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[PATCH] fbcon: fix integer overflow in fbcon_do_set_font

Samasth Norway Ananda posted 1 patch 2 weeks, 5 days ago 
drivers/video/fbdev/core/fbcon.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
[PATCH] fbcon: fix integer overflow in fbcon_do_set_font
Posted by Samasth Norway Ananda 2 weeks, 5 days ago 
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>
---
 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;
-- 
2.50.1
Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
Posted by Jani Nikula 1 week, 3 days ago 
On Fri, 12 Sep 2025, Samasth Norway Ananda <samasth.norway.ananda@oracle.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@oracle.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
Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
Posted by Jani Nikula 1 week, 3 days ago 
On Mon, 22 Sep 2025, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Fri, 12 Sep 2025, Samasth Norway Ananda <samasth.norway.ananda@oracle.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@oracle.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
Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
Posted by Thomas Zimmermann 1 week, 3 days ago 

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)
Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
Posted by Thomas Zimmermann 1 week, 3 days ago 

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>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.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)

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