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

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Message-ID: <9c7e4c1213c6a77680d8d5f454e1b7c27fc5db62@intel.com>
Date: Mon, 22 Sep 2025 15:49:33 +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 Mon, 22 Sep 2025, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> 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].

The overflow of 16 bytes matches FONT_EXTRA_WORDS * sizeof(int):

memcmp: detected buffer overflow: 8208 byte read of buffer size 8192


> 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