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SubjectRe: [PATCH v3] pty: Fix input race when closing
On 04/13/2015 01:24 PM, Peter Hurley wrote:
> 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>
> ---
>
> v3: Fix race w/ pty_open()
> Fix ordering of TTY_OTHER_DONE wrt head index

Please let me know if anyone needs the analysis of how this patch
fixes these two problems (and whether I should include that analysis
in the already-too-long commit log).

Regards,
Peter Hurley

>
> v2: Clear TTY_OTHER_DONE on pty re-open
>
> Documentation/serial/tty.txt | 3 +++
> drivers/tty/n_hdlc.c | 4 ++--
> drivers/tty/n_tty.c | 22 ++++++++++++++++++----
> drivers/tty/pty.c | 5 +++--
> drivers/tty/tty_buffer.c | 41 +++++++++++++++++++++++++++--------------
> include/linux/tty.h | 2 +-
> 6 files changed, 54 insertions(+), 23 deletions(-)
>
> diff --git a/Documentation/serial/tty.txt b/Documentation/serial/tty.txt
> index 1e52d67..dbe6623 100644
> --- a/Documentation/serial/tty.txt
> +++ b/Documentation/serial/tty.txt
> @@ -198,6 +198,9 @@ 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 644ddb8..bbc4ce6 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 54da8f4..85e4642 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1946,6 +1946,18 @@ 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
> @@ -2164,7 +2176,7 @@ 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;
> @@ -2232,8 +2244,10 @@ 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;
> }
> @@ -2440,12 +2454,12 @@ 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 6fffb53..4d5937c 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -59,9 +59,8 @@ 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);
> @@ -249,7 +248,9 @@ 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 7566164..2f78b77 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
> *
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index f9fbdf1..0f29f31 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -339,6 +339,7 @@ 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 */
> @@ -462,7 +463,6 @@ extern int tty_hung_up_p(struct file *filp);
> extern void do_SAK(struct tty_struct *tty);
> extern void __do_SAK(struct tty_struct *tty);
> extern void no_tty(void);
> -extern void tty_flush_to_ldisc(struct tty_struct *tty);
> extern void tty_buffer_free_all(struct tty_port *port);
> extern void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld);
> extern void tty_buffer_init(struct tty_port *port);
>



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