From 8c28051cdcbe9dfcec6bd0a4709d67a09df6edae Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Date: Wed, 8 Sep 2021 19:27:49 +0900
Subject: fbmem: don't allow too huge resolutions
Git-commit: 8c28051cdcbe9dfcec6bd0a4709d67a09df6edae
Patch-mainline: v5.15-rc1
References: bsc#1129770

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
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 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
@@ -954,6 +954,7 @@ fb_set_var(struct fb_info *info, struct
 {
 	int flags = info->flags;
 	int ret = 0;
+	u32 unused;
 
 	if (var->activate & FB_ACTIVATE_INV_MODE) {
 		struct fb_videomode mode1, mode2;
@@ -1006,6 +1007,11 @@ 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)