Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font

[Thomas Zimmermann <tzimmermann@suse.de>]

Am 12.09.25 um 19:00 schrieb Samasth Norway Ananda:
> Fix integer overflow vulnerabilities in fbcon_do_set_font() where font
> size calculations could overflow when handling user-controlled font
> parameters.
>
> The vulnerabilities occur when:
> 1. CALC_FONTSZ(h, pitch, charcount) performs h * pith * charcount
>     multiplication with user-controlled values that can overflow.
> 2. FONT_EXTRA_WORDS * sizeof(int) + size addition can also overflow
> 3. This results in smaller allocations than expected, leading to buffer
>     overflows during font data copying.
>
> Add explicit overflow checking using check_mul_overflow() and
> check_add_overflow() kernel helpers to safety validate all size
> calculations before allocation.
>
> Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>

Fixes: 39b3cffb8cf3 ("fbcon: prevent user font height or width change 
from causing potential out-of-bounds access")
Cc: George Kennedy <george.kennedy@oracle.com>
Cc: stable <stable@vger.kernel.org>
Cc: syzbot+38a3699c7eaf165b97a6@syzkaller.appspotmail.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Qianqiang Liu <qianqiang.liu@163.com>
Cc: Shixiong Ou <oushixiong@kylinos.cn>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org> # v5.9+


> ---
>   drivers/video/fbdev/core/fbcon.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 55f5731e94c3..a507d05f8fea 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2531,9 +2531,16 @@ static int fbcon_set_font(struct vc_data *vc, const struct console_font *font,
>   	if (fbcon_invalid_charcount(info, charcount))
>   		return -EINVAL;
>   
> -	size = CALC_FONTSZ(h, pitch, charcount);
> +	/* Check for integer overflow in font size calculation */
> +	if (check_mul_overflow(h, pitch, &size) ||
> +	    check_mul_overflow(size, charcount, &size))
> +		return -EINVAL;
> +
> +	/* Check for overflow in allocation size calculation */
> +	if (check_add_overflow(FONT_EXTRA_WORDS * sizeof(int), size, &size))
> +		return -EINVAL;
>   
> -	new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
> +	new_data = kmalloc(size, GFP_USER);
>   
>   	if (!new_data)
>   		return -ENOMEM;

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)