Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: 2021-08-30 12:00:56
Also in:
dri-devel, lkml
Hi Testsuo, On Mon, Aug 30, 2021 at 4:27 AM Tetsuo Handa [off-list ref] wrote:
quoted hunk ↗ jump to hunk
On 2021/08/30 9:24, Randy Dunlap wrote:quoted
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) {
Mindlessly changing the sizes is not the solution. Please use e.g. the array_size() helper from <linux/overflow.h> instead.
quoted hunk ↗ jump to hunk
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; +
Same comment here, of course.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds