Re: [PATCH] tty: fix data race in flush_to_ldisc

From: Dmitry Vyukov
Date: Wed Sep 02 2015 - 13:55:47 EST


On Wed, Sep 2, 2015 at 5:28 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> On 09/02/2015 10:41 AM, Dmitry Vyukov wrote:
>> On Wed, Sep 2, 2015 at 4:18 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>> On 09/01/2015 04:11 PM, Dmitry Vyukov wrote:
>>>> The data race is found with KernelThreadSanitizer (on rev 21bdb584af8c):
>>>>
>>>> ThreadSanitizer: data-race in release_tty
>>>> Write of size 8 by thread T325 (K2579):
>>>> release_tty+0xf3/0x1c0 drivers/tty/tty_io.c:1688
>>>> tty_release+0x698/0x7c0 drivers/tty/tty_io.c:1920
>>>> __fput+0x15f/0x310 fs/file_table.c:207
>>>> ____fput+0x1d/0x30 fs/file_table.c:243
>>>> task_work_run+0x115/0x130 kernel/task_work.c:123
>>>> do_notify_resume+0x73/0x80
>>>> tracehook_notify_resume include/linux/tracehook.h:190
>>>> do_notify_resume+0x73/0x80 arch/x86/kernel/signal.c:757
>>>> int_signal+0x12/0x17 arch/x86/entry/entry_64.S:326
>>>> Previous read of size 8 by thread T19 (K16):
>>>> flush_to_ldisc+0x29/0x300 drivers/tty/tty_buffer.c:472
>>>> process_one_work+0x47e/0x930 kernel/workqueue.c:2036
>>>> worker_thread+0xb0/0x900 kernel/workqueue.c:2170
>>>> kthread+0x150/0x170 kernel/kthread.c:207
>>>>
>>>> flush_to_ldisc reads port->itty and checks that it is not NULL,
>>>> concurrently release_tty sets port->itty to NULL. It is possible
>>>> that flush_to_ldisc loads port->itty once, ensures that it is
>>>> not NULL, but then reloads it again and uses. The second load
>>>> can already return NULL, which will cause a crash.
>>>>
>>>> Set port->itty to NULL after shutting down flush_to_ldisc work.
>>>>
>>>> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>>> ---
>>>> drivers/tty/tty_io.c | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>>>> index 57fc6ee..0df24c1 100644
>>>> --- a/drivers/tty/tty_io.c
>>>> +++ b/drivers/tty/tty_io.c
>>>> @@ -1684,9 +1684,9 @@ static void release_tty(struct tty_struct *tty, int idx)
>>>> tty_free_termios(tty);
>>>> tty_driver_remove_tty(tty->driver, tty);
>>>> tty->port->itty = NULL;
>>>> + cancel_work_sync(&tty->port->buf.work);
>>>> if (tty->link)
>>>> tty->link->port->itty = NULL;
>>>> - cancel_work_sync(&tty->port->buf.work);
>>>
>>> This patch doesn't do anything.
>>
>> It sets tty->link->port->itty to NULL _after_ waiting for
>> flush_to_ldisc work to finish. At least this is the intention.
>> Can you explain why it does not do anything?
>
> Because tty->link is a different tty than that corresponding
> to the buffer work being cancelled.

Got it.
Please take a look at the updated version of the patch.
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