linux-kernel - [PATCH 1/2] fbcon: prevent user font height or width change from causing potential out-of-bounds access

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Message-Id: <1596213192-6635-1-git-send-email-george.kennedy@oracle.com>
Date:   Fri, 31 Jul 2020 12:33:11 -0400
From:   George Kennedy <george.kennedy@...cle.com>
To:     george.kennedy@...cle.com, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, b.zolnierkie@...sung.com,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        dan.carpenter@...cle.com, dhaval.giani@...cle.com
Subject: [PATCH 1/2] fbcon: prevent user font height or width change from causing potential out-of-bounds access

Add a check to fbcon_resize() to ensure that a possible change to user font
height or user font width will not allow a font data out-of-bounds access.
NOTE: must use original charcount in calculation as font charcount can
change and cannot be used to determine the font data allocated size.

Signed-off-by: George Kennedy <george.kennedy@...cle.com>
Reported-by: syzbot+38a3699c7eaf165b97a6@...kaller.appspotmail.com
---
 drivers/video/fbdev/core/fbcon.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 8a31fc2..6616783 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2191,6 +2191,9 @@ static void updatescrollmode(struct fbcon_display *p,
 	}
 }
 
+#define PITCH(w) (((w) + 7) >> 3)
+#define CALC_FONTSZ(h, p, c) ((h) * (p) * (c)) /* size = height * pitch * charcount */
+
 static int fbcon_resize(struct vc_data *vc, unsigned int width, 
 			unsigned int height, unsigned int user)
 {
@@ -2200,6 +2203,24 @@ static int fbcon_resize(struct vc_data *vc, unsigned int width,
 	struct fb_var_screeninfo var = info->var;
 	int x_diff, y_diff, virt_w, virt_h, virt_fw, virt_fh;
 
+	if (ops->p && ops->p->userfont && FNTSIZE(vc->vc_font.data)) {
+		int size;
+		int pitch = PITCH(vc->vc_font.width);
+
+		/*
+		 * If user font, ensure that a possible change to user font
+		 * height or width will not allow a font data out-of-bounds access.
+		 * NOTE: must use original charcount in calculation as font
+		 * charcount can change and cannot be used to determine the
+		 * font data allocated size.
+		 */
+		if (pitch <= 0)
+			return -EINVAL;
+		size = CALC_FONTSZ(vc->vc_font.height, pitch, FNTCHARCNT(vc->vc_font.data));
+		if (size > FNTSIZE(vc->vc_font.data))
+			return -EINVAL;
+	}
+
 	virt_w = FBCON_SWAP(ops->rotate, width, height);
 	virt_h = FBCON_SWAP(ops->rotate, height, width);
 	virt_fw = FBCON_SWAP(ops->rotate, vc->vc_font.width,
@@ -2652,7 +2673,7 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
 	int size;
 	int i, csum;
 	u8 *new_data, *data = font->data;
-	int pitch = (font->width+7) >> 3;
+	int pitch = PITCH(font->width);
 
 	/* Is there a reason why fbconsole couldn't handle any charcount >256?
 	 * If not this check should be changed to charcount < 256 */
@@ -2668,7 +2689,7 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
 	if (fbcon_invalid_charcount(info, charcount))
 		return -EINVAL;
 
-	size = h * pitch * charcount;
+	size = CALC_FONTSZ(h, pitch, charcount);
 
 	new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
 
-- 
1.8.3.1