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    Subject[PATCH 3.10 03/48] n_tty: Fix n_tty_write crash when echoing in raw mode
    Date
    3.10-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Peter Hurley <peter@hurleysoftware.com>

    commit 4291086b1f081b869c6d79e5b7441633dc3ace00 upstream.

    The tty atomic_write_lock does not provide an exclusion guarantee for
    the tty driver if the termios settings are LECHO & !OPOST. And since
    it is unexpected and not allowed to call TTY buffer helpers like
    tty_insert_flip_string concurrently, this may lead to crashes when
    concurrect writers call pty_write. In that case the following two
    writers:
    * the ECHOing from a workqueue and
    * pty_write from the process
    race and can overflow the corresponding TTY buffer like follows.

    If we look into tty_insert_flip_string_fixed_flag, there is:
    int space = __tty_buffer_request_room(port, goal, flags);
    struct tty_buffer *tb = port->buf.tail;
    ...
    memcpy(char_buf_ptr(tb, tb->used), chars, space);
    ...
    tb->used += space;

    so the race of the two can result in something like this:
    A B
    __tty_buffer_request_room
    __tty_buffer_request_room
    memcpy(buf(tb->used), ...)
    tb->used += space;
    memcpy(buf(tb->used), ...) ->BOOM

    B's memcpy is past the tty_buffer due to the previous A's tb->used
    increment.

    Since the N_TTY line discipline input processing can output
    concurrently with a tty write, obtain the N_TTY ldisc output_lock to
    serialize echo output with normal tty writes. This ensures the tty
    buffer helper tty_insert_flip_string is not called concurrently and
    everything is fine.

    Note that this is nicely reproducible by an ordinary user using
    forkpty and some setup around that (raw termios + ECHO). And it is
    present in kernels at least after commit
    d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty layer to
    use the normal buffering logic) in 2.6.31-rc3.

    js: add more info to the commit log
    js: switch to bool
    js: lock unconditionally
    js: lock only the tty->ops->write call

    References: CVE-2014-0196
    Reported-and-tested-by: Jiri Slaby <jslaby@suse.cz>
    Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/n_tty.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/drivers/tty/n_tty.c
    +++ b/drivers/tty/n_tty.c
    @@ -2066,8 +2066,12 @@ static ssize_t n_tty_write(struct tty_st
    if (tty->ops->flush_chars)
    tty->ops->flush_chars(tty);
    } else {
    + struct n_tty_data *ldata = tty->disc_data;
    +
    while (nr > 0) {
    + mutex_lock(&ldata->output_lock);
    c = tty->ops->write(tty, b, nr);
    + mutex_unlock(&ldata->output_lock);
    if (c < 0) {
    retval = c;
    goto break_out;



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