Commit 2b48f52f authored by Matthew Rosato's avatar Matthew Rosato Committed by Alex Williamson
Browse files

vfio: fix deadlock between group lock and kvm lock 



After 51cdc8bc, 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: 51cdc8bc ("kvm/vfio: Fix potential deadlock on vfio group_lock")
Reported-by: default avatarAnthony Krowiak <akrowiak@linux.ibm.com>
Suggested-by: default avatarJason Gunthorpe <jgg@nvidia.com>
Signed-off-by: default avatarMatthew Rosato <mjrosato@linux.ibm.com>
Tested-by: default avatarTony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: default avatarKevin Tian <kevin.tian@intel.com>
Reviewed-by: default avatarYi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230203215027.151988-2-mjrosato@linux.ibm.com


Signed-off-by: default avatarAlex Williamson <alex.williamson@redhat.com>
parent e592296c

drivers/vfio/group.c

+37 −7
Original line number Diff line number Diff line
@@ -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);

drivers/vfio/vfio.h

+15 −0
Original line number Diff line number Diff line
@@ -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,
@@ -244,4 +245,18 @@ 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

drivers/vfio/vfio_main.c

+56 −7
Original line number Diff line number Diff line
@@ -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);

}



/*

include/linux/vfio.h

+1 −1
Original line number Diff line number Diff line
@@ -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;