Re: [PATCH 4.19] fbmem: add margin check to fb_check_caps()

[Geert Uytterhoeven]

Hi Dongliang,

On Thu, Sep 2, 2021 at 8:04 AM Dongliang Mu <mudongliangabcd@xxxxxxxxx> wrote:
> [ Upstream commit a49145acfb975d921464b84fe00279f99827d816 ]

Oops, looks like I missed when that one was submitted for review...

> A fb_ioctl() FBIOPUT_VSCREENINFO call with invalid xres setting
> or yres setting in struct fb_var_screeninfo will result in a
> KASAN: vmalloc-out-of-bounds failure in bitfill_aligned() as
> the margins are being cleared. The margins are cleared in
> chunks and if the xres setting or yres setting is a value of
> zero upto the chunk size, the failure will occur.
>
> Add a margin check to validate xres and yres settings.

Shouldn't (the caller of) bitfill_aligned() be fixed instead?
Can this be triggered by e.g. using the mini_4x6 font?

> Note that, this patch needs special handling to backport it to linux
> kernel 4.19, 4.14, 4.9, 4.4.
>
> Signed-off-by: George Kennedy <george.kennedy@xxxxxxxxxx>
> Reported-by: syzbot+e5fd3e65515b48c02a30@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reviewed-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Dhaval Giani <dhaval.giani@xxxxxxxxxx>
> Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Link: https://patchwork.freedesktop.org/patch/msgid/1594149963-13801-1-git-send-email-george.kennedy@xxxxxxxxxx
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  drivers/video/fbdev/core/fbmem.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 84845275dbef..de04c097d67c 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -991,6 +991,10 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>                         goto done;
>                 }
>
> +               /* bitfill_aligned() assumes that it's at least 8x8 */
> +               if (var->xres < 8 || var->yres < 8)
> +                       return -EINVAL;

Are you sure there don't exist such small displays (e.g. OLED)?

> +
>                 ret = info->fbops->fb_check_var(var, info);
>
>                 if (ret)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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