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Subject: Re: [syzbot] BUG: unable to handle kernel paging request in
 vga16fb_fillrect
To:     Randy Dunlap <rdunlap@infradead.org>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        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
References: <000000000000815b9605c70e74f8@google.com>
 <131b24e5-ee31-6f7b-42b4-c34583711913@infradead.org>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <2fccb5d3-191c-924e-159f-1c9d423e282f@i-love.sakura.ne.jp>
Date:   Mon, 30 Aug 2021 11:27:27 +0900
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On 2021/08/30 9:24, Randy Dunlap wrote:
> Note that yres_virtual is set to 0x10000000. Is there no practical limit
> (hence limit check) that can be used here?
> 
> Also, in vga16fb_check_var(), beginning at line 404:
> 
>   404        if (yres > vyres)
>   405            vyres = yres;
>   406        if (vxres * vyres > maxmem) {
>   407            vyres = maxmem / vxres;
>   408            if (vyres < yres)
>   409                return -ENOMEM;
>   410        }
> 
> At line 406, the product of vxres * vyres overflows 32 bits (is 0 in this
> case/example), so any protection from this block is lost.

OK. Then, we can check overflow like below.

diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index e2757ff1c23d..e483a3f5fd47 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c
@@ -403,7 +403,7 @@ static int vga16fb_check_var(struct fb_var_screeninfo *var,
 
 	if (yres > vyres)
 		vyres = yres;
-	if (vxres * vyres > maxmem) {
+	if ((u64) vxres * vyres > (u64) maxmem) {
 		vyres = maxmem / vxres;
 		if (vyres < yres)
 			return -ENOMEM;

But I think we can check overflow in the common code like below. (Both patch fixed the oops.)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1c855145711b..8899679bbc46 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 > (u64) UINT_MAX ||
+	    (u64) var->xres_virtual * var->yres_virtual > (u64) UINT_MAX)
+		return -EINVAL;
+
 	ret = info->fbops->fb_check_var(var, info);
 
 	if (ret)

> 
> But even if yres_virtual (aka vyres) is "only" 0x01000000, so no
> multiplication overflow occurs, the resulting value of vyres "seems"
> to still be too large and can cause an error [I'm not sure about this
> last part -- I need to use a new gcc so that KASAN will work.]