linux-kernel - Re: UBSAN: shift-out-of-bounds in fbcon_set

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Date:   Thu, 26 Jan 2023 09:46:55 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
Cc:     "deller@....de" <deller@....de>,
        "sam@...nborg.org" <sam@...nborg.org>,
        "tzimmermann@...e.de" <tzimmermann@...e.de>,
        "geert+renesas@...der.be" <geert+renesas@...der.be>,
        "samuel.thibault@...-lyon.org" <samuel.thibault@...-lyon.org>,
        "penguin-kernel@...ove.SAKURA.ne.jp" 
        <penguin-kernel@...ove.sakura.ne.jp>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "contact@...zz.com" <contact@...zz.com>,
        "syzkaller@...glegroups.com" <syzkaller@...glegroups.com>
Subject: Re: UBSAN: shift-out-of-bounds in fbcon_set_font

Hi Sanan,

On Thu, Jan 26, 2023 at 12:58 AM Sanan Hasanov
<sanan.hasanov@...ghts.ucf.edu> wrote:
> We found a bug using a modified kernel configuration file used by syzbot.
>
> We enhanced the coverage of the configuration file using our tool, klocalizer.
>
> Kernel Branch: 6.2.0-rc5-next-20230124
> Kernel config: https://drive.google.com/file/d/1F-LszDAizEEH0ZX0HcSR06v5q8FPl2Uv/view?usp=sharing
> Reproducer: https://drive.google.com/file/d/1mP1jcLBY7vWCNM60OMf-ogw-urQRjNrm/view?usp=sharing
>
> Thank you!
>
> Best regards,
> Sanan Hasanov
>
> ================================================================================
> UBSAN: shift-out-of-bounds in drivers/video/fbdev/core/fbcon.c:2489:33
> shift exponent 35 is too large for 32-bit type 'int'

This looks valid to me: con_font_set() checks the font width and height
against max_font_width (64) and max_font_height (128), but fbcon has
stricter limits.

fbcon_set_font() should reject fonts with width > 32 or height > 32.

Thanks!

Gr{oetje,eeting}s,

                        Geert

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