mic: vop: Fix use-after-free on remove

KASAN detects a use-after-free when vop devices are removed.

This problem was introduced by commit 0063e8bbd2b62d136 ("virtio_vop:
don't kfree device on register failure"). That patch moved the freeing
of the struct _vop_vdev to the release function, but failed to ensure
that vop holds a reference to the device when it doesn't want it to go
away. A kfree() was replaced with a put_device() in the unregistration
path, but the last reference to the device is already dropped in
unregister_virtio_device() so the struct is freed before vop is done
with it.

Fix it by holding a reference until cleanup is done. This is similar to
the fix in virtio_pci in commit 2989be09a8a9d6 ("virtio_pci: fix use
after free on release").

==================================================================
BUG: KASAN: use-after-free in vop_scan_devices+0xc6c/0xe50 [vop]
Read of size 8 at addr ffff88800da18580 by task kworker/0:1/12

CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 5.0.0-rc4+ #53
Workqueue: events vop_hotplug_devices [vop]
Call Trace:
dump_stack+0x74/0xbb
print_address_description+0x5d/0x2b0
? vop_scan_devices+0xc6c/0xe50 [vop]
kasan_report+0x152/0x1aa
? vop_scan_devices+0xc6c/0xe50 [vop]
? vop_scan_devices+0xc6c/0xe50 [vop]
vop_scan_devices+0xc6c/0xe50 [vop]
? vop_loopback_free_irq+0x160/0x160 [vop_loopback]
process_one_work+0x7c0/0x14b0
? pwq_dec_nr_in_flight+0x2d0/0x2d0
? do_raw_spin_lock+0x120/0x280
worker_thread+0x8f/0xbf0
? __kthread_parkme+0x78/0xf0
? process_one_work+0x14b0/0x14b0
kthread+0x2ae/0x3a0
? kthread_park+0x120/0x120
ret_from_fork+0x3a/0x50

Allocated by task 12:
kmem_cache_alloc_trace+0x13a/0x2a0
vop_scan_devices+0x473/0xe50 [vop]
process_one_work+0x7c0/0x14b0
worker_thread+0x8f/0xbf0
kthread+0x2ae/0x3a0
ret_from_fork+0x3a/0x50

Freed by task 12:
kfree+0x104/0x310
device_release+0x73/0x1d0
kobject_put+0x14f/0x420
unregister_virtio_device+0x32/0x50
vop_scan_devices+0x19d/0xe50 [vop]
process_one_work+0x7c0/0x14b0
worker_thread+0x8f/0xbf0
kthread+0x2ae/0x3a0
ret_from_fork+0x3a/0x50

The buggy address belongs to the object at ffff88800da18008
which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 1400 bytes inside of
2048-byte region [ffff88800da18008, ffff88800da18808)
The buggy address belongs to the page:
page:ffffea0000368600 count:1 mapcount:0 mapping:ffff88801440dbc0 index:0x0 compound_mapcount: 0
flags: 0x4000000000010200(slab|head)
raw: 4000000000010200 ffffea0000378608 ffffea000037a008 ffff88801440dbc0
raw: 0000000000000000 00000000000d000d 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff88800da18480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88800da18500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88800da18580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff88800da18600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88800da18680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Fixes: 0063e8bbd2b62d136 ("virtio_vop: don't kfree device on register failure")
Signed-off-by: Vincent Whitchurch <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/mic/vop/vop_main.c

···
 	int ret = -1;
 
 	if (ioread8(&dc->config_change) == MIC_VIRTIO_PARAM_DEV_REMOVE) {
+		struct device *dev = get_device(&vdev->vdev.dev);
+
 		dev_dbg(&vpdev->dev,
 			"%s %d config_change %d type %d vdev %p\n",
 			__func__, __LINE__,
···
 		iowrite8(-1, &dc->h2c_vdev_db);
 		if (status & VIRTIO_CONFIG_S_DRIVER_OK)
 			wait_for_completion(&vdev->reset_done);
-		put_device(&vdev->vdev.dev);
+		put_device(dev);
 		iowrite8(1, &dc->guest_ack);
 		dev_dbg(&vpdev->dev, "%s %d guest_ack %d\n",
 			__func__, __LINE__, ioread8(&dc->guest_ack));