• Peter Hurley's avatar
    n_tty: Fix n_tty_write crash when echoing in raw mode · 664c0fc6
    Peter Hurley authored
    commit 4291086b 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
    d945cb9c (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: default avatarJiri Slaby <jslaby@suse.cz>
    Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
    Signed-off-by: default avatarJiri Slaby <jslaby@suse.cz>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    [bwh: Backported to 3.2: output_lock is a member of struct tty_struct]
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    664c0fc6
n_tty.c 52.1 KB