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Subject: [PATCH] fbmem: don't allow too huge resolutions
To:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>,
        akpm@linux-foundation.org, b.zolnierkie@samsung.com,
        colin.king@canonical.com, dri-devel@lists.freedesktop.org,
        linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        masahiroy@kernel.org, syzkaller-bugs@googlegroups.com,
        Randy Dunlap <rdunlap@infradead.org>
References: <000000000000815b9605c70e74f8@google.com>
 <131b24e5-ee31-6f7b-42b4-c34583711913@infradead.org>
 <2fccb5d3-191c-924e-159f-1c9d423e282f@i-love.sakura.ne.jp>
 <339bfb21-8e80-c7d9-46dd-c416f87c50c0@infradead.org>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <535e404d-03bf-8e7a-b296-132a2a98c599@i-love.sakura.ne.jp>
Date:   Tue, 31 Aug 2021 01:05:05 +0900
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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@syzkaller.appspotmail.com>
Debugged-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.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