Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock

[Alex Williamson]

On Thu, 12 Jan 2023 15:38:44 -0500
Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:

> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation.  This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
> 
> kvm_put_kvm
>  -> kvm_destroy_vm
>   -> kvm_destroy_devices
>    -> kvm_vfio_destroy
>     -> kvm_vfio_file_set_kvm
>      -> vfio_file_set_kvm
>       -> group->group_lock/group_rwsem  
> 
> Avoid this scenario by having vfio core code acquire a KVM reference
> the first time a device is opened and hold that reference until the
> device fd is closed, at a point after the group lock has been released.
> 
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> ---
> Changes from v1:
> * Re-write using symbol get logic to get kvm ref during first device
>   open, release the ref during device fd close after group lock is
>   released
> * Drop kvm get/put changes to drivers; now that vfio core holds a
>   kvm ref until sometime after the device_close op is called, it
>   should be fine for drivers to get and put their own references to it.
> ---
>  drivers/vfio/group.c     |  6 ++---
>  drivers/vfio/vfio_main.c | 48 +++++++++++++++++++++++++++++++++++++---
>  include/linux/vfio.h     |  1 -
>  3 files changed, 48 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..2b0da82f82f4 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
>  	}
>  
>  	/*
> -	 * 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.
> +	 * Here we pass the KVM pointer with the group under the lock.  A
> +	 * reference will be obtained the first time the device is opened and
> +	 * will be held until the device fd is closed.
>  	 */
>  	ret = vfio_device_open(device, device->group->iommufd,
>  			       device->group->kvm);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..c969e2a0ecd3 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,7 @@
>  #include <linux/fs.h>
>  #include <linux/idr.h>
>  #include <linux/iommu.h>
> +#include <linux/kvm_host.h>
>  #include <linux/list.h>
>  #include <linux/miscdevice.h>
>  #include <linux/module.h>
> @@ -344,6 +345,35 @@ static bool vfio_assert_device_open(struct vfio_device *device)
>  	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
>  }
>  
> +static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
> +{
> +	bool (*fn)(struct kvm *kvm);
> +	bool ret;
> +
> +	fn = symbol_get(kvm_get_kvm_safe);
> +	if (WARN_ON(!fn))
> +		return false;
> +
> +	ret = fn(kvm);
> +
> +	symbol_put(kvm_get_kvm_safe);
> +
> +	return ret;
> +}
> +
> +static void vfio_kvm_put_kvm(struct kvm *kvm)
> +{
> +	void (*fn)(struct kvm *kvm);
> +
> +	fn = symbol_get(kvm_put_kvm);
> +	if (WARN_ON(!fn))
> +		return;
> +
> +	fn(kvm);
> +
> +	symbol_put(kvm_put_kvm);
> +}
> +
>  static int vfio_device_first_open(struct vfio_device *device,
>  				  struct iommufd_ctx *iommufd, struct kvm *kvm)
>  {
> @@ -361,16 +391,24 @@ static int vfio_device_first_open(struct vfio_device *device,
>  	if (ret)
>  		goto err_module_put;
>  
> +	if (kvm && !vfio_kvm_get_kvm_safe(kvm)) {
> +		ret = -ENOENT;
> +		goto err_unuse_iommu;
> +	}
>  	device->kvm = kvm;

This could just as easily be:

	if (kvm && vfio_kvm_get_kvm_safe(kvm))
		device->kvm = kvm;

Right?  The error path would test device->kvm and we already use
device->kvm in the release path.

Otherwise, in the off chance userspace hits this error, what's the
value in generating a failure here for a device that may or may not
have a kvm dependency.  A driver with a dependency should fail if
device->kvm is NULL.

>  	if (device->ops->open_device) {
>  		ret = device->ops->open_device(device);
>  		if (ret)
> -			goto err_unuse_iommu;
> +			goto err_put_kvm;
>  	}
>  	return 0;
>  
> +err_put_kvm:
> +	if (kvm) {
> +		vfio_kvm_put_kvm(kvm);
> +		device->kvm = NULL;
> +	}
>  err_unuse_iommu:
> -	device->kvm = NULL;
>  	if (iommufd)
>  		vfio_iommufd_unbind(device);
>  	else
> @@ -387,7 +425,6 @@ 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
> @@ -465,6 +502,11 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
>  
>  	vfio_device_group_close(device);
>  
> +	if (device->open_count == 0 && device->kvm) {
> +		vfio_kvm_put_kvm(device->kvm);
> +		device->kvm = NULL;
> +	}

IIUC, device->open_count is protected by device->dev_set->lock.  Thanks,

Alex

> +
>  	vfio_device_put_registration(device);
>  
>  	return 0;
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 35be78e9ae57..3ff7e9302cc1 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 */