memory leak in usbtest_probe

[Kyungtae Kim]

We report a bug (in linux-5.6.11) found by FuzzUSB (a modified version
of syzkaller)

The buf of an usbtest_dev instance (dev->buf) allocated in
usbtest_probe() leaked.

The usbtest_dev instance holding the buf is attached to a
corresponding device instance
through usb_set_intfdata().
But later, the usbtest_dev instance seems to be freed without
deallocating its buf field ahead.

kernel config: https://kt0755.github.io/etc/config_v5.6.11

==================================================================
BUG: memory leak
unreferenced object 0xffff888055046e00 (size 256):
  comm "kworker/2:9", pid 2570, jiffies 4294942129 (age 1095.500s)
  hex dump (first 32 bytes):
    00 70 04 55 80 88 ff ff 18 bb 5a 81 ff ff ff ff  .p.U......Z.....
    f5 96 78 81 ff ff ff ff 37 de 8e 81 ff ff ff ff  ..x.....7.......
  backtrace:
    [<00000000d121dccf>] kmemleak_alloc_recursive
include/linux/kmemleak.h:43 [inline]
    [<00000000d121dccf>] slab_post_alloc_hook mm/slab.h:586 [inline]
    [<00000000d121dccf>] slab_alloc_node mm/slub.c:2786 [inline]
    [<00000000d121dccf>] slab_alloc mm/slub.c:2794 [inline]
    [<00000000d121dccf>] kmem_cache_alloc_trace+0x15e/0x2d0 mm/slub.c:2811
    [<000000005c3c3381>] kmalloc include/linux/slab.h:555 [inline]
    [<000000005c3c3381>] usbtest_probe+0x286/0x19d0
drivers/usb/misc/usbtest.c:2790
    [<000000001cec6910>] usb_probe_interface+0x2bd/0x870
drivers/usb/core/driver.c:361
    [<000000007806c118>] really_probe+0x48d/0x8f0 drivers/base/dd.c:551
    [<00000000a3308c3e>] driver_probe_device+0xfc/0x2a0 drivers/base/dd.c:724
    [<000000003ef66004>] __device_attach_driver+0x1b6/0x240
drivers/base/dd.c:831
    [<00000000eee53e97>] bus_for_each_drv+0x14e/0x1e0 drivers/base/bus.c:431
    [<00000000bb0648d0>] __device_attach+0x1f9/0x350 drivers/base/dd.c:897
    [<00000000838b324a>] device_initial_probe+0x1a/0x20 drivers/base/dd.c:944
    [<0000000030d501c1>] bus_probe_device+0x1e1/0x280 drivers/base/bus.c:491
    [<000000005bd7adef>] device_add+0x131d/0x1c40 drivers/base/core.c:2504
    [<00000000a0937814>] usb_set_configuration+0xe84/0x1ab0
drivers/usb/core/message.c:2030
    [<00000000e3934741>] generic_probe+0x6a/0xe0 drivers/usb/core/generic.c:210
    [<0000000098ade0f1>] usb_probe_device+0x90/0xd0
drivers/usb/core/driver.c:266
    [<000000007806c118>] really_probe+0x48d/0x8f0 drivers/base/dd.c:551
    [<00000000a3308c3e>] driver_probe_device+0xfc/0x2a0 drivers/base/dd.c:724
==================================================================