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Commit 42fd552e authored by Alan Cox's avatar Alan Cox Committed by Linus Torvalds
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fix serial buffer memory leak 



Patch c5c34d48 (tty: flush flip buffer on
ldisc input queue flush) introduces a race condition which can lead to memory
leaks.

The problem can be triggered when tcflush() is called when data are being
pushed to the line discipline driver by flush_to_ldisc().

flush_to_ldisc() releases tty->buf.lock when calling the line discipline
receive_buf function. At that poing tty_buffer_flush() kicks in and sets both
tty->buf.head and tty->buf.tail to NULL. When flush_to_ldisc() finishes, it
restores tty->buf.head but doesn't touch tty->buf.tail. This corrups the
buffer queue, and the next call to tty_buffer_request_room() will allocate a
new buffer and overwrite tty->buf.head. The previous buffer is then lost
forever without being released.

(Thanks to Laurent for the above text, for finding, disgnosing and reporting
the bug)

- Use tty->flags bits for the flush status.

- Wait for the flag to clear again before returning

- Fix the doc error noted

- Fix flush of empty queue leaving stale flushpending

[akpm@linux-foundation.org: cleanup]
Signed-off-by: default avatarAlan Cox <alan@redhat.com>
Acked-by: default avatarPaul Fulghum <paulkf@microgate.com>
Cc: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f8a74594

drivers/char/tty_io.c

+50 −6
Original line number Diff line number Diff line
@@ -369,25 +369,54 @@ static void tty_buffer_free(struct tty_struct *tty, struct tty_buffer *b) 
}



/**

 *	tty_buffer_flush		-	flush full tty buffers

 *	__tty_buffer_flush		-	flush full tty buffers

 *	@tty: tty to flush

 *

 *	flush all the buffers containing receive data

 *	flush all the buffers containing receive data. Caller must

 *	hold the buffer lock and must have ensured no parallel flush to

 *	ldisc is running.

 *

 *	Locking: none

 *	Locking: Caller must hold tty->buf.lock

 */



static void tty_buffer_flush(struct tty_struct *tty)

static void __tty_buffer_flush(struct tty_struct *tty)

{

	struct tty_buffer *thead;

	unsigned long flags;



	spin_lock_irqsave(&tty->buf.lock, flags);

	while((thead = tty->buf.head) != NULL) {

		tty->buf.head = thead->next;

		tty_buffer_free(tty, thead);

	}

	tty->buf.tail = NULL;

}



/**

 *	tty_buffer_flush		-	flush full tty buffers

 *	@tty: tty to flush

 *

 *	flush all the buffers containing receive data. If the buffer is

 *	being processed by flush_to_ldisc then we defer the processing

 *	to that function

 *

 *	Locking: none

 */



static void tty_buffer_flush(struct tty_struct *tty)

{

	unsigned long flags;

	spin_lock_irqsave(&tty->buf.lock, flags);



	/* If the data is being pushed to the tty layer then we can't

	   process it here. Instead set a flag and the flush_to_ldisc

	   path will process the flush request before it exits */

	if (test_bit(TTY_FLUSHING, &tty->flags)) {

		set_bit(TTY_FLUSHPENDING, &tty->flags);

		spin_unlock_irqrestore(&tty->buf.lock, flags);

		wait_event(tty->read_wait,

				test_bit(TTY_FLUSHPENDING, &tty->flags) == 0);

		return;

	} else

		__tty_buffer_flush(tty);

	spin_unlock_irqrestore(&tty->buf.lock, flags);

}
@@ -3594,6 +3623,7 @@ static void flush_to_ldisc(struct work_struct *work) 
		return;



	spin_lock_irqsave(&tty->buf.lock, flags);

	set_bit(TTY_FLUSHING, &tty->flags);	/* So we know a flush is running */

	head = tty->buf.head;

	if (head != NULL) {

		tty->buf.head = NULL;
@@ -3607,6 +3637,11 @@ static void flush_to_ldisc(struct work_struct *work) 
				tty_buffer_free(tty, tbuf);

				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(TTY_FLUSHPENDING, &tty->flags))

				break;

			if (!tty->receive_room) {

				schedule_delayed_work(&tty->buf.work, 1);

				break;
@@ -3620,8 +3655,17 @@ static void flush_to_ldisc(struct work_struct *work) 
			disc->receive_buf(tty, char_buf, flag_buf, count);

			spin_lock_irqsave(&tty->buf.lock, flags);

		}

		/* Restore the queue head */

		tty->buf.head = head;

	}

	/* 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(TTY_FLUSHPENDING, &tty->flags)) {

		__tty_buffer_flush(tty);

		clear_bit(TTY_FLUSHPENDING, &tty->flags);

		wake_up(&tty->read_wait);

	}

	clear_bit(TTY_FLUSHING, &tty->flags);

	spin_unlock_irqrestore(&tty->buf.lock, flags);



	tty_ldisc_deref(disc);

include/linux/tty.h

+2 −0
Original line number Diff line number Diff line
@@ -274,6 +274,8 @@ struct tty_struct { 
#define TTY_PTY_LOCK 		16	/* pty private */

#define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */

#define TTY_HUPPED 		18	/* Post driver->hangup() */

#define TTY_FLUSHING		19	/* Flushing to ldisc in progress */

#define TTY_FLUSHPENDING	20	/* Queued buffer flush pending */



#define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))