> 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