drm/fb-helper: Fix out-of-bounds access

Clip memory range to screen-buffer size to avoid out-of-bounds access
in fbdev deferred I/O's damage handling.

Fbdev's deferred I/O can only track pages. From the range of pages, the
damage handler computes the clipping rectangle for the display update.
If the fbdev screen buffer ends near the beginning of a page, that page
could contain more scanlines. The damage handler would then track these
non-existing scanlines as dirty and provoke an out-of-bounds access
during the screen update. Hence, clip the maximum memory range to the
size of the screen buffer.

While at it, rename the variables min/max to min_off/max_off in
drm_fb_helper_deferred_io(). This avoids confusion with the macros of
the same name.

Reported-by: Nuno Gonçalves <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Tested-by: Nuno Gonçalves <[e-mail]>
Fixes: 67b723f5b742 ("drm/fb-helper: Calculate damaged area in separate helper")
Cc: Thomas Zimmermann <[e-mail]>
Cc: Javier Martinez Canillas <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: <[e-mail]> # v5.18+
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_fb_helper.c

···
 	schedule_work(&helper->damage_work);
 }
 
-/* Convert memory region into area of scanlines and pixels per scanline */
+/*
+ * Convert memory region into area of scanlines and pixels per
+ * scanline. The parameters off and len must not reach beyond
+ * the end of the framebuffer.
+ */
 static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off, size_t len,
 					       struct drm_rect *clip)
 {
···
  */
 void drm_fb_helper_deferred_io(struct fb_info *info, struct list_head *pagereflist)
 {
-	unsigned long start, end, min, max;
+	unsigned long start, end, min_off, max_off;
 	struct fb_deferred_io_pageref *pageref;
 	struct drm_rect damage_area;
 
-	min = ULONG_MAX;
-	max = 0;
+	min_off = ULONG_MAX;
+	max_off = 0;
 	list_for_each_entry(pageref, pagereflist, list) {
 		start = pageref->offset;
 		end = start + PAGE_SIZE;
-		min = min(min, start);
-		max = max(max, end);
+		min_off = min(min_off, start);
+		max_off = max(max_off, end);
 	}
-	if (min >= max)
+	if (min_off >= max_off)
 		return;
 
-	drm_fb_helper_memory_range_to_clip(info, min, max - min, &damage_area);
+	/*
+	 * As we can only track pages, we might reach beyond the end
+	 * of the screen and account for non-existing scanlines. Hence,
+	 * keep the covered memory area within the screen buffer.
+	 */
+	max_off = min(max_off, info->screen_size);
+
+	drm_fb_helper_memory_range_to_clip(info, min_off, max_off - min_off, &damage_area);
 	drm_fb_helper_damage(info, damage_area.x1, damage_area.y1,
 			     drm_rect_width(&damage_area),
 			     drm_rect_height(&damage_area));