[PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers

[Peilin Ye]

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 at 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