Re: [PATCH v4 1/2] vfio: fix deadlock between group lock and kvm lock

Alex Williamson
2023-02-04T14:26:13Z

On Sat, 4 Feb 2023 06:21:48 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> > From: Matthew Rosato <mjrosato@linux.ibm.com>
> > Sent: Saturday, February 4, 2023 5:50 AM
> > To: alex.williamson@redhat.com; pbonzini@redhat.com; Liu, Yi L
> >
> > 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@linux.ibm.com>
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > ---
> > drivers/vfio/group.c | 44 +++++++++++++++++++++++-----
> > drivers/vfio/vfio.h | 15 ++++++++++
> > drivers/vfio/vfio_main.c | 63
> > +++++++++++++++++++++++++++++++++++-----
> > include/linux/vfio.h | 2 +-
> > 4 files changed, 109 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index bb24b2f0271e..98621ac082f0 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -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
>
> Nit: a redundant space before "Save". Other part looks good to me.

Two spaces between sentences is a common standard. Not everyone
prefers this, but I do. Thanks,

Alex

> Reviewed-by: Yi Liu <yi.l.liu@intel.com>
>
> Regards,
> Yi Liu
>
> > + * 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);
> >
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index f8219a438bfb..24d6cd285945 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -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,
> > @@ -251,4 +252,18 @@ 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
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index 5177bb061b17..28c47cd6a6b5 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>
> > @@ -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);
> > }
> >
> > /*
> > 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
>