Re: [PATCH] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

Jiri Slaby
2020-03-18T13:15:05Z

On 24. 02. 20, 9:19, Eric Biggers wrote:
> On Mon, Feb 24, 2020 at 09:04:33AM +0100, Jiri Slaby wrote:
>>> KASAN report:
>>> BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
>>> Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129
>>>
>>> CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
>>> Call Trace:
>>> [...]
>>> con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
>>> release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514
>>> tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629
>>> tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789
>>> [...]
>>>
>>> Allocated by task 129:
>>> [...]
>>> kzalloc include/linux/slab.h:669 [inline]
>>> vc_allocate drivers/tty/vt/vt.c:1085 [inline]
>>> vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066
>>> con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229
>>> tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline]
>>> tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341
>>> tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
>>> tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
>>> [...]
>>>
>>> Freed by task 130:
>>> [...]
>>> kfree+0xbf/0x1e0 mm/slab.c:3757
>>> vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline]
>>> vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818
>>> tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
>>
>> That means the associated tty_port is destroyed while the tty layer
>> still has a tty on the top of it. That is a BUG anyway.

...

>> Locking tty_mutex here does not sound quite right. What about switching
>> vc_data to proper refcounting based on tty_port? (Instead of doing
>> tty_port_destroy and kfree in vt_disallocate*.)
>>
>
> How would that work? We could make struct vc_data refcounted such that
> VT_DISALLOCATE doesn't free it right away but rather it's freed in the next
> con_shutdown(). But release_tty() still accesses tty->port afterwards, which is
> part of the 'struct vc_data' that would have just been freed.

Yes, but if it does the same as pty, i.e. throwing tty_port in
->cleanup, not ->shutdown, that would work, right?

The initial requirement from tty_port is that it outlives tty. This was
later lifted by pty to live at least till ->cleanup.

thanks,
--
js
suse labs