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

[Dmitry Vyukov]

On Thu, Sep 3, 2015 at 2:50 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> On 09/02/2015 01:53 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
>
> The stack traces are not really helpful in describing how the race
> occurs; I would leave it out of the changelog.

ok

>> 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.
>>
>> Use READ_ONCE/WRITE_ONCE to read/update port->itty.
>
> See below.
>
>> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> ---
>>  drivers/tty/tty_buffer.c | 2 +-
>>  drivers/tty/tty_io.c     | 2 +-
>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
>> index 4cf263d..1f1031d 100644
>> --- a/drivers/tty/tty_buffer.c
>> +++ b/drivers/tty/tty_buffer.c
>> @@ -469,7 +469,7 @@ static void flush_to_ldisc(struct work_struct *work)
>>       struct tty_struct *tty;
>>       struct tty_ldisc *disc;
>>
>> -     tty = port->itty;
>> +     tty = READ_ONCE(port->itty);
>
> This is fine.
>
>>       if (tty == NULL)
>>               return;
>>
>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>> index 57fc6ee..aad47df 100644
>> --- a/drivers/tty/tty_io.c
>> +++ b/drivers/tty/tty_io.c
>> @@ -1685,7 +1685,7 @@ static void release_tty(struct tty_struct *tty, int idx)
>>       tty_driver_remove_tty(tty->driver, tty);
>>       tty->port->itty = NULL;
>>       if (tty->link)
>> -             tty->link->port->itty = NULL;
>> +             WRITE_ONCE(tty->link->port->itty, NULL);
>
> This isn't doing anything useful.
>
> 1. The compiler can't push the store past the cancel_work_sync() (because the
>    compiler has no visibility into cancel_work_sync()), and,
> 2. There's no effect if the compiler hoists the store higher in the release_tty()
>    because the line discipline has already been closed and killed (so the
>    tty_ldisc_ref() in flush_to_ldisc() returns NULL anyway).

OK, let me do one try at convincing you that WRITE_ONCE here is a good
idea. If you are not convinced then I will remove it.

WRITE_ONCE/READ_ONCE for all shared memory accesses:
1. Make the code more readable but highlighting important aspects.
2. Required by relevant standards and relieve you, me and everybody
else reading this code from spending time on proving that it cannot
break (think of multi-file compilation mode, store tearing which
compilers indeed known to do in some contexts, and compiler
transformations that we don't know of).
3. Allow tooling that finds undoubtedly harmful bugs, like this one,
or in fact, I've just mailed another fix for missed memory barriers in
this file (also found by KTSAN).

I've described these aspects in more detail here:
https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE

I don't see any negative aspects to it. Do you see any? Because if you
see at least some value in at least on these points and don't see any
negative aspects, then it is worth doing.


-- 
Dmitry Vyukov, Software Engineer, dvyukov@xxxxxxxxxx
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