufd_ctx		*iommufd;
> +	spinlock_t			kvm_ref_lock;
>   };
>   
>   int vfio_device_set_group(struct vfio_device *device,
> @@ -251,4 +252,18 @@ extern bool vfio_noiommu __read_mostly;
>   enum { vfio_noiommu = false };
>   #endif
>   
> +#ifdef CONFIG_HAVE_KVM
> +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm);
> +void vfio_device_put_kvm(struct vfio_device *device);
> +#else
> +static inline void _vfio_device_get_kvm_safe(struct vfio_device *device,
> +					     struct kvm *kvm)
> +{
> +}
> +
> +static inline void vfio_device_put_kvm(struct vfio_device *device)
> +{
> +}
> +#endif
> +
>   #endif
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..28c47cd6a6b5 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,9 @@
>   #include <linux/fs.h>
>   #include <linux/idr.h>
>   #include <linux/iommu.h>
> +#ifdef CONFIG_HAVE_KVM
> +#include <linux/kvm_host.h>
> +#endif
>   #include <linux/list.h>
>   #include <linux/miscdevice.h>
>   #include <linux/module.h>
> @@ -338,6 +341,55 @@ void vfio_unregister_group_dev(struct vfio_device *device)
>   }
>   EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
>   
> +#ifdef CONFIG_HAVE_KVM
> +void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
> +{
> +	void (*pfn)(struct kvm *kvm);
> +	bool (*fn)(struct kvm *kvm);
> +	bool ret;
> +
> +	lockdep_assert_held(&device->dev_set->lock);
> +
> +	pfn = symbol_get(kvm_put_kvm);
> +	if (WARN_ON(!pfn))
> +		return;
> +
> +	fn = symbol_get(kvm_get_kvm_safe);
> +	if (WARN_ON(!fn)) {
> +		symbol_put(kvm_put_kvm);
> +		return;
> +	}
> +
> +	ret = fn(kvm);
> +	symbol_put(kvm_get_kvm_safe);
> +	if (!ret) {
> +		symbol_put(kvm_put_kvm);
> +		return;
> +	}
> +
> +	device->put_kvm = pfn;
> +	device->kvm = kvm;
> +}
> +
> +void vfio_device_put_kvm(struct vfio_device *device)
> +{
> +	lockdep_assert_held(&device->dev_set->lock);
> +
> +	if (!device->kvm)
> +		return;
> +
> +	if (WARN_ON(!device->put_kvm))
> +		goto clear;
> +
> +	device->put_kvm(device->kvm);
> +	device->put_kvm = NULL;
> +	symbol_put(kvm_put_kvm);
> +
> +clear:
> +	device->kvm = NULL;
> +}
> +#endif
> +
>   /* true if the vfio_device has open_device() called but not close_device() */
>   static bool vfio_assert_device_open(struct vfio_device *device)
>   {
> @@ -361,7 +413,6 @@ static int vfio_device_first_open(struct vfio_device *device,
>   	if (ret)
>   		goto err_module_put;
>   
> -	device->kvm = kvm;
>   	if (device->ops->open_device) {
>   		ret = device->ops->open_device(device);
>   		if (ret)
> @@ -370,7 +421,6 @@ static int vfio_device_first_open(struct vfio_device *device,
>   	return 0;
>   
>   err_unuse_iommu:
> -	device->kvm = NULL;
>   	if (iommufd)
>   		vfio_iommufd_unbind(device);
>   	else
> @@ -387,7 +437,6 @@ static void vfio_device_last_close(struct vfio_device *device,
>   
>   	if (device->ops->close_device)
>   		device->ops->close_device(device);
> -	device->kvm = NULL;
>   	if (iommufd)
>   		vfio_iommufd_unbind(device);
>   	else
> @@ -400,14 +449,14 @@ int vfio_device_open(struct vfio_device *device,
>   {
>   	int ret = 0;
>   
> -	mutex_lock(&device->dev_set->lock);
> +	lockdep_assert_held(&device->dev_set->lock);
> +
>   	device->open_count++;
>   	if (device->open_count == 1) {
>   		ret = vfio_device_first_open(device, iommufd, kvm);
>   		if (ret)
>   			device->open_count--;
>   	}
> -	mutex_unlock(&device->dev_set->lock);
>   
>   	return ret;
>   }
> @@ -415,12 +464,12 @@ int vfio_device_open(struct vfio_device *device,
>   void vfio_device_close(struct vfio_device *device,
>   		       struct iommufd_ctx *iommufd)
>   {
> -	mutex_lock(&device->dev_set->lock);
> +	lockdep_assert_held(&device->dev_set->lock);
> +
>   	vfio_assert_device_open(device);
>   	if (device->open_count == 1)
>   		vfio_device_last_close(device, iommufd);
>   	device->open_count--;
> -	mutex_unlock(&device->dev_set->lock);
>   }
>   
>   /*
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 35be78e9ae57..87ff862ff555 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -46,7 +46,6 @@ struct vfio_device {
>   	struct vfio_device_set *dev_set;
>   	struct list_head dev_set_list;
>   	unsigned int migration_flags;
> -	/* Driver must reference the kvm during open_device or never touch it */
>   	struct kvm *kvm;
>   
>   	/* Members below here are private, not for driver use */
> @@ -58,6 +57,7 @@ struct vfio_device {
>   	struct list_head group_next;
>   	struct list_head iommu_entry;
>   	struct iommufd_access *iommufd_access;
> +	void (*put_kvm)(struct kvm *kvm);
>   #if IS_ENABLED(CONFIG_IOMMUFD)
>   	struct iommufd_device *iommufd_device;
>   	struct iommufd_ctx *iommufd_ictx;