Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts

syzbot has reported an issue in the framebuffer layer, where a malicious
user may overflow our built-in font data buffers.

In order to perform a reliable range check, subsystems need to know
`FONTDATAMAX` for each built-in font. Unfortunately, our font descriptor,
`struct console_font` does not contain `FONTDATAMAX`, and is part of the
UAPI, making it infeasible to modify it.

For user-provided fonts, the framebuffer layer resolves this issue by
reserving four extra words at the beginning of data buffers. Later,
whenever a function needs to access them, it simply uses the following
macros:

Recently we have gathered all the above macros to <linux/font.h>. Let us
do the same thing for built-in fonts, prepend four extra words (including
`FONTDATAMAX`) to their data buffers, so that subsystems can use these
macros for all fonts, no matter built-in or user-provided.

This patch depends on patch "fbdev, newport_con: Move FONT_EXTRA_WORDS
macros into linux/font.h".

Cc: stable@vger.kernel.org
Link: https://syzkaller.appspot.com/bug?id=08b8be45afea11888776f897895aef9ad1c3ecfd
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/ef18af00c35fb3cc826048a5f70924ed6ddce95b.1600953813.git.yepeilin.cs@gmail.com

1 files changed, 4 insertions, 5 deletions

diff --git a/lib/fonts/font_8x8.c b/lib/fonts/font_8x8.c
index 2328ebc8bab5..8ab695538395 100644
--- a/lib/fonts/font_8x8.c
+++ b/lib/fonts/font_8x8.c
@@ -9,8 +9,8 @@
 
 #define FONTDATAMAX 2048
 
-static const unsigned char fontdata_8x8[FONTDATAMAX] = {
-
+static struct font_data fontdata_8x8 = {
+	{ 0, 0, FONTDATAMAX, 0 }, {
 	/* 0 0x00 '^@' */
 	0x00, /* 00000000 */
 	0x00, /* 00000000 */
@@ -2570,8 +2570,7 @@ static const unsigned char fontdata_8x8[FONTDATAMAX] = {
 	0x00, /* 00000000 */
 	0x00, /* 00000000 */
 	0x00, /* 00000000 */
-
-};
+} };
 
 
 const struct font_desc font_vga_8x8 = {
@@ -2579,6 +2578,6 @@ const struct font_desc font_vga_8x8 = {
 	.name	= "VGA8x8",
 	.width	= 8,
 	.height	= 8,
-	.data	= fontdata_8x8,
+	.data	= fontdata_8x8.data,
 	.pref	= 0,
 };