[BUG] drm: vc4: refcount_t: increment on 0; use-after-free.

[Stefan Wahren]

Hi Boris,
if i boot Raspberry Pi 3 (ARM64, defconfig, linux-next-20171122) with sufficient CMA memory (32 MB), i'll get this warning during boot:

[    7.623510] vc4-drm soc:gpu: bound 3f902000.hdmi (ops vc4_hdmi_ops [vc4])
[    7.632453] vc4-drm soc:gpu: bound 3f806000.vec (ops vc4_vec_ops [vc4])
[    7.639707] vc4-drm soc:gpu: bound 3f400000.hvs (ops vc4_hvs_ops [vc4])
[    7.647364] vc4-drm soc:gpu: bound 3f206000.pixelvalve (ops vc4_crtc_ops [vc4])
[    7.655451] vc4-drm soc:gpu: bound 3f207000.pixelvalve (ops vc4_crtc_ops [vc4])
[    7.663415] vc4-drm soc:gpu: bound 3f807000.pixelvalve (ops vc4_crtc_ops [vc4])
[    7.730580] vc4-drm soc:gpu: bound 3fc00000.v3d (ops vc4_v3d_ops [vc4])
[    7.743965] [drm] Initialized vc4 0.0.0 20140616 for soc:gpu on minor 0
[    7.750841] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    7.757620] [drm] Driver supports precise vblank timestamp query.
[    7.811775] ------------[ cut here ]------------
[    7.811780] refcount_t: increment on 0; use-after-free.
[    7.811881] WARNING: CPU: 2 PID: 2188 at lib/refcount.c:153 refcount_inc+0x44/0x50
[    7.811884] Modules linked in: vc4(+) cfg80211 cec drm_kms_helper smsc95xx usbnet drm rfkill brcmutil bcm2835_rng rng_core bcm2835_dma crc32_ce i2c_bcm2835 pwm_bcm2835 ip_tables x_tables ipv6
[    7.811950] CPU: 2 PID: 2188 Comm: systemd-udevd Not tainted 4.14.0-next-20171122 #1
[    7.811953] Hardware name: Raspberry Pi 3 Model B Rev 1.2 (DT)
[    7.811958] task: ffff800036b91c00 task.stack: ffff00000d6f0000
[    7.811967] pstate: 20000005 (nzCv daif -PAN -UAO)
[    7.811974] pc : refcount_inc+0x44/0x50
[    7.811981] lr : refcount_inc+0x44/0x50
[    7.811984] sp : ffff00000d6f3300
[    7.811988] x29: ffff00000d6f3300 x28: 0000000000000000 
[    7.811996] x27: 0000000000000003 x26: ffff800037107800 
[    7.812004] x25: 0000000000000001 x24: ffff800035afdc00 
[    7.812012] x23: 0000000000000000 x22: ffff800035dfa600 
[    7.812020] x21: ffff800035afd9b0 x20: ffff800035afd9a4 
[    7.812027] x19: 0000000000000000 x18: 0000000000000000 
[    7.812034] x17: 0000000000000001 x16: 0000000000000019 
[    7.812042] x15: 0000000000000001 x14: 00000000fffffff0 
[    7.812049] x13: ffff0000090ae840 x12: ffff000008fa2d50 
[    7.812057] x11: ffff000008fa2000 x10: ffff0000090ad000 
[    7.812064] x9 : 0000000000000000 x8 : ffff0000090b5c2f 
[    7.812072] x7 : 0000000000000000 x6 : 000000000015ee00 
[    7.812079] x5 : 0000000000000000 x4 : 0000000000000000 
[    7.812087] x3 : ffffffffffffffff x2 : 0000800030047000 
[    7.812094] x1 : ffff800036b91c00 x0 : 000000000000002b 
[    7.812102] Call trace:
[    7.812109]  refcount_inc+0x44/0x50
[    7.812226]  vc4_bo_inc_usecnt+0x84/0x88 [vc4]
[    7.812310]  vc4_prepare_fb+0x40/0xf0 [vc4]
[    7.812460]  drm_atomic_helper_prepare_planes+0x54/0xf0 [drm_kms_helper]
[    7.812543]  vc4_atomic_commit+0x88/0x130 [vc4]
[    7.812868]  drm_atomic_commit+0x48/0x68 [drm]
[    7.813002]  restore_fbdev_mode_atomic+0x1d8/0x1e8 [drm_kms_helper]
[    7.813113]  restore_fbdev_mode+0x28/0x160 [drm_kms_helper]
[    7.813223]  drm_fb_helper_restore_fbdev_mode_unlocked.part.24+0x28/0x90 [drm_kms_helper]
[    7.813331]  drm_fb_helper_set_par+0x54/0xa8 [drm_kms_helper]
[    7.813346]  fbcon_init+0x4e8/0x538
[    7.813357]  visual_init+0xb4/0x108
[    7.813366]  do_bind_con_driver+0x1b8/0x3a0
[    7.813373]  do_take_over_console+0x150/0x1d0
[    7.813380]  do_fbcon_takeover+0x6c/0xf0
[    7.813387]  fbcon_event_notify+0x8fc/0x928
[    7.813399]  notifier_call_chain+0x50/0x90
[    7.813406]  __blocking_notifier_call_chain+0x4c/0x90
[    7.813413]  blocking_notifier_call_chain+0x14/0x20
[    7.813420]  fb_notifier_call_chain+0x1c/0x28
[    7.813426]  register_framebuffer+0x1d0/0x2d8
[    7.813533]  __drm_fb_helper_initial_config_and_unlock+0x1e8/0x350 [drm_kms_helper]
[    7.813639]  drm_fb_helper_initial_config+0x40/0x58 [drm_kms_helper]
[    7.813747]  drm_fbdev_cma_init_with_funcs+0x88/0x158 [drm_kms_helper]
[    7.813855]  drm_fbdev_cma_init+0x14/0x28 [drm_kms_helper]
[    7.813943]  vc4_kms_load+0xa4/0xf0 [vc4]
[    7.814026]  vc4_drm_bind+0x100/0x168 [vc4]
[    7.814038]  try_to_bring_up_master+0x144/0x1a8
[    7.814044]  component_master_add_with_match+0x9c/0xe0
[    7.814128]  vc4_platform_drm_probe+0xb4/0xf0 [vc4]
[    7.814137]  platform_drv_probe+0x58/0xc0
[    7.814146]  driver_probe_device+0x224/0x308
[    7.814153]  __driver_attach+0xac/0xb0
[    7.814161]  bus_for_each_dev+0x64/0xa0
[    7.814169]  driver_attach+0x20/0x28
[    7.814177]  bus_add_driver+0x108/0x228
[    7.814184]  driver_register+0x60/0xf8
[    7.814190]  __platform_driver_register+0x40/0x48
[    7.814274]  vc4_drm_register+0x38/0x1000 [vc4]
[    7.814283]  do_one_initcall+0x38/0x120
[    7.814295]  do_init_module+0x58/0x1b8
[    7.814304]  load_module+0x1fa8/0x2280
[    7.814311]  SyS_finit_module+0xc0/0xd0
[    7.814318]  __sys_trace_return+0x0/0x4
[    7.814325] ---[ end trace 3348554eb91e19a1 ]---