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

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Message-ID: <20200924165919.GA880812@PWN>
Date:   Thu, 24 Sep 2020 12:59:19 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data
 buffers

On Thu, Sep 24, 2020 at 06:45:16PM +0300, Dan Carpenter wrote:
> Smatch has a tool to show where struct members are set.
> 
> `~/smatch/smatch_data/db/smdb.py where console_font height`
> 
> It's not perfect and this output comes from allmodconfig on yesterday's
> linux-next.
> 
> regards,
> dan carpenter

Wow, thank you for the really interesting information!

> drivers/video/console/vgacon.c | vgacon_init                    | (struct console_font)->height | 0-32
> drivers/video/console/vgacon.c | vgacon_adjust_height           | (struct console_font)->height | 1-32
> drivers/video/fbdev/core/fbcon.c | fbcon_startup                  | (struct console_font)->height | 6,8,10-11,14,16,18,22,32
> drivers/video/fbdev/core/fbcon.c | fbcon_init                     | (struct console_font)->height | 6,8,10-11,14,16,18,22,32
> drivers/video/fbdev/core/fbcon.c | fbcon_do_set_font              | (struct console_font)->height | 0-u32max
> drivers/video/fbdev/core/fbcon.c | fbcon_set_def_font             | (struct console_font)->height | 6,8,10-11,14,16,18,22,32
> drivers/usb/misc/sisusbvga/sisusb_con.c | sisusbcon_init                 | (struct console_font)->height | 0-u32max

In looking at this one,
	c->vc_font.height = sisusb->current_font_height;

`sisusb->current_font_height` is only set in sisusbcon_do_font_op():
		sisusb->current_font_height = fh;

and...

> drivers/usb/misc/sisusbvga/sisusb_con.c | sisusbcon_do_font_op           | (struct console_font)->height | 1-32

...sisusbcon_do_font_op() is called in four places, with an `fh` of either
16, `sisusb->font_backup_height`, or `font->height`. The latter two cases
all come from sisusbcon_font_set(), whose dispatcher, con_font_set()
gurantees that `font->height` is less than or equal to 32, as shown by
Smatch here.

> drivers/tty/vt/vt_ioctl.c      | vt_k_ioctl                     | (struct console_font)->height | ignore
> drivers/tty/vt/vt_ioctl.c      | vt_resizex                     | (struct console_font)->height | 0-u32max
> drivers/tty/vt/vt_ioctl.c      | vt_ioctl                       | (struct console_font)->height | ignore
> drivers/tty/vt/vt_ioctl.c      | vt_compat_ioctl                | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | vc_allocate                    | (struct console_font)->height | 0
> drivers/tty/vt/vt.c            | vt_resize                      | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | do_con_write                   | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | con_unthrottle                 | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | con_flush_chars                | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | con_shutdown                   | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | con_cleanup                    | (struct console_font)->height | ignore
> drivers/tty/vt/vt.c            | con_init                       | (struct console_font)->height | 0
> drivers/tty/vt/vt.c            | con_font_set                   | (struct console_font)->height | 1-32
> drivers/tty/vt/vt.c            | con_font_default               | (struct console_font)->height | 0-u32max
> drivers/tty/vt/selection.c     | paste_selection                | (struct console_font)->height | ignore

I will go through the list starting from these "0-u32max" cases. Thanks
again!

Peilin Ye