Check fops_get() return value

Several subsystem open handlers dereference the fops_get() return value
without checking it for nullness. This opens a race condition between the
open handler and module unloading.

A module can be marked as being unloaded (MODULE_STATE_GOING) before its
exit function is called and gets the chance to unregister the driver.
During that window open handlers can still be called, and fops_get() will
fail in try_module_get() and return a NULL pointer.

This change checks the fops_get() return value and returns -ENODEV if NULL.

Reported-by: Alan Jenkins <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>
Acked-by: Takashi Iwai <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Acked-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/gpu/drm/drm_fops.c

···
 
 	old_fops = filp->f_op;
 	filp->f_op = fops_get(&dev->driver->fops);
+	if (filp->f_op == NULL) {
+		filp->f_op = old_fops;
+		goto out;
+	}
 	if (filp->f_op->open && (err = filp->f_op->open(inode, filp))) {
 		fops_put(filp->f_op);
 		filp->f_op = fops_get(old_fops);

drivers/media/dvb/dvb-core/dvbdev.c

···
 		file->private_data = dvbdev;
 		old_fops = file->f_op;
 		file->f_op = fops_get(dvbdev->fops);
+		if (file->f_op == NULL) {
+			file->f_op = old_fops;
+			goto fail;
+		}
 		if(file->f_op->open)
 			err = file->f_op->open(inode,file);
 		if (err) {
···
 		unlock_kernel();
 		return err;
 	}
+fail:
 	up_read(&minor_rwsem);
 	unlock_kernel();
 	return -ENODEV;

sound/core/sound.c

···
 	}
 	old_fops = file->f_op;
 	file->f_op = fops_get(mptr->f_ops);
+	if (file->f_op == NULL) {
+		file->f_op = old_fops;
+		return -ENODEV;
+	}
 	if (file->f_op->open)
 		err = file->f_op->open(inode, file);
 	if (err) {