drm: Protect drm_connector_register_all() under DRIVER_MODESET

0-day kbuilder found

[    1.360244] BUG: unable to handle kernel NULL pointer dereference at   (null)
[    1.360972] IP: [<c14db9ad>] mutex_lock_nested+0x11f/0x2c3
[    1.361512] *pde = 00000000
[    1.361827] Oops: 0002 [#1]
[    1.362123] Modules linked in:
[    1.362451] CPU: 0 PID: 1 Comm: swapper Not tainted 4.7.0-rc2-00564-ge28cd4d0 #1
[    1.363202] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
[    1.364105] task: c03d0000 ti: d28da000 task.ti: d28da000
[    1.364636] EIP: 0060:[<c14db9ad>] EFLAGS: 00210096 CPU: 0
[    1.365215] EIP is at mutex_lock_nested+0x11f/0x2c3
[    1.365703] EAX: 00000000 EBX: d39e8ae8 ECX: d39e8b14 EDX: c1361cf9
[    1.366351] ESI: c03d0000 EDI: d28dbed0 EBP: d28dbeec ESP: d28dbec0
[    1.367010]  DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
[    1.367534] CR0: 80050033 CR2: 00000000 CR3: 019a9000 CR4: 00000690
[    1.368152] Stack:
[    1.368356]  d39e8b14 d39e8b24 c1361cf9 00200246 d39e8b14 00000000 11111111 d28dbed0
[    1.369235]  d39e8800 d39e8ae8 00000000 d28dbf08 c1361cf9 d28dbf0c c10b25be d39e8800
[    1.370087]  00000000 00000000 d28dbf1c c135e37d fffffff4 ffffffff 00000000 d28dbf28
[    1.371012] Call Trace:
[    1.371272]  [<c1361cf9>] ? drm_connector_register_all+0x1a/0x92
[    1.371847]  [<c1361cf9>] drm_connector_register_all+0x1a/0x92
[    1.372421]  [<c10b25be>] ? kstrdup+0x25/0x3a
[    1.372863]  [<c135e37d>] drm_dev_register+0x59/0x99
[    1.373358]  [<c195ea3e>] vgem_init+0x34/0x49
[    1.373770]  [<c195ea0a>] ? mipi_dsi_bus_init+0xf/0xf
[    1.374257]  [<c100048f>] do_one_initcall+0x7c/0xfd
[    1.374754]  [<c104b409>] ? parse_args+0x1fd/0x314
[    1.375259]  [<c1939c10>] ? kernel_init_freeable+0xd0/0x179
[    1.375837]  [<c1939c2c>] kernel_init_freeable+0xec/0x179
[    1.376371]  [<c14d66ea>] kernel_init+0x8/0xcb
[    1.376806]  [<c14debce>] ret_from_kernel_thread+0xe/0x30
[    1.377322]  [<c14d66e2>] ? rest_init+0x10e/0x10e
[    1.377754] Code: 89 fa e8 71 c5 b7 ff 8b 4e 04 89 fa 89 d8 e8 8e c6 b7 ff 8d 43 2c 89 45 d4 8b 43 30 8d 4b 2c 89 45 e8 89 7b 30 89 4d e4 8b 55 dc <89> 38 8d 43 3c 89 75 ec e8 c9 dd b7 ff eb 0c 31 c0 87 03 48
+75
[    1.380442] EIP: [<c14db9ad>] mutex_lock_nested+0x11f/0x2c3 SS:ESP 0068:d28dbec0
[    1.381174] CR2: 0000000000000000

when loading the non-modesetting vGEM module. To prevent use of the
uninitialised dev->mode_config from drm_dev_register() we move the
drm_connector_register_all() under a DRIVER_MODESET guard. Longer term,
we probably want to initialise the embedded dev->mode_config automatically
from drm_dev_init() for all DRIVER_MODESET drivers.

v2: Also protect drm_dev_unregister.

Fixes: e28cd4d0 ("drm: Automatically register/unregister all connectors")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Testcase: igt/vgem_reload_basic
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466257601-5656-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_drv.c

@@ -687,6 +687,7 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 			goto err_minors;
 	}
 
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
 		drm_connector_register_all(dev);
 
 	ret = 0;
@@ -719,6 +720,7 @@ void drm_dev_unregister(struct drm_device *dev)
 
 	drm_lastclose(dev);
 
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
 		drm_connector_unregister_all(dev);
 
 	if (dev->driver->unload)