fbdev: pxafb: Fix possible use after free in pxafb_task()

fbdev: pxafb: Fix possible use after free in pxafb_task()

In the pxafb_probe function, it calls the pxafb_init_fbinfo function,
after which &fbi->task is associated with pxafb_task. Moreover,
within this pxafb_init_fbinfo function, the pxafb_blank function
within the &pxafb_ops struct is capable of scheduling work.

If we remove the module which will call pxafb_remove to make cleanup,
it will call unregister_framebuffer function which can call
do_unregister_framebuffer to free fbi->fb through
put_fb_info(fb_info), while the work mentioned above will be used.
The sequence of operations that may lead to a UAF bug is as follows:

CPU0                                                CPU1

                                   | pxafb_task
pxafb_remove                       |
unregister_framebuffer(info)       |
do_unregister_framebuffer(fb_info) |
put_fb_info(fb_info)               |
// free fbi->fb                    | set_ctrlr_state(fbi, state)
                                   | __pxafb_lcd_power(fbi, 0)
                                   | fbi->lcd_power(on, &fbi->fb.var)
                                   | //use fbi->fb

Fix it by ensuring that the work is canceled before proceeding
with the cleanup in pxafb_remove.

Note that only root user can remove the driver at runtime.

Signed-off-by: Kaixin Wang <kxwang23@m.fudan.edu.cn>
Signed-off-by: Helge Deller <deller@gmx.de>