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Subject[PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
Date
Hi all,

syzbot has reported [1] a global out-of-bounds read issue in
fbcon_get_font(). A malicious user may resize `vc_font.height` to a large
value in vt_ioctl(), causing fbcon_get_font() to overflow our built-in
font data buffers, declared in lib/fonts/font_*.c:

(e.g. lib/fonts/font_8x8.c)
#define FONTDATAMAX 2048

static const unsigned char fontdata_8x8[FONTDATAMAX] = {

/* 0 0x00 '^@' */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
0x00, /* 00000000 */
[...]

In order to perform a reliable range check, fbcon_get_font() needs to know
`FONTDATAMAX` for each built-in font under lib/fonts/. Unfortunately, we
do not keep that information in our font descriptor,
`struct console_font`:

(include/uapi/linux/kd.h)
struct console_font {
unsigned int width, height; /* font size */
unsigned int charcount;
unsigned char *data; /* font data with height fixed to 32 */
};

To make things worse, `struct console_font` is part of the UAPI, so we
cannot add a new field to keep track of `FONTDATAMAX`.

Fortunately, the framebuffer layer itself gives us a hint of how to
resolve this issue without changing UAPI. When allocating a buffer for a
user-provided font, fbcon_set_font() reserves four "extra words" at the
beginning of the buffer:

(drivers/video/fbdev/core/fbcon.c)
new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
[...]
new_data += FONT_EXTRA_WORDS * sizeof(int);
FNTSIZE(new_data) = size;
FNTCHARCNT(new_data) = charcount;
REFCOUNT(new_data) = 0; /* usage counter */
[...]
FNTSUM(new_data) = csum;

Later, to get the size of a data buffer, the framebuffer layer simply
calls FNTSIZE() on it:

(drivers/video/fbdev/core/fbcon.h)
/* Font */
#define REFCOUNT(fd) (((int *)(fd))[-1])
#define FNTSIZE(fd) (((int *)(fd))[-2])
#define FNTCHARCNT(fd) (((int *)(fd))[-3])
#define FNTSUM(fd) (((int *)(fd))[-4])
#define FONT_EXTRA_WORDS 4

Currently, this is only done for user-provided fonts. Let us do the same
thing for built-in fonts, prepend these "extra words" (including
`FONTDATAMAX`) to their data buffers, so that other subsystems, like the
framebuffer layer, can use these macros on all fonts, no matter built-in
or user-provided. As an example, this series fixes the syzbot issue in
fbcon_get_font():

(drivers/video/fbdev/core/fbcon.c)
if (font->width <= 8) {
j = vc->vc_font.height;
+ if (font->charcount * j > FNTSIZE(fontdata))
+ return -EINVAL;
[...]

Similarly, newport_con also use these macros. It only uses three of them:

(drivers/video/console/newport_con.c)
/* borrowed from fbcon.c */
#define REFCOUNT(fd) (((int *)(fd))[-1])
#define FNTSIZE(fd) (((int *)(fd))[-2])
#define FNTCHARCNT(fd) (((int *)(fd))[-3])
#define FONT_EXTRA_WORDS 3

To keep things simple, move all these macro definitions to <linux/font.h>,
use four words instead of three, and initialize the fourth word in
newport_set_font() properly.

Many thanks to Greg Kroah-Hartman <gregkh@linuxfoundation.org>, who
reviewed and improved this series!

[1]: KASAN: global-out-of-bounds Read in fbcon_get_font
https://syzkaller.appspot.com/bug?id=08b8be45afea11888776f897895aef9ad1c3ecfd

Peilin Ye (3):
fbdev, newport_con: Move FONT_EXTRA_WORDS macros into linux/font.h
Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts
fbcon: Fix global-out-of-bounds read in fbcon_get_font()

drivers/video/console/newport_con.c | 7 +------
drivers/video/fbdev/core/fbcon.c | 12 ++++++++++++
drivers/video/fbdev/core/fbcon.h | 7 -------
drivers/video/fbdev/core/fbcon_rotate.c | 1 +
drivers/video/fbdev/core/tileblit.c | 1 +
include/linux/font.h | 13 +++++++++++++
lib/fonts/font_10x18.c | 9 ++++-----
lib/fonts/font_6x10.c | 9 +++++----
lib/fonts/font_6x11.c | 9 ++++-----
lib/fonts/font_7x14.c | 9 ++++-----
lib/fonts/font_8x16.c | 9 ++++-----
lib/fonts/font_8x8.c | 9 ++++-----
lib/fonts/font_acorn_8x8.c | 9 ++++++---
lib/fonts/font_mini_4x6.c | 8 ++++----
lib/fonts/font_pearl_8x8.c | 9 ++++-----
lib/fonts/font_sun12x22.c | 9 ++++-----
lib/fonts/font_sun8x16.c | 7 ++++---
lib/fonts/font_ter16x32.c | 9 ++++-----
18 files changed, 79 insertions(+), 67 deletions(-)

--
2.25.1

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