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SubjectRe: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
On Thu, Sep 24, 2020 at 09:38:22AM -0400, Peilin Ye wrote:
> 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(-)

Gotta love going backwards in arrays :)

Nice work, whole series is:

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


Daniel, can you take this through your tree?

thanks,

greg k-h

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