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

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Message-ID: <20250912170023.3931881-1-samasth.norway.ananda@oracle.com>
Date: Fri, 12 Sep 2025 10:00:23 -0700
From: Samasth Norway Ananda <samasth.norway.ananda@...cle.com>
To: simona@...ll.ch, deller@....de
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [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 <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))
+		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