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 <samasth.norway.ananda@oracle.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 39b3cff ("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+
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250912170023.3931881-1-samasth.norway.ananda@oracle.com

Author: samasth-norway

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;