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SubjectRe: [PATCH 3.18 34/59] tty: Prevent ldisc drivers from re-using stale tty fields
On Tue, 23 May 2017 22:10:02 +0200
Greg Kroah-Hartman <gregkh@linuxfoundation.org> 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@hurleysoftware.com>
>
> 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@imap.cc>
> 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@oracle.com>
> [ 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@imap.cc>
> Cc: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
> Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>
> ---
> 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;
> }
>
> /**
>

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