This is a note to let you know that I've just added the patch titled vfio: Fix group release deadlock to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: vfio-fix-group-release-deadlock.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 811642d8d8a82c0cce8dc2debfdaf23c5a144839 Mon Sep 17 00:00:00 2001 From: Alex Williamson <alex.williamson@xxxxxxxxxx> Date: Mon, 19 Jun 2017 09:10:32 -0600 Subject: vfio: Fix group release deadlock From: Alex Williamson <alex.williamson@xxxxxxxxxx> commit 811642d8d8a82c0cce8dc2debfdaf23c5a144839 upstream. If vfio_iommu_group_notifier() acquires a group reference and that reference becomes the last reference to the group, then vfio_group_put introduces a deadlock code path where we're trying to unregister from the iommu notifier chain from within a callout of that chain. Use a work_struct to release this reference asynchronously. Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx> Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx> Tested-by: Eric Auger <eric.auger@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/vfio/vfio.c | 37 ++++++++++++++++++++++++++++++++++++- 1 file changed, 36 insertions(+), 1 deletion(-) --- a/drivers/vfio/vfio.c +++ b/drivers/vfio/vfio.c @@ -296,6 +296,34 @@ static void vfio_group_put(struct vfio_g kref_put_mutex(&group->kref, vfio_group_release, &vfio.group_lock); } +struct vfio_group_put_work { + struct work_struct work; + struct vfio_group *group; +}; + +static void vfio_group_put_bg(struct work_struct *work) +{ + struct vfio_group_put_work *do_work; + + do_work = container_of(work, struct vfio_group_put_work, work); + + vfio_group_put(do_work->group); + kfree(do_work); +} + +static void vfio_group_schedule_put(struct vfio_group *group) +{ + struct vfio_group_put_work *do_work; + + do_work = kmalloc(sizeof(*do_work), GFP_KERNEL); + if (WARN_ON(!do_work)) + return; + + INIT_WORK(&do_work->work, vfio_group_put_bg); + do_work->group = group; + schedule_work(&do_work->work); +} + /* Assume group_lock or group reference is held */ static void vfio_group_get(struct vfio_group *group) { @@ -620,7 +648,14 @@ static int vfio_iommu_group_notifier(str break; } - vfio_group_put(group); + /* + * If we're the last reference to the group, the group will be + * released, which includes unregistering the iommu group notifier. + * We hold a read-lock on that notifier list, unregistering needs + * a write-lock... deadlock. Release our reference asynchronously + * to avoid that situation. + */ + vfio_group_schedule_put(group); return NOTIFY_OK; } Patches currently in stable-queue which might be from alex.williamson@xxxxxxxxxx are queue-4.4/vfio-new-external-user-group-file-match.patch queue-4.4/vfio-fix-group-release-deadlock.patch