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

Daniel Vetter daniel at ffwll.ch
Fri Sep 25 08:38:42 UTC 2020


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 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(-)
> 
> Gotta love going backwards in arrays :)
> 
> Nice work, whole series is:
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh at 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch


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