Re: [PATCH 2/2] vt: selection, close sel_buffer race

[Greg KH]

On Thu, Feb 13, 2020 at 08:09:31AM +0100, Jiri Slaby wrote:
> On 12. 02. 20, 20:59, Greg KH wrote:
> > On Mon, Feb 10, 2020 at 09:11:31AM +0100, Jiri Slaby wrote:
> >> syzkaller reported this UAF:
> >> BUG: KASAN: use-after-free in n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741
> >> Read of size 1 at addr ffff8880089e40e9 by task syz-executor.1/13184
> >>
> >> CPU: 0 PID: 13184 Comm: syz-executor.1 Not tainted 5.4.7 #1
> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> >> Call Trace:
> >> ...
> >>  kasan_report+0xe/0x20 mm/kasan/common.c:634
> >>  n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741
> >>  tty_ldisc_receive_buf+0xac/0x190 drivers/tty/tty_buffer.c:461
> >>  paste_selection+0x297/0x400 drivers/tty/vt/selection.c:372
> >>  tioclinux+0x20d/0x4e0 drivers/tty/vt/vt.c:3044
> >>  vt_ioctl+0x1bcf/0x28d0 drivers/tty/vt/vt_ioctl.c:364
> >>  tty_ioctl+0x525/0x15a0 drivers/tty/tty_io.c:2657
> >>  vfs_ioctl fs/ioctl.c:47 [inline]
> >>
> >> It is due to a race between parallel paste_selection (TIOCL_PASTESEL)
> >> and set_selection_user (TIOCL_SETSEL) invocations. One uses sel_buffer,
> >> while the other frees it and reallocates a new one for another
> >> selection. Add a mutex to close this race.
> >>
> >> The mutex takes care properly of sel_buffer and sel_buffer_lth only. The
> >> other selection global variables (like sel_start, sel_end, and sel_cons)
> >> are protected only in set_selection_user. The other functions need quite
> >> some more work to close the races of the variables there. This is going
> >> to happen later.
> >>
> >> This likely fixes (I am unsure as there is no reproducer provided) bug
> >> 206361 too. It was marked as CVE-2020-8648.
> >>
> >> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
> >> Reported-by: syzbot+59997e8d5cbdc486e6f6@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> References: https://bugzilla.kernel.org/show_bug.cgi?id=206361
> > 
> > This needs patch 1 in order to work properly, right?
> 
> Not necessarily -- the patches fix two different bugs (endless loop in
> kernel vs crash). If you want to apply them in the opposite order, just
> let me know.
> 
> BTW I completely forgot to add Fixes and Cc: stable tags. Both of the
> issues come from 1.x times. (But the crash obviously needs
> SMP/preemption, i.e. 2.x.*.)

No worries, I'll go add cc: stable to them now, thanks.

greg k-h