fbdev: smscufx: Fix several use-after-free bugs

Several types of UAFs can occur when physically removing a USB device.

Adds ufx_ops_destroy() function to .fb_destroy of fb_ops, and
in this function, there is kref_put() that finally calls ufx_free().

This fix prevents multiple UAFs.

Signed-off-by: Hyunwoo Kim <[e-mail]>
Link: https://lore.kernel.org/linux-fbdev/20221011153436.GA4446@ubuntu/
Cc: <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/smscufx.c

···
 	struct kref kref;
 	int fb_count;
 	bool virtualized; /* true when physical usb device not present */
-	struct delayed_work free_framebuffer_work;
 	atomic_t usb_active; /* 0 = update virtual buffer, but no usb traffic */
 	atomic_t lost_pixels; /* 1 = a render op failed. Need screen refresh */
 	u8 *edid; /* null until we read edid from hw or get from sysfs */
···
 {
 	struct ufx_data *dev = container_of(kref, struct ufx_data, kref);
 
-	/* this function will wait for all in-flight urbs to complete */
-	if (dev->urbs.count > 0)
-		ufx_free_urb_list(dev);
-
-	pr_debug("freeing ufx_data %p", dev);
-
 	kfree(dev);
 }
+
+static void ufx_ops_destory(struct fb_info *info)
+{
+	struct ufx_data *dev = info->par;
+	int node = info->node;
+
+	/* Assume info structure is freed after this point */
+	framebuffer_release(info);
+
+	pr_debug("fb_info for /dev/fb%d has been freed", node);
+
+	/* release reference taken by kref_init in probe() */
+	kref_put(&dev->kref, ufx_free);
+}
+
 
 static void ufx_release_urb_work(struct work_struct *work)
 {
···
 	up(&unode->dev->urbs.limit_sem);
 }
 
-static void ufx_free_framebuffer_work(struct work_struct *work)
+static void ufx_free_framebuffer(struct ufx_data *dev)
 {
-	struct ufx_data *dev = container_of(work, struct ufx_data,
-					    free_framebuffer_work.work);
 	struct fb_info *info = dev->info;
-	int node = info->node;
-
-	unregister_framebuffer(info);
 
 	if (info->cmap.len != 0)
 		fb_dealloc_cmap(&info->cmap);
···
 	fb_destroy_modelist(&info->modelist);
 
 	dev->info = NULL;
-
-	/* Assume info structure is freed after this point */
-	framebuffer_release(info);
-
-	pr_debug("fb_info for /dev/fb%d has been freed", node);
 
 	/* ref taken in probe() as part of registering framebfufer */
 	kref_put(&dev->kref, ufx_free);
···
 {
 	struct ufx_data *dev = info->par;
 
+	mutex_lock(&disconnect_mutex);
+
 	dev->fb_count--;
 
 	/* We can't free fb_info here - fbmem will touch it when we return */
 	if (dev->virtualized && (dev->fb_count == 0))
-		schedule_delayed_work(&dev->free_framebuffer_work, HZ);
+		ufx_free_framebuffer(dev);
 
 	if ((dev->fb_count == 0) && (info->fbdefio)) {
 		fb_deferred_io_cleanup(info);
···
 		  info->node, user, dev->fb_count);
 
 	kref_put(&dev->kref, ufx_free);
+
+	mutex_unlock(&disconnect_mutex);
 
 	return 0;
 }
···
 	.fb_blank = ufx_ops_blank,
 	.fb_check_var = ufx_ops_check_var,
 	.fb_set_par = ufx_ops_set_par,
+	.fb_destroy = ufx_ops_destory,
 };
 
 /* Assumes &info->lock held by caller
···
 		goto destroy_modedb;
 	}
 
-	INIT_DELAYED_WORK(&dev->free_framebuffer_work,
-			  ufx_free_framebuffer_work);
-
 	retval = ufx_reg_read(dev, 0x3000, &id_rev);
 	check_warn_goto_error(retval, "error %d reading 0x3000 register from device", retval);
 	dev_dbg(dev->gdev, "ID_REV register value 0x%08x", id_rev);
···
 static void ufx_usb_disconnect(struct usb_interface *interface)
 {
 	struct ufx_data *dev;
+	struct fb_info *info;
 
 	mutex_lock(&disconnect_mutex);
 
 	dev = usb_get_intfdata(interface);
+	info = dev->info;
 
 	pr_debug("USB disconnect starting\n");
 
···
 
 	/* if clients still have us open, will be freed on last close */
 	if (dev->fb_count == 0)
-		schedule_delayed_work(&dev->free_framebuffer_work, 0);
+		ufx_free_framebuffer(dev);
 
-	/* release reference taken by kref_init in probe() */
-	kref_put(&dev->kref, ufx_free);
+	/* this function will wait for all in-flight urbs to complete */
+	if (dev->urbs.count > 0)
+		ufx_free_urb_list(dev);
 
-	/* consider ufx_data freed */
+	pr_debug("freeing ufx_data %p", dev);
+
+	unregister_framebuffer(info);
 
 	mutex_unlock(&disconnect_mutex);
 }