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 nunojpg@gmail.com Signed-off-by: Thomas Zimmermann tzimmermann@suse.de Tested-by: Nuno Gonçalves nunojpg@gmail.com Fixes: 67b723f5b742 ("drm/fb-helper: Calculate damaged area in separate helper") Cc: Thomas Zimmermann tzimmermann@suse.de Cc: Javier Martinez Canillas javierm@redhat.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Maxime Ripard mripard@kernel.org Cc: stable@vger.kernel.org # v5.18+ --- drivers/gpu/drm/drm_fb_helper.c | 27 +++++++++++++++++++-------- 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c index 5ad2b6a2778c..1705e8d345ab 100644 --- a/drivers/gpu/drm/drm_fb_helper.c +++ b/drivers/gpu/drm/drm_fb_helper.c @@ -680,7 +680,11 @@ static void drm_fb_helper_damage(struct fb_info *info, u32 x, u32 y, 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) { @@ -715,22 +719,29 @@ static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off, */ 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));