Messages in this thread | | | Date | Wed, 18 Mar 2020 15:27:04 -0700 | From | Eric Biggers <> | Subject | Re: [PATCH] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console |
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On Wed, Mar 18, 2020 at 02:15:00PM +0100, Jiri Slaby wrote: > 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. >
Yes, it looks like that will work. I didn't know about tty_operations::cleanup(). I'll update the patch.
- Eric
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