Re: [PATCH] fbcon: fix integer overflow in fbcon_do_set_font
From: Thomas Zimmermann <tzimmermann@suse.de>
Date: 2025-09-22 06:34:54
Also in:
dri-devel, lkml
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+
quoted hunk ↗ jump to hunk
--- 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)