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; + device->kvm = NULL; + } if (iommufd) vfio_iommufd_unbind(device); else @@ -387,7 +454,11 @@ static void vfio_device_last_close(struct vfio_device *device, if (device->ops->close_device) device->ops->close_device(device); - device->kvm = NULL; + if (device->kvm) { + vfio_kvm_put_kvm(device); + device->kvm = NULL; + device->put_kvm = NULL; + } if (iommufd) vfio_iommufd_unbind(device); else @@ -395,15 +466,14 @@ static void vfio_device_last_close(struct vfio_device *device, module_put(device->dev->driver->owner); } -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) { int ret = 0; mutex_lock(&device->dev_set->lock); device->open_count++; if (device->open_count == 1) { - ret = vfio_device_first_open(device, iommufd, kvm); + ret = vfio_device_first_open(device, group); if (ret) device->open_count--; } 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