fbmem: fix remove_conflicting_framebuffers races

When a register_framebuffer() call results in us removing old
conflicting framebuffers, the new registration_lock doesn't protect that
situation.  And we can't just add the same locking to the function,
because these functions call each other: register_framebuffer() calls
remove_conflicting_framebuffers, which in turn calls
unregister_framebuffer for any conflicting entry.

In order to fix it, this just creates wrapper functions around all three
functions and makes the versions that actually do the work be called
"do_xxx()", leaving just the wrapper that gets the lock and calls the
worker function.

So the rule becomes simply that "do_xxxx()" has to be called with the
lock held, and now do_register_framebuffer() can just call
do_remove_conflicting_framebuffers(), and that in turn can call
_do_unregister_framebuffer(), and there is no deadlock, and we can hold
the registration lock over the whole sequence, fixing the races.

It also makes error cases simpler, and fixes one situation where we
would return from unregister_framebuffer() without releasing the lock,
pointed out by Bruno Prémont.
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Tested-by: Anca Emanuel <anca.emanuel@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/fbmem.c

@@ -1537,8 +1537,10 @@ static bool fb_do_apertures_overlap(struct apertures_struct *gena,
 	return false;
 }
 
+static int do_unregister_framebuffer(struct fb_info *fb_info);
+
 #define VGA_FB_PHYS 0xA0000
-void remove_conflicting_framebuffers(struct apertures_struct *a,
+static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
 				     const char *name, bool primary)
 {
 	int i;
@@ -1560,24 +1562,12 @@ void remove_conflicting_framebuffers(struct apertures_struct *a,
 			printk(KERN_INFO "fb: conflicting fb hw usage "
 			       "%s vs %s - removing generic driver\n",
 			       name, registered_fb[i]->fix.id);
-			unregister_framebuffer(registered_fb[i]);
+			do_unregister_framebuffer(registered_fb[i]);
 		}
 	}
 }
-EXPORT_SYMBOL(remove_conflicting_framebuffers);
-
-/**
- *	register_framebuffer - registers a frame buffer device
- *	@fb_info: frame buffer info structure
- *
- *	Registers a frame buffer device @fb_info.
- *
- *	Returns negative errno on error, or zero for success.
- *
- */
 
-int
-register_framebuffer(struct fb_info *fb_info)
+static int do_register_framebuffer(struct fb_info *fb_info)
 {
 	int i;
 	struct fb_event event;
@@ -1589,10 +1579,9 @@ register_framebuffer(struct fb_info *fb_info)
 	if (fb_check_foreignness(fb_info))
 		return -ENOSYS;
 
-	remove_conflicting_framebuffers(fb_info->apertures, fb_info->fix.id,
+	do_remove_conflicting_framebuffers(fb_info->apertures, fb_info->fix.id,
 					 fb_is_primary_device(fb_info));
 
-	mutex_lock(&registration_lock);
 	num_registered_fb++;
 	for (i = 0 ; i < FB_MAX; i++)
 		if (!registered_fb[i])
@@ -1635,7 +1624,6 @@ register_framebuffer(struct fb_info *fb_info)
 	fb_var_to_videomode(&mode, &fb_info->var);
 	fb_add_videomode(&mode, &fb_info->modelist);
 	registered_fb[i] = fb_info;
-	mutex_unlock(&registration_lock);
 
 	event.info = fb_info;
 	if (!lock_fb_info(fb_info))
@@ -1645,37 +1633,14 @@ register_framebuffer(struct fb_info *fb_info)
 	return 0;
 }
 
-
-/**
- *	unregister_framebuffer - releases a frame buffer device
- *	@fb_info: frame buffer info structure
- *
- *	Unregisters a frame buffer device @fb_info.
- *
- *	Returns negative errno on error, or zero for success.
- *
- *      This function will also notify the framebuffer console
- *      to release the driver.
- *
- *      This is meant to be called within a driver's module_exit()
- *      function. If this is called outside module_exit(), ensure
- *      that the driver implements fb_open() and fb_release() to
- *      check that no processes are using the device.
- */
-
-int
-unregister_framebuffer(struct fb_info *fb_info)
+static int do_unregister_framebuffer(struct fb_info *fb_info)
 {
 	struct fb_event event;
 	int i, ret = 0;
 
-	mutex_lock(&registration_lock);
 	i = fb_info->node;
-	if (!registered_fb[i]) {
-		ret = -EINVAL;
-		goto done;
-	}
-
+	if (!registered_fb[i])
+		return -EINVAL;
 
 	if (!lock_fb_info(fb_info))
 		return -ENODEV;
@@ -1683,10 +1648,8 @@ unregister_framebuffer(struct fb_info *fb_info)
 	ret = fb_notifier_call_chain(FB_EVENT_FB_UNBIND, &event);
 	unlock_fb_info(fb_info);
 
-	if (ret) {
-		ret = -EINVAL;
-		goto done;
-	}
+	if (ret)
+		return -EINVAL;
 
 	if (fb_info->pixmap.addr &&
 	    (fb_info->pixmap.flags & FB_PIXMAP_DEFAULT))
@@ -1701,8 +1664,64 @@ unregister_framebuffer(struct fb_info *fb_info)
 
 	/* this may free fb info */
 	put_fb_info(fb_info);
-done:
+	return 0;
+}
+
+void remove_conflicting_framebuffers(struct apertures_struct *a,
+				     const char *name, bool primary)
+{
+	mutex_lock(&registration_lock);
+	do_remove_conflicting_framebuffers(a, name, primary);
 	mutex_unlock(&registration_lock);
+}
+EXPORT_SYMBOL(remove_conflicting_framebuffers);
+
+/**
+ *	register_framebuffer - registers a frame buffer device
+ *	@fb_info: frame buffer info structure
+ *
+ *	Registers a frame buffer device @fb_info.
+ *
+ *	Returns negative errno on error, or zero for success.
+ *
+ */
+int
+register_framebuffer(struct fb_info *fb_info)
+{
+	int ret;
+
+	mutex_lock(&registration_lock);
+	ret = do_register_framebuffer(fb_info);
+	mutex_unlock(&registration_lock);
+
+	return ret;
+}
+
+/**
+ *	unregister_framebuffer - releases a frame buffer device
+ *	@fb_info: frame buffer info structure
+ *
+ *	Unregisters a frame buffer device @fb_info.
+ *
+ *	Returns negative errno on error, or zero for success.
+ *
+ *      This function will also notify the framebuffer console
+ *      to release the driver.
+ *
+ *      This is meant to be called within a driver's module_exit()
+ *      function. If this is called outside module_exit(), ensure
+ *      that the driver implements fb_open() and fb_release() to
+ *      check that no processes are using the device.
+ */
+int
+unregister_framebuffer(struct fb_info *fb_info)
+{
+	int ret;
+
+	mutex_lock(&registration_lock);
+	ret = do_unregister_framebuffer(fb_info);
+	mutex_unlock(&registration_lock);
+
 	return ret;
 }