linux-kernel - [PATCH] fbmem: don't allow too huge resolutions

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Message-ID: <535e404d-03bf-8e7a-b296-132a2a98c599@i-love.sakura.ne.jp>
Date:   Tue, 31 Aug 2021 01:05:05 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, b.zolnierkie@...sung.com,
        colin.king@...onical.com, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        masahiroy@...nel.org, syzkaller-bugs@...glegroups.com,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH] fbmem: don't allow too huge resolutions

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.

This patch does not use array_size(), for array_size() is allowed to
return UINT_MAX on 32bits even if overflow did not happen. We want to
detect only overflow here, for individual module will recheck with more
strict limits as needed.

Link: https://syzkaller.appspot.com/bug?extid=04168c8063cfdde1db5e [1]
Reported-by: syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>
Debugged-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>
---
 drivers/video/fbdev/core/fbmem.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1c855145711b..9f5075dc2345 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1008,6 +1008,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	if (var->xres < 8 || var->yres < 8)
 		return -EINVAL;
 
+	/* Don't allow u32 * u32 to overflow. */
+	if ((u64) var->xres * var->yres > UINT_MAX ||
+	    (u64) var->xres_virtual * var->yres_virtual > UINT_MAX)
+		return -EINVAL;
+
 	ret = info->fbops->fb_check_var(var, info);
 
 	if (ret)
-- 
2.18.4