[PATCH] Fix OpenSSH pty regression on close

From: Brian Bloniarz
Date: Tue Mar 01 2016 - 01:41:14 EST

OpenSSH expects the (non-blocking) read() of pty master to return
EAGAIN only if it has received all of the slave-side output after
it has received SIGCHLD. This used to work on pre-3.12 kernels.

This fix effectively forces non-blocking read() and poll() to
block for parallel i/o to complete for all ttys. It unwinds these
changes:

1) f95499c3030fe1bfad57745f2db1959c5b43dca8
n_tty: Don't wait for buffer work in read() loop

2) f8747d4a466ab2cafe56112c51b3379f9fdb7a12
tty: Fix pty master read() after slave closes

3) 52bce7f8d4fc633c9a9d0646eef58ba6ae9a3b73
pty, n_tty: Simplify input processing on final close

4) 1a48632ffed61352a7810ce089dc5a8bcd505a60
pty: Fix input race when closing

Inspired by analysis and patch from Marc Aurele La France <tsi@xxxxxxxxxx>

Reported-by: Volth <openssh@xxxxxxxxx>
Reported-by: Marc Aurele La France <tsi@xxxxxxxxxx>
BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=52
BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=2492
Signed-off-by: Brian Bloniarz <brian.bloniarz@xxxxxxxxx>
---
Documentation/serial/tty.txt | 3 --
drivers/tty/n_hdlc.c | 4 +--
drivers/tty/n_tty.c | 70 +++++++++++++++++++++-----------------------
drivers/tty/pty.c | 4 +--
drivers/tty/tty_buffer.c | 34 ++++-----------------
include/linux/tty.h | 2 +-
6 files changed, 43 insertions(+), 74 deletions(-)

diff --git a/Documentation/serial/tty.txt b/Documentation/serial/tty.txt
index bc3842d..e2dea3d 100644
--- a/Documentation/serial/tty.txt
+++ b/Documentation/serial/tty.txt
@@ -213,9 +213,6 @@ TTY_IO_ERROR If set, causes all subsequent userspace read/write

TTY_OTHER_CLOSED Device is a pty and the other side has closed.

-TTY_OTHER_DONE Device is a pty and the other side has closed and
- all pending input processing has been completed.
-
TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into
smaller chunks.

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index bbc4ce6..644ddb8 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_DONE, &tty->flags)) {
+ if (test_bit(TTY_OTHER_CLOSED, &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_DONE, &tty->flags))
+ if (test_bit(TTY_OTHER_CLOSED, &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 b280abaa..c12def7 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1963,18 +1963,6 @@ 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
@@ -2170,7 +2158,7 @@ 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, done;
+ int c;
int minimum, time;
ssize_t retval = 0;
long timeout;
@@ -2238,32 +2226,35 @@ 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 (done) {
- retval = -EIO;
- break;
- }
- if (tty_hung_up_p(file))
- break;
- if (!timeout)
- break;
- if (file->f_flags & O_NONBLOCK) {
- retval = -EAGAIN;
- break;
- }
- if (signal_pending(current)) {
- retval = -ERESTARTSYS;
- break;
- }
up_read(&tty->termios_rwsem);
+ tty_buffer_flush_work(tty->port);
+ down_read(&tty->termios_rwsem);
+ if (!input_available_p(tty, 0)) {
+ if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
+ retval = -EIO;
+ break;
+ }
+ if (tty_hung_up_p(file))
+ break;
+ if (!timeout)
+ break;
+ if (file->f_flags & O_NONBLOCK) {
+ retval = -EAGAIN;
+ break;
+ }
+ if (signal_pending(current)) {
+ retval = -ERESTARTSYS;
+ break;
+ }
+ up_read(&tty->termios_rwsem);

- timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
- timeout);
+ timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
+ timeout);

- down_read(&tty->termios_rwsem);
- continue;
+ down_read(&tty->termios_rwsem);
+ continue;
+ }
}

if (ldata->icanon && !L_EXTPROC(tty)) {
@@ -2445,12 +2436,17 @@ 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;
+ else {
+ tty_buffer_flush_work(tty->port);
+ 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 2348fa6..6427a39 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -59,7 +59,7 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
if (!tty->link)
return;
set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
- tty_flip_buffer_push(tty->link->port);
+ wake_up_interruptible(&tty->link->read_wait);
wake_up_interruptible(&tty->link->write_wait);
if (tty->driver->subtype == PTY_TYPE_MASTER) {
set_bit(TTY_OTHER_CLOSED, &tty->flags);
@@ -247,9 +247,7 @@ 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 3cd31e0..fb31eec 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -37,29 +37,6 @@

#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
* tty_buffer_unlock_exclusive - release exclusive access
@@ -254,8 +231,6 @@ 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);
}
@@ -505,10 +480,8 @@ static void flush_to_ldisc(struct work_struct *work)
*/
count = smp_load_acquire(&head->commit) - head->read;
if (!count) {
- if (next == NULL) {
- check_other_closed(tty);
+ if (next == NULL)
break;
- }
buf->head = next;
tty_buffer_free(port, head);
continue;
@@ -597,3 +570,8 @@ bool tty_buffer_cancel_work(struct tty_port *port)
{
return cancel_work_sync(&port->buf.work);
}
+
+void tty_buffer_flush_work(struct tty_port *port)
+{
+ flush_work(&port->buf.work);
+}
diff --git a/include/linux/tty.h b/include/linux/tty.h
index d9fb4b0..663155b 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -338,7 +338,6 @@ struct tty_file_private {
#define TTY_EXCLUSIVE 3 /* Exclusive open mode */
#define TTY_DEBUG 4 /* Debugging */
#define TTY_DO_WRITE_WAKEUP 5 /* Call write_wakeup after queuing new */
-#define TTY_OTHER_DONE 6 /* Closed pty has completed input processing */
#define TTY_LDISC_OPEN 11 /* Line discipline is open */
#define TTY_PTY_LOCK 16 /* pty private */
#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
@@ -464,6 +463,7 @@ extern void tty_buffer_init(struct tty_port *port);
extern void tty_buffer_set_lock_subclass(struct tty_port *port);
extern bool tty_buffer_restart_work(struct tty_port *port);
extern bool tty_buffer_cancel_work(struct tty_port *port);
+extern void tty_buffer_flush_work(struct tty_port *port);
extern speed_t tty_termios_baud_rate(struct ktermios *termios);
extern speed_t tty_termios_input_baud_rate(struct ktermios *termios);
extern void tty_termios_encode_baud_rate(struct ktermios *termios,
--
2.5.0