[PATCH] fbcon: fix integer overflow in fbcon_do_set_font

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 <redacted>
---
 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

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 <redacted>

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

Thanks a lot for the patch.

-- 
--
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)

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 <redacted>

Fixes: 39b3cffb8cf3 ("fbcon: prevent user font height or width change 
from causing potential out-of-bounds access")
Cc: George Kennedy <redacted>
Cc: stable <redacted>
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" <redacted>
Cc: Sam Ravnborg <redacted>
Cc: Qianqiang Liu <redacted>
Cc: Shixiong Ou <redacted>
Cc: Kees Cook <kees@kernel.org>
Cc: <redacted> # v5.9+

-- 
--
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)

On Fri, 12 Sep 2025, Samasth Norway Ananda [off-list ref] wrote:

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

On Mon, 22 Sep 2025, Jani Nikula [off-list ref] wrote:

On Fri, 12 Sep 2025, Samasth Norway Ananda [off-list ref] wrote:

quoted

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 <redacted>
---
 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

quoted

+		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