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

[Greg KH]

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?

thanks,

greg k-h