Hi Matthew,
> From: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Sent: Friday, February 3, 2023 12:25 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>
> ---
> Changes from v2:
> * Relocate the new functions back to vfio_main and externalize to call
> from group (Kevin) since cdev will need this too
> * s/vfio_kvm_*_kvm/vfio_device_*_kvm/ and only pass device as input.
> Handle new kvm_ref_lock directly inside vfio_device_get_kvm (Alex)
> * Add assert_lockdep_held for dev_set lock (Alex)
> * Internalize error paths for vfio_device_get_kvm_safe and now return
> void - either device->kvm is set with a ref taken or is NULL (Alex)
> * Other flow suggestions to make the call path cleaner - Thanks! (Alex)
> * Can't pass group->kvm to vfio_device_open, as it references the value
> outside of new lock. Pass device->kvm to minimize changes in this
> fix (Alex, Yi)
> Changes from v1:
> * use spin_lock instead of spin_lock_irqsave (Jason)
> * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
> * Re-arrange code to avoid referencing the group contents from within
> vfio_main (Kevin) which meant moving most of the code in this patch
> to group.c along with getting/dropping of the dev_set lock
> ---
> drivers/vfio/group.c | 32 ++++++++++++++----
> drivers/vfio/vfio.h | 14 ++++++++
> drivers/vfio/vfio_main.c | 70
> ++++++++++++++++++++++++++++++++++++----
> include/linux/vfio.h | 2 +-
> 4 files changed, 103 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..7fed4233ca23 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -164,13 +164,23 @@ 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_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 +190,14 @@ 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 +467,7 @@ 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 +821,9 @@ 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..20d715b0a3a8 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 +252,17 @@ 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);
> +void vfio_device_put_kvm(struct vfio_device *device);
> +#else
> +static inline void vfio_device_get_kvm_safe(struct vfio_device *device)
> +{
> +}
> +
> +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..4762550e9f42 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 +341,62 @@ 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)
> +{
> + void (*pfn)(struct kvm *kvm);
> + bool (*fn)(struct kvm *kvm);
> + bool ret;
> +
> + lockdep_assert_held(&device->dev_set->lock);
> +
> + spin_lock(&device->group->kvm_ref_lock);
> + if (!device->group->kvm)
> + goto unlock;
> +
> + pfn = symbol_get(kvm_put_kvm);
> + if (WARN_ON(!pfn))
> + goto unlock;
> +
> + fn = symbol_get(kvm_get_kvm_safe);
> + if (WARN_ON(!fn)) {
> + symbol_put(kvm_put_kvm);
> + goto unlock;
> + }
> +
> + ret = fn(device->group->kvm);
> + symbol_put(kvm_get_kvm_safe);
> + if (!ret) {
> + symbol_put(kvm_put_kvm);
> + goto unlock;
> + }
> +
> + device->put_kvm = pfn;
> + device->kvm = device->group->kvm;
> +
> +unlock:
> + spin_unlock(&device->group->kvm_ref_lock);
> +}
> +
> +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 +420,6 @@ static int vfio_device_first_open(struct vfio_device
> *device,
> if (ret)
> goto err_module_put;
>
> - device->kvm = kvm;
Since you've deleted the only usage of kvm pointer in this function, I
guess you can remove the kvm parameter from vfio_device_open()
and vfio_device_first_open(). :-) if it makes this patch too big, may
just have another patch to do it.
Regards,
Yi Liu
> if (device->ops->open_device) {
> ret = device->ops->open_device(device);
> if (ret)
> @@ -370,7 +428,6 @@ 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 +444,6 @@ 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 +456,14 @@ 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 +471,12 @@ 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
