Commit 39f610e4 authored Mar 20, 2013 by Peter Hurley Committed by Greg Kroah-Hartman Apr 09, 2013

tty: Fix race condition if flushing tty flip buffers



As Ilya Zykov identified in his patch 'PROBLEM: Race condition in
tty buffer's function flush_to_ldisc()', a race condition exists
which allows a parallel flush_to_ldisc() to flush and free the tty
flip buffers while those buffers are in-use. For example,

  CPU 0                         |  CPU 1                                  |  CPU 2
                                | flush_to_ldisc()                        |
                                |  grab spin lock                         |
tty_buffer_flush()              |                                         | flush_to_ldisc()
 wait for spin lock             |                                         |  wait for spin lock
                                |   if (!test_and_set_bit(TTYP_FLUSHING)) |
                                |    while (next flip buffer)             |
                                |     ...                                 |
                                |     drop spin lock                      |
  grab spin lock                |                                         |
   if (test_bit(TTYP_FLUSHING)) |                                         |
    set_bit(TTYP_FLUSHPENDING)  |      receive_buf()                      |
    drop spin lock              |                                         |
                                |                                         |   grab spin lock
                                |                                         |    if (!test_and_set_bit(TTYP_FLUSHING))
                                |                                         |    if (test_bit(TTYP_FLUSHPENDING))
                                |                                         |    __tty_buffer_flush()

CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is
transferring data from the head flip buffer.

The original patch was rejected under the assumption that parallel
flush_to_ldisc() was not possible. Because of necessary changes to
the workqueue api, work items can execute in parallel on SMP.

This patch differs slightly from the original patch by testing for
a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING
can only be set while the lock is dropped around receive_buf().

Reported-by: Ilya Zykov <linux@izyk.ru>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Ilya Zykov <linux@izyk.ru>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 31bdfc64

drivers/tty/tty_buffer.c

+10 −12

Original line number	Diff line number	Diff line
		@@ -449,11 +449,6 @@ static void flush_to_ldisc(struct work_struct *work)
		tty_buffer_free(port, head);
		continue;
		}
		/* Ldisc or user is trying to flush the buffers
		we are feeding to the ldisc, stop feeding the
		line discipline as we want to empty the queue */
		if (test_bit(TTYP_FLUSHPENDING, &port->iflags))
		break;
		if (!tty->receive_room)
		break;
		if (count > tty->receive_room)
		@@ -465,17 +460,20 @@ static void flush_to_ldisc(struct work_struct *work)
		disc->ops->receive_buf(tty, char_buf,
		flag_buf, count);
		spin_lock_irqsave(&buf->lock, flags);
		}
		clear_bit(TTYP_FLUSHING, &port->iflags);
		}

		/* We may have a deferred request to flush the input buffer,
		if so pull the chain under the lock and empty the queue */
		/* Ldisc or user is trying to flush the buffers.
		We may have a deferred request to flush the
		input buffer, if so pull the chain under the lock
		and empty the queue */
		if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
		__tty_buffer_flush(port);
		clear_bit(TTYP_FLUSHPENDING, &port->iflags);
		wake_up(&tty->read_wait);
		break;
		}
		}
		clear_bit(TTYP_FLUSHING, &port->iflags);
		}

		spin_unlock_irqrestore(&buf->lock, flags);

		tty_ldisc_deref(disc);

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