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syzkaller reported use-after-free in load_firmware_cb() [1].
The reason is because the module allocated a struct tuner in tuner_probe(),
and then the module initialization failed, the struct tuner was released.
A worker which created during module initialization accesses this struct
tuner later, it caused use-after-free.

The process is as follows:

task-6504           worker_thread
tuner_probe                             <= alloc dvb_frontend [2]
...
request_firmware_nowait                 <= create a worker
...
tuner_remove                            <= free dvb_frontend
...
                    request_firmware_work_func  <= the firmware is ready
                    load_firmware_cb    <= but now the dvb_frontend has been freed

To fix the issue, check the dvd_frontend in load_firmware_cb(), if it is
null, report a warning and just return.

[1]:
    ==================================================================
     BUG: KASAN: use-after-free in load_firmware_cb+0x1310/0x17a0
     Read of size 8 at addr ffff8000d7ca2308 by task kworker/2:3/6504

     Call trace:
      load_firmware_cb+0x1310/0x17a0
      request_firmware_work_func+0x128/0x220
      process_one_work+0x770/0x1824
      worker_thread+0x488/0xea0
      kthread+0x300/0x430
      ret_from_fork+0x10/0x20

     Allocated by task 6504:
      kzalloc
      tuner_probe+0xb0/0x1430
      i2c_device_probe+0x92c/0xaf0
      really_probe+0x678/0xcd0
      driver_probe_device+0x280/0x370
      __device_attach_driver+0x220/0x330
      bus_for_each_drv+0x134/0x1c0
      __device_attach+0x1f4/0x410
      device_initial_probe+0x20/0x30
      bus_probe_device+0x184/0x200
      device_add+0x924/0x12c0
      device_register+0x24/0x30
      i2c_new_device+0x4e0/0xc44
      v4l2_i2c_new_subdev_board+0xbc/0x290
      v4l2_i2c_new_subdev+0xc8/0x104
      em28xx_v4l2_init+0x1dd0/0x3770

     Freed by task 6504:
      kfree+0x238/0x4e4
      tuner_remove+0x144/0x1c0
      i2c_device_remove+0xc8/0x290
      __device_release_driver+0x314/0x5fc
      device_release_driver+0x30/0x44
      bus_remove_device+0x244/0x490
      device_del+0x350/0x900
      device_unregister+0x28/0xd0
      i2c_unregister_device+0x174/0x1d0
      v4l2_device_unregister+0x224/0x380
      em28xx_v4l2_init+0x1d90/0x3770

     The buggy address belongs to the object at ffff8000d7ca2000
      which belongs to the cache kmalloc-2k of size 2048
     The buggy address is located 776 bytes inside of
      2048-byte region [ffff8000d7ca2000, ffff8000d7ca2800)
     The buggy address belongs to the page:
     page:ffff7fe00035f280 count:1 mapcount:0 mapping:ffff8000c001f000 index:0x0
     flags: 0x7ff800000000100(slab)
     raw: 07ff800000000100 ffff7fe00049d880 0000000300000003 ffff8000c001f000
     raw: 0000000000000000 0000000080100010 00000001ffffffff 0000000000000000
     page dumped because: kasan: bad access detected

     Memory state around the buggy address:
      ffff8000d7ca2200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
      ffff8000d7ca2280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
     >ffff8000d7ca2300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                           ^
      ffff8000d7ca2380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
      ffff8000d7ca2400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
     ==================================================================

[2]
    Actually, it is allocated for struct tuner, and dvb_frontend is inside.
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>