Re: [PATCH 3.18 34/59] tty: Prevent ldisc drivers from re-using stale tty fields

[Alan Cox]

On Tue, 23 May 2017 22:10:02 +0200
Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

> 3.18-stable review patch.  If anyone has any objections, please let me know.
> 

Thiis is a patch designed to cause a crash in order to stop future errors
occurring. It seems less than ideal as a stable candidate.

> ------------------
> 
> From: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> 
> commit dd42bf1197144ede075a9d4793123f7689e164bc upstream.
> 
> Line discipline drivers may mistakenly misuse ldisc-related fields
> when initializing. For example, a failure to initialize tty->receive_room
> in the N_GIGASET_M101 line discipline was recently found and fixed [1].
> Now, the N_X25 line discipline has been discovered accessing the previous
> line discipline's already-freed private data [2].
> 
> Harden the ldisc interface against misuse by initializing revelant
> tty fields before instancing the new line discipline.
> 
> [1]
>     commit fd98e9419d8d622a4de91f76b306af6aa627aa9c
>     Author: Tilman Schmidt <tilman@xxxxxxx>
>     Date:   Tue Jul 14 00:37:13 2015 +0200
> 
>     isdn/gigaset: reset tty->receive_room when attaching ser_gigaset
> 
> [2] Report from Sasha Levin <sasha.levin@xxxxxxxxxx>
>     [  634.336761] ==================================================================
>     [  634.338226] BUG: KASAN: use-after-free in x25_asy_open_tty+0x13d/0x490 at addr ffff8800a743efd0
>     [  634.339558] Read of size 4 by task syzkaller_execu/8981
>     [  634.340359] =============================================================================
>     [  634.341598] BUG kmalloc-512 (Not tainted): kasan: bad access detected
>     ...
>     [  634.405018] Call Trace:
>     [  634.405277] dump_stack (lib/dump_stack.c:52)
>     [  634.405775] print_trailer (mm/slub.c:655)
>     [  634.406361] object_err (mm/slub.c:662)
>     [  634.406824] kasan_report_error (mm/kasan/report.c:138 mm/kasan/report.c:236)
>     [  634.409581] __asan_report_load4_noabort (mm/kasan/report.c:279)
>     [  634.411355] x25_asy_open_tty (drivers/net/wan/x25_asy.c:559 (discriminator 1))
>     [  634.413997] tty_ldisc_open.isra.2 (drivers/tty/tty_ldisc.c:447)
>     [  634.414549] tty_set_ldisc (drivers/tty/tty_ldisc.c:567)
>     [  634.415057] tty_ioctl (drivers/tty/tty_io.c:2646 drivers/tty/tty_io.c:2879)
>     [  634.423524] do_vfs_ioctl (fs/ioctl.c:43 fs/ioctl.c:607)
>     [  634.427491] SyS_ioctl (fs/ioctl.c:622 fs/ioctl.c:613)
>     [  634.427945] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:188)
> 
> Cc: Tilman Schmidt <tilman@xxxxxxx>
> Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Amit Pundir <amit.pundir@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> ---
>  drivers/tty/tty_ldisc.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -414,6 +414,10 @@ EXPORT_SYMBOL_GPL(tty_ldisc_flush);
>   *	they are not on hot paths so a little discipline won't do
>   *	any harm.
>   *
> + *	The line discipline-related tty_struct fields are reset to
> + *	prevent the ldisc driver from re-using stale information for
> + *	the new ldisc instance.
> + *
>   *	Locking: takes termios_rwsem
>   */
>  
> @@ -422,6 +426,9 @@ static void tty_set_termios_ldisc(struct
>  	down_write(&tty->termios_rwsem);
>  	tty->termios.c_line = num;
>  	up_write(&tty->termios_rwsem);
> +
> +	tty->disc_data = NULL;
> +	tty->receive_room = 0;
>  }
>  
>  /**
>