On Thu, Sep 24, 2020 at 04:09:37PM +0200, Greg Kroah-Hartman wrote:
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=08b8be45afea11888776f897895aef9ad1c3ecf...
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?
Applied to drm-misc-fixes, but just missed the pull request train for -rc7. Should land in Linus' tree next week.
But I did look at the code, and I have regrets. Macros into untyped arrays and negative indices is very old skool C. It's definitely neater than before, but also can't deny that we're doing dental surgery on a living and fire breathing dragon here :-/
I guess I'll just add this to the list of requirements anyone has to resolve before we're going to resurrect scrollback.
Cheers, Daniel
thanks,
greg k-h