RE: [PATCH] vfio: fix deadlock between group lock and kvm lock

[Liu, Yi L]

> From: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Sent: Wednesday, February 1, 2023 4:07 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@xxxxxxxxxxxxx>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> ---
>  drivers/vfio/group.c     | 14 +++----
>  drivers/vfio/vfio.h      |  5 ++-
>  drivers/vfio/vfio_main.c | 88
> ++++++++++++++++++++++++++++++++++++----
>  include/linux/vfio.h     |  2 +-
>  4 files changed, 88 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..8f67d33e0e8d 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -164,13 +164,7 @@ static int vfio_device_group_open(struct
> vfio_device *device)
>  		goto out_unlock;
>  	}
> 
> -	/*
> -	 * 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.
> -	 */
> -	ret = vfio_device_open(device, device->group->iommufd,
> -			       device->group->kvm);
> +	ret = vfio_device_open(device, device->group);
> 
>  out_unlock:
>  	mutex_unlock(&device->group->group_lock);
> @@ -450,6 +444,7 @@ 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;
> @@ -799,13 +794,14 @@
> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
>  void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
>  {
>  	struct vfio_group *group = file->private_data;
> +	unsigned long flags;
> 
>  	if (!vfio_file_is_group(file))
>  		return;
> 
> -	mutex_lock(&group->group_lock);
> +	spin_lock_irqsave(&group->kvm_ref_lock, flags);
>  	group->kvm = kvm;
> -	mutex_unlock(&group->group_lock);
> +	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
>  }
>  EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
> 
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index f8219a438bfb..d153ad35de24 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -15,11 +15,11 @@ struct iommufd_ctx;
>  struct iommu_group;
>  struct vfio_device;
>  struct vfio_container;
> +struct vfio_group;
> 
>  void vfio_device_put_registration(struct vfio_device *device);
>  bool vfio_device_try_get_registration(struct vfio_device *device);
> -int vfio_device_open(struct vfio_device *device,
> -		     struct iommufd_ctx *iommufd, struct kvm *kvm);
> +int vfio_device_open(struct vfio_device *device, struct vfio_group
> *group);
>  void vfio_device_close(struct vfio_device *device,
>  		       struct iommufd_ctx *iommufd);
> 
> @@ -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,
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..b0defb0d0d87 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>
> @@ -344,9 +347,59 @@ static bool vfio_assert_device_open(struct
> vfio_device *device)
>  	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
>  }
> 
> +#ifdef CONFIG_HAVE_KVM
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm)
> +{
> +	void (*pfn)(struct kvm *kvm);
> +	bool (*fn)(struct kvm *kvm);
> +	bool ret;
> +
> +	pfn = symbol_get(kvm_put_kvm);
> +	if (WARN_ON(!pfn))
> +		return false;
> +
> +	fn = symbol_get(kvm_get_kvm_safe);
> +	if (WARN_ON(!fn)) {
> +		symbol_put(kvm_put_kvm);
> +		return false;
> +	}
> +
> +	ret = fn(kvm);
> +	if (ret)
> +		device->put_kvm = pfn;
> +	else
> +		symbol_put(kvm_put_kvm);
> +
> +	symbol_put(kvm_get_kvm_safe);
> +
> +	return ret;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> +	if (WARN_ON(!device->kvm || !device->put_kvm))
> +		return;
> +
> +	device->put_kvm(device->kvm);
> +	symbol_put(kvm_put_kvm);
> +}
> +
> +#else
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm)
> +{
> +	return false;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> +}
> +#endif
> +
>  static int vfio_device_first_open(struct vfio_device *device,
> -				  struct iommufd_ctx *iommufd, struct kvm
> *kvm)
> +				  struct vfio_group *group)
>  {
> +	struct iommufd_ctx *iommufd = group->iommufd;
> +	unsigned long flags;
>  	int ret;
> 
>  	lockdep_assert_held(&device->dev_set->lock);
> @@ -361,16 +414,30 @@ static int vfio_device_first_open(struct
> vfio_device *device,
>  	if (ret)
>  		goto err_module_put;
> 
> -	device->kvm = kvm;
> +	/*
> +	 * 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.  Save the pointer in the device for use by
> +	 * drivers.
> +	 */
> +	spin_lock_irqsave(&group->kvm_ref_lock, flags);
> +	if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
> +		device->kvm = group->kvm;
> +	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> +
>  	if (device->ops->open_device) {
>  		ret = device->ops->open_device(device);
>  		if (ret)
> -			goto err_unuse_iommu;
> +			goto err_put_kvm;
>  	}
>  	return 0;
> 
> -err_unuse_iommu:
> -	device->kvm = NULL;
> +err_put_kvm:
> +	if (device->kvm) {
> +		vfio_kvm_put_kvm(device);
> +		device->put_kvm = NULL;

Can "device->put_kvm = NULL;" be part of vfio_kvm_put_kvm()?

Regards,
Yi Liu