pty: Fix input race when closing

A read() from a pty master may mistakenly indicate EOF (errno == -EIO)
after the pty slave has closed, even though input data remains to be read.
For example,

       pty slave       |        input worker        |    pty master
                       |                            |
                       |                            |   n_tty_read()
pty_write()            |                            |     input avail? no
  add data             |                            |     sleep
  schedule worker  --->|                            |     .
                       |---> flush_to_ldisc()       |     .
pty_close()            |       fill read buffer     |     .
  wait for worker      |       wakeup reader    --->|     .
                       |       read buffer full?    |---> input avail ? yes
                       |<---   yes - exit worker    |     copy 4096 bytes to user
  TTY_OTHER_CLOSED <---|                            |<--- kick worker
                       |                            |

		                **** New read() before worker starts ****

                       |                            |   n_tty_read()
                       |                            |     input avail? no
                       |                            |     TTY_OTHER_CLOSED? yes
                       |                            |     return -EIO

Several conditions are required to trigger this race:
1. the ldisc read buffer must become full so the input worker exits
2. the read() count parameter must be >= 4096 so the ldisc read buffer
   is empty
3. the subsequent read() occurs before the kicked worker has processed
   more input

However, the underlying cause of the race is that data is pipelined, while
tty state is not; ie., data already written by the pty slave end is not
yet visible to the pty master end, but state changes by the pty slave end
are visible to the pty master end immediately.

Pipeline the TTY_OTHER_CLOSED state through input worker to the reader.
1. Introduce TTY_OTHER_DONE which is set by the input worker when
   TTY_OTHER_CLOSED is set and either the input buffers are flushed or
   input processing has completed. Readers/polls are woken when
   TTY_OTHER_DONE is set.
2. Reader/poll checks TTY_OTHER_DONE instead of TTY_OTHER_CLOSED.
3. A new input worker is started from pty_close() after setting
   TTY_OTHER_CLOSED, which ensures the TTY_OTHER_DONE state will be
   set if the last input worker is already finished (or just about to
   exit).

Remove tty_flush_to_ldisc(); no in-tree callers.

Fixes: 52bce7f8d4fc ("pty, n_tty: Simplify input processing on final close")
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96311
BugLink: http://bugs.launchpad.net/bugs/1429756
Cc: <stable@vger.kernel.org> # 3.19+
Reported-by: Andy Whitcroft <apw@canonical.com>
Reported-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 50 insertions, 22 deletions

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 644ddb841d9f..bbc4ce66c2c1 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -600,7 +600,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 	add_wait_queue(&tty->read_wait, &wait);
 
 	for (;;) {
-		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
+		if (test_bit(TTY_OTHER_DONE, &tty->flags)) {
 			ret = -EIO;
 			break;
 		}
@@ -828,7 +828,7 @@ static unsigned int n_hdlc_tty_poll(struct tty_struct *tty, struct file *filp,
 		/* set bits for operations that won't block */
 		if (n_hdlc->rx_buf_list.head)
 			mask |= POLLIN | POLLRDNORM;	/* readable */
-		if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
+		if (test_bit(TTY_OTHER_DONE, &tty->flags))
 			mask |= POLLHUP;
 		if (tty_hung_up_p(filp))
 			mask |= POLLHUP;
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index cf6e0f2e1331..cc57a3a6b02b 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1949,6 +1949,18 @@ static inline int input_available_p(struct tty_struct *tty, int poll)
 		return ldata->commit_head - ldata->read_tail >= amt;
 }
 
+static inline int check_other_done(struct tty_struct *tty)
+{
+	int done = test_bit(TTY_OTHER_DONE, &tty->flags);
+	if (done) {
+		/* paired with cmpxchg() in check_other_closed(); ensures
+		 * read buffer head index is not stale
+		 */
+		smp_mb__after_atomic();
+	}
+	return done;
+}
+
 /**
  *	copy_from_read_buf	-	copy read data directly
  *	@tty: terminal device
@@ -2167,7 +2179,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 	struct n_tty_data *ldata = tty->disc_data;
 	unsigned char __user *b = buf;
 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
-	int c;
+	int c, done;
 	int minimum, time;
 	ssize_t retval = 0;
 	long timeout;
@@ -2235,8 +2247,10 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 		    ((minimum - (b - buf)) >= 1))
 			ldata->minimum_to_wake = (minimum - (b - buf));
 
+		done = check_other_done(tty);
+
 		if (!input_available_p(tty, 0)) {
-			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
+			if (done) {
 				retval = -EIO;
 				break;
 			}
@@ -2443,12 +2457,12 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
 
 	poll_wait(file, &tty->read_wait, wait);
 	poll_wait(file, &tty->write_wait, wait);
+	if (check_other_done(tty))
+		mask |= POLLHUP;
 	if (input_available_p(tty, 1))
 		mask |= POLLIN | POLLRDNORM;
 	if (tty->packet && tty->link->ctrl_status)
 		mask |= POLLPRI | POLLIN | POLLRDNORM;
-	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
-		mask |= POLLHUP;
 	if (tty_hung_up_p(file))
 		mask |= POLLHUP;
 	if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index e72ee629cead..4d5e8409769c 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -53,9 +53,8 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
 	/* Review - krefs on tty_link ?? */
 	if (!tty->link)
 		return;
-	tty_flush_to_ldisc(tty->link);
 	set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
-	wake_up_interruptible(&tty->link->read_wait);
+	tty_flip_buffer_push(tty->link->port);
 	wake_up_interruptible(&tty->link->write_wait);
 	if (tty->driver->subtype == PTY_TYPE_MASTER) {
 		set_bit(TTY_OTHER_CLOSED, &tty->flags);
@@ -243,7 +242,9 @@ static int pty_open(struct tty_struct *tty, struct file *filp)
 		goto out;
 
 	clear_bit(TTY_IO_ERROR, &tty->flags);
+	/* TTY_OTHER_CLOSED must be cleared before TTY_OTHER_DONE */
 	clear_bit(TTY_OTHER_CLOSED, &tty->link->flags);
+	clear_bit(TTY_OTHER_DONE, &tty->link->flags);
 	set_bit(TTY_THROTTLED, &tty->flags);
 	return 0;
 
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 75661641f5fe..2f78b77f0f81 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -37,6 +37,28 @@
 
 #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
 
+/*
+ * If all tty flip buffers have been processed by flush_to_ldisc() or
+ * dropped by tty_buffer_flush(), check if the linked pty has been closed.
+ * If so, wake the reader/poll to process
+ */
+static inline void check_other_closed(struct tty_struct *tty)
+{
+	unsigned long flags, old;
+
+	/* transition from TTY_OTHER_CLOSED => TTY_OTHER_DONE must be atomic */
+	for (flags = ACCESS_ONCE(tty->flags);
+	     test_bit(TTY_OTHER_CLOSED, &flags);
+	     ) {
+		old = flags;
+		__set_bit(TTY_OTHER_DONE, &flags);
+		flags = cmpxchg(&tty->flags, old, flags);
+		if (old == flags) {
+			wake_up_interruptible(&tty->read_wait);
+			break;
+		}
+	}
+}
 
 /**
  *	tty_buffer_lock_exclusive	-	gain exclusive access to buffer
@@ -229,6 +251,8 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
 	if (ld && ld->ops->flush_buffer)
 		ld->ops->flush_buffer(tty);
 
+	check_other_closed(tty);
+
 	atomic_dec(&buf->priority);
 	mutex_unlock(&buf->lock);
 }
@@ -471,8 +495,10 @@ static void flush_to_ldisc(struct work_struct *work)
 		smp_rmb();
 		count = head->commit - head->read;
 		if (!count) {
-			if (next == NULL)
+			if (next == NULL) {
+				check_other_closed(tty);
 				break;
+			}
 			buf->head = next;
 			tty_buffer_free(port, head);
 			continue;
@@ -489,19 +515,6 @@ static void flush_to_ldisc(struct work_struct *work)
 }
 
 /**
- *	tty_flush_to_ldisc
- *	@tty: tty to push
- *
- *	Push the terminal flip buffers to the line discipline.
- *
- *	Must not be called from IRQ context.
- */
-void tty_flush_to_ldisc(struct tty_struct *tty)
-{
-	flush_work(&tty->port->buf.work);
-}
-
-/**
  *	tty_flip_buffer_push	-	terminal
  *	@port: tty port to push
  *