From: Peilin Ye <yepeilin.cs@gmail.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Peilin Ye <yepeilin.cs@gmail.com>,
Jiri Slaby <jirislaby@kernel.org>,
Daniel Vetter <daniel.vetter@ffwll.ch>,
dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
linux-kernel-mentees@lists.linuxfoundation.org,
syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org
Subject: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
Date: Thu, 24 Sep 2020 09:38:22 -0400 [thread overview]
Message-ID: <cover.1600953813.git.yepeilin.cs@gmail.com> (raw)
In-Reply-To: <0000000000006b9e8d059952095e@google.com>
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
next prev parent reply other threads:[~2020-09-24 13:40 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-10 4:35 KASAN: global-out-of-bounds Read in fbcon_get_font syzbot
2020-01-01 17:40 ` syzbot
2020-09-24 13:38 ` Peilin Ye [this message]
2020-09-24 13:40 ` [PATCH 1/3] fbdev, newport_con: Move FONT_EXTRA_WORDS macros into linux/font.h Peilin Ye
2020-09-24 13:42 ` [PATCH 2/3] Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts Peilin Ye
2020-09-24 13:43 ` [PATCH 3/3] fbcon: Fix global-out-of-bounds read in fbcon_get_font() Peilin Ye
2020-09-24 14:09 ` [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers Greg Kroah-Hartman
2020-09-24 14:25 ` Peilin Ye
2020-09-24 14:42 ` David Laight
2020-09-24 15:30 ` Peilin Ye
2020-09-24 15:45 ` Dan Carpenter
2020-09-24 16:59 ` Peilin Ye
2020-09-25 8:38 ` Daniel Vetter
2020-09-25 6:46 ` Jiri Slaby
2020-09-25 10:13 ` Peilin Ye
2020-09-25 13:25 ` Daniel Vetter
2020-09-25 15:35 ` Peilin Ye
2020-09-29 9:09 ` Daniel Vetter
2020-09-29 9:44 ` Peilin Ye
2020-09-29 12:34 ` Peilin Ye
2020-09-29 14:38 ` Daniel Vetter
2020-09-30 7:11 ` Peilin Ye
2020-09-30 9:53 ` Daniel Vetter
2020-09-30 10:55 ` Peilin Ye
2020-09-30 11:25 ` Daniel Vetter
2020-09-30 11:52 ` Greg Kroah-Hartman
2020-09-30 12:58 ` Peilin Ye
2020-09-30 5:26 ` Jiri Slaby
2020-09-30 7:16 ` Peilin Ye
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