linux-kernel - Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set

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Message-ID: <dfc5f39f-7743-400c-a602-55a569da7411@suse.de>
Date: Mon, 22 Sep 2025 08:24:52 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Samasth Norway Ananda <samasth.norway.ananda@...cle.com>,
 simona@...ll.ch, deller@....de
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font



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@...cle.com>

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

Thanks a lot for the patch.

> ---
>   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)