[PATCH 5.10 234/236] fbmem: dont allow too huge resolutions

Greg Kroah-Hartman
2021-09-13T13:54:04Z

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

commit 8c28051cdcbe9dfcec6bd0a4709d67a09df6edae upstream.

syzbot is reporting page fault at vga16fb_fillrect() [1], for
vga16fb_check_var() is failing to detect multiplication overflow.

if (vxres * vyres > maxmem) {
vyres = maxmem / vxres;
if (vyres < yres)
return -ENOMEM;
}

Since no module would accept too huge resolutions where multiplication
overflow happens, let's reject in the common path.

Link: https://syzkaller.appspot.com/bug?extid=04168c8063cfdde1db5e [1]
Reported-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>
Debugged-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/185175d6-227a-7b55-433d-b070929b262c@i-love.sakura.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/video/fbdev/core/fbmem.c | 6 ++++++
1 file changed, 6 insertions(+)

--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -962,6 +962,7 @@ fb_set_var(struct fb_info *info, struct
struct fb_var_screeninfo old_var;
struct fb_videomode mode;
struct fb_event event;
+ u32 unused;

if (var->activate & FB_ACTIVATE_INV_MODE) {
struct fb_videomode mode1, mode2;
@@ -1008,6 +1009,11 @@ fb_set_var(struct fb_info *info, struct
if (var->xres < 8 || var->yres < 8)
return -EINVAL;

+ /* Too huge resolution causes multiplication overflow. */
+ if (check_mul_overflow(var->xres, var->yres, &unused) ||
+ check_mul_overflow(var->xres_virtual, var->yres_virtual, &unused))
+ return -EINVAL;
+
ret = info->fbops->fb_check_var(var, info);

if (ret)