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

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Message-ID: <12cfe7be56a4eeed0f32d8da69d06f0490a9eec9@intel.com>
Date: Mon, 22 Sep 2025 15:43:29 +0300
From: Jani Nikula <jani.nikula@...ux.intel.com>
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, tzimmermann@...e.de
Subject: Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font

On Fri, 12 Sep 2025, Samasth Norway Ananda <samasth.norway.ananda@...cle.com> wrote:
> 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>
> ---
>  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))

This change stores the intermediate value into size, but fails to take
into account that size is used just a bit later in the function,
expecting the original size:

	new_data += FONT_EXTRA_WORDS * sizeof(int);
	FNTSIZE(new_data) = size;
	REFCOUNT(new_data) = 0;	/* usage counter */
	for (i=0; i< charcount; i++) {
		memcpy(new_data + i*h*pitch, data +  i*vpitch*pitch, h*pitch);
	}

	/* Since linux has a nice crc32 function use it for counting font
	 * checksums. */
	csum = crc32(0, new_data, size);

What was supposed to address an unlikely integer overflow seems to have
caused a real buffer overflow [1].

BR,
Jani.


[1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15020

> +		return -EINVAL;
>  
> -	new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
> +	new_data = kmalloc(size, GFP_USER);
>  
>  	if (!new_data)
>  		return -ENOMEM;

-- 
Jani Nikula, Intel