[PATCH 4.9 51/78] fbcon: prevent user font height or width change from causing potential out-of-bounds access

[Greg Kroah-Hartman]

From: George Kennedy <george.kennedy@xxxxxxxxxx>

commit 39b3cffb8cf3111738ea993e2757ab382253d86a upstream.

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@xxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx>
Reported-by: syzbot+38a3699c7eaf165b97a6@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/1596213192-6635-1-git-send-email-george.kennedy@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/video/console/fbcon.c |   25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

--- a/drivers/video/console/fbcon.c
+++ b/drivers/video/console/fbcon.c
@@ -2116,6 +2116,9 @@ static void updatescrollmode(struct disp
 	}
 }
 
+#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)
 {
@@ -2125,6 +2128,24 @@ static int fbcon_resize(struct vc_data *
 	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,
@@ -2586,7 +2607,7 @@ static int fbcon_set_font(struct vc_data
 	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 */
@@ -2602,7 +2623,7 @@ static int fbcon_set_font(struct vc_data
 	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);