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Date: Fri, 4 Sep 2015 21:37:13 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Peter Hurley <peter@...leysoftware.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, jslaby@...e.com,
LKML <linux-kernel@...r.kernel.org>, Jiri Slaby <jslaby@...e.cz>,
Andrey Konovalov <andreyknvl@...gle.com>,
Kostya Serebryany <kcc@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
Hans Boehm <hboehm@...gle.com>
Subject: Re: [PATCH] tty: fix data races on tty_buffer.commit
On Fri, Sep 4, 2015 at 9:34 PM, Peter Hurley <peter@...leysoftware.com> wrote:
> 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.
OK, I will split.
To answer this particular question: you need release/write barrier
_before_ the synchronizing store, not _after_. Once the store to
commit happened, another thread can start reading buffer data, this
thread has not yet executed schedule_work at this point.
>> }
>> 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);
>>
>
--
Dmitry Vyukov, Software Engineer, dvyukov@...gle.com
Google Germany GmbH, Dienerstraße 12, 80331, München
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