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Date: Fri, 4 Sep 2015 15:34:13 -0400
From: Peter Hurley <peter@...leysoftware.com>
To: Dmitry Vyukov <dvyukov@...gle.com>, gregkh@...uxfoundation.org
Cc: jslaby@...e.com, linux-kernel@...r.kernel.org, jslaby@...e.cz,
andreyknvl@...gle.com, kcc@...gle.com, glider@...gle.com,
paulmck@...ux.vnet.ibm.com, hboehm@...gle.com
Subject: Re: [PATCH] tty: fix data races on tty_buffer.commit
Hi Dmitry,
On 09/04/2015 03:09 PM, Dmitry Vyukov wrote:
> Race on buffer data happens in the following scenario:
> __tty_buffer_request_room does a plain write of tail->commit,
> no barriers were executed before that.
> At this point flush_to_ldisc reads this new value of commit,
> and reads buffer data, no barriers in between.
> The committed buffer data is not necessary visible to flush_to_ldisc.
Please submit one patch for each "fix", because it is not possible
to review what you believe you're fixing.
See below for an example.
> Similar bug happens when tty_schedule_flip commits data.
>
> Another race happens in tty_buffer_flush. It uses plain reads
> to read tty_buffer.next, as the result it can free a buffer
> which has pending writes in __tty_buffer_request_room thread.
> For example, tty_buffer_flush calls tty_buffer_free which
> reads b->size, the size may not be visible to this thread.
> As the result a large buffer can hang in the freelist.
>
> Update commit with smp_store_release and read commit with
> smp_load_acquire, as it is commit that signals data readiness.
> This is orthogonal to the existing synchronization on tty_buffer.next,
> which is required to not dismiss a buffer with unconsumed data.
>
> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> ---
> drivers/tty/tty_buffer.c | 38 ++++++++++++++++++++++++--------------
> 1 file changed, 24 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 4cf263d..4fae5d1 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -89,7 +89,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
> struct tty_bufhead *buf = &port->buf;
> int restart;
>
> - restart = buf->head->commit != buf->head->read;
> + restart = READ_ONCE(buf->head->commit) != buf->head->read;
>
> atomic_dec(&buf->priority);
> mutex_unlock(&buf->lock);
> @@ -242,11 +242,14 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
> atomic_inc(&buf->priority);
>
> mutex_lock(&buf->lock);
> - while ((next = buf->head->next) != NULL) {
> + /* paired with smp_store_release in __tty_buffer_request_room();
> + * ensures there are no outstanding writes to buf->head when we free it
> + */
> + while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
> tty_buffer_free(port, buf->head);
> buf->head = next;
> }
> - buf->head->read = buf->head->commit;
> + buf->head->read = READ_ONCE(buf->head->commit);
>
> if (ld && ld->ops->flush_buffer)
> ld->ops->flush_buffer(tty);
> @@ -290,13 +293,15 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
> if (n != NULL) {
> n->flags = flags;
> buf->tail = n;
> - b->commit = b->used;
> - /* paired w/ barrier in flush_to_ldisc(); ensures the
> - * latest commit value can be read before the head is
> - * advanced to the next buffer
> + /* paired with smp_load_acquire in flush_to_ldisc();
> + * ensures flush_to_ldisc() sees buffer data.
> */
> - smp_wmb();
> - b->next = n;
> + smp_store_release(&b->commit, b->used);
> + /* paired with smp_load_acquire in flush_to_ldisc();
> + * ensures the latest commit value can be read before
> + * the head is advanced to the next buffer
> + */
> + smp_store_release(&b->next, n);
> } else if (change)
> size = 0;
> else
> @@ -394,7 +399,10 @@ void tty_schedule_flip(struct tty_port *port)
> {
> struct tty_bufhead *buf = &port->buf;
>
> - buf->tail->commit = buf->tail->used;
> + /* paired with smp_load_acquire in flush_to_ldisc(); ensures the
> + * committed data is visible to flush_to_ldisc()
> + */
> + smp_store_release(&buf->tail->commit, buf->tail->used);
> schedule_work(&buf->work);
schedule_work() is an implied barrier for obvious reasons.
Regards.
Peter Hurley
> }
> EXPORT_SYMBOL(tty_schedule_flip);
> @@ -488,13 +496,15 @@ static void flush_to_ldisc(struct work_struct *work)
> if (atomic_read(&buf->priority))
> break;
>
> - next = head->next;
> - /* paired w/ barrier in __tty_buffer_request_room();
> + /* paired with smp_store_release in __tty_buffer_request_room();
> * ensures commit value read is not stale if the head
> * is advancing to the next buffer
> */
> - smp_rmb();
> - count = head->commit - head->read;
> + next = smp_load_acquire(&head->next);
> + /* paired with smp_store_release in __tty_buffer_request_room();
> + * ensures we see the committed buffer data
> + */
> + count = smp_load_acquire(&head->commit) - head->read;
> if (!count) {
> if (next == NULL) {
> check_other_closed(tty);
>
--
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