drm: bochs: Prevent double-free of fb helper

If fbdev registration fails for whatever reason, the error path of
bochs_fbdev_init() will call bochs_fbdev_fini(), but since an fbdev
initialization error is not fatal to the probe function, a subsequent
device removal will try to call bochs_fbdev_fini() again, hitting the
Oops below.

This was detected by 0-day with a failing framebuffer registration and
CONFIG_DEBUG_TEST_DRIVER_REMOVE=y.  This reproduces the scenario I
mentioned above at insmod time, because the test attempts to remove the
device right after probing.

root@debian:~# insmod bochs-drm.ko
[   17.609635] [drm] Found bochs VGA, ID 0xb0c0.
[   17.612974] [drm] Framebuffer size 16384 kB @ 0xfa000000, mmio @ 0xfebf2000.
[   17.613938] [TTM] Zone  kernel: Available graphics memory: 1022244 kiB
[   17.614701] [TTM] Initializing pool allocator
[   17.615427] [TTM] Initializing DMA pool allocator
[   17.619143] fbcon: bochsdrmfb (fb0) is primary device
[   17.619428] Console: switching to colour frame buffer device 128x48
[   17.621047] bochs-drm 0000:00:02.0: fb0: bochsdrmfb frame buffer device
[   17.641111] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 0
[   17.642380] general protection fault: 0000 [#1] SMP
[   17.642985] Modules linked in: bochs_drm(+)
[   17.643259] CPU: 4 PID: 3279 Comm: insmod Tainted: G        W       4.11.0-rc1+ #119
[   17.643259] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-20161025_171302-gandalf 04/01/2014
[   17.643259] task: ffff88007af35e00 task.stack: ffffc90000d84000
[   17.643259] RIP: 0010:drm_fb_helper_fini+0x8e/0x110
[   17.643259] RSP: 0018:ffffc90000d87ad0 EFLAGS: 00010202
[   17.643259] RAX: dead000000000200 RBX: ffff8800790d5770 RCX: 0000000000000000
[   17.652101] RDX: dead000000000100 RSI: 000000007fffffff RDI: ffffffff81eaf820
[   17.652101] RBP: ffffc90000d87ae0 R08: 0000000000000000 R09: ffff88007271d918
[   17.652101] R10: ffffc90000d87a88 R11: 0000000000000000 R12: 0000000000000000
[   17.652101] R13: ffff8800790d56d0 R14: 0000000000000000 R15: 0000000000000000
[   17.652101] FS:  00007f9285995700(0000) GS:ffff88007cf00000(0000) knlGS:0000000000000000
[   17.652101] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   17.652101] CR2: 0000564f1cf9f1e8 CR3: 0000000079686000 CR4: 00000000000006e0
[   17.652101] Call Trace:
[   17.652101]  bochs_fbdev_fini+0x24/0x90 [bochs_drm]
[   17.652101]  bochs_unload+0x16/0x50 [bochs_drm]
[   17.652101]  drm_dev_unregister+0x37/0xd0
[   17.652101]  drm_put_dev+0x31/0x60
[   17.652101]  bochs_pci_remove+0x10/0x20 [bochs_drm]
[   17.652101]  pci_device_remove+0x34/0xb0
[   17.652101]  driver_probe_device+0xd0/0x370
[   17.652101]  __driver_attach+0x96/0xa0
[   17.652101]  ? driver_probe_device+0x370/0x370
[   17.652101]  bus_for_each_dev+0x5b/0x90
[   17.652101]  driver_attach+0x19/0x20
[   17.652101]  bus_add_driver+0x11c/0x220
[   17.652101]  driver_register+0x5b/0xd0
[   17.652101]  ? 0xffffffffa0006000
[   17.652101]  __pci_register_driver+0x47/0x50
[   17.652101]  drm_pci_init+0xe1/0xf0
[   17.652101]  ? 0xffffffffa0006000
[   17.652101]  bochs_init+0x3c/0x1000 [bochs_drm]
[   17.652101]  do_one_initcall+0x3e/0x180
[   17.652101]  ? kmem_cache_alloc_trace+0x33/0x150
[   17.652101]  do_init_module+0x5a/0x1eb
[   17.652101]  load_module+0x1ea0/0x2650
[   17.652101]  ? __symbol_put+0x40/0x40
[   17.652101]  ? kernel_read_file+0x19e/0x1c0
[   17.652101]  ? kernel_read_file_from_fd+0x44/0x70
[   17.652101]  SYSC_finit_module+0xba/0xc0
[   17.652101]  SyS_finit_module+0x9/0x10
[   17.652101]  entry_SYSCALL_64_fastpath+0x1a/0xa9
[   17.652101] RIP: 0033:0x7f92854da119
[   17.652101] RSP: 002b:00007ffcd0390498 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[   17.652101] RAX: ffffffffffffffda RBX: 00007f928578eb58 RCX: 00007f92854da119
[   17.652101] RDX: 0000000000000000 RSI: 0000564f1c8bd638 RDI: 0000000000000003
[   17.652101] RBP: 000000000000270e R08: 0000000000000000 R09: 00007f9285790ea0
[   17.652101] R10: 0000000000000003 R11: 0000000000000246 R12: 00007f928578eb58
[   17.652101] R13: 0000000000001020 R14: 0000564f1cf9e1c0 R15: 00007f928578eb00
[   17.652101] Code: c7 20 f8 ea 81 e8 b3 3e 50 00 48 8b 83 d0 00 00 00 48 8d 93 d0 00 00 00 48 39 c2 74 46 48 8b 83 d8 00 00 00 48 8b 93 d0 00 00 00 <48> 89 42 08 48 89 10 48 b8 00 01 00 00 0000 ad de 48 89 83 d0
[   17.652101] RIP: drm_fb_helper_fini+0x8e/0x110 RSP: ffffc90000d87ad0
[   17.653331] ---[ end trace 542fd75a2e60a6a4 ]---
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324045444.11912-1-krisman@collabora.co.ukSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/bochs/bochs_fbdev.c

@@ -192,6 +192,8 @@ void bochs_fbdev_fini(struct bochs_device *bochs)
 	if (bochs->fb.initialized)
 		bochs_fbdev_destroy(bochs);
 
-	drm_fb_helper_fini(&bochs->fb.helper);
+	if (bochs->fb.helper.fbdev)
+		drm_fb_helper_fini(&bochs->fb.helper);
+
 	bochs->fb.initialized = false;
 }