linux-kernel - Re: [PATCH] tty: fix data race in tty_buffer

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Message-ID: <CACT4Y+agZDEnAv9jz-7BkZBybFu+nr8EfzFpoDK3DrmgucRDNw@mail.gmail.com>
Date:	Wed, 16 Sep 2015 20:20:44 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Linux kernel mailing list <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 race in tty_buffer_flush

Sorry for the delay. It is starred in my inbox and I plan to resend
the patches. Just did not have time yet.


On Wed, Sep 16, 2015 at 3:38 AM, Peter Hurley <peter@...leysoftware.com> wrote:
> On Tue, Sep 8, 2015 at 8:48 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> tty_buffer_flush frees not acquired buffers.
>> As the result, for example, read of b->size in tty_buffer_free
>> can return garbage value which will lead to a huge buffer
>> hanging in the freelist. This is just the benignest
>> manifestation of freeing of a not acquired object.
>> If the object is passed to kfree, heap can be corrupted.
>>
>> Acquire visibility over the buffer before freeing it.
>>
>> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Dmitry,
>
> Looks good. Thanks for splitting this from the other fix.
>
> Like Greg said, the patch needs revision info to help maintainers, et al
> track which is most current/relevant/etc.
>
> Typical format for $subject is something like:
>
> [PATCH v4] tty: fix data race in tty_buffer_flush
>
>
> Notes re: changes from other patch versions are added below the
> git changelog separator (example below).
>
>> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
>> ---
>
> v4: Corrected commit log and patch revision notes
>
> v3: Added code comment re: paired smp barrier
>
> v2: Split from 'tty: fix data races on tty_buffer.commit'
>
>>  drivers/tty/tty_buffer.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
>> index 5a3fa89..a2a8cd0 100644
>> --- a/drivers/tty/tty_buffer.c
>> +++ b/drivers/tty/tty_buffer.c
>> @@ -242,7 +242,10 @@ 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 w/ release in __tty_buffer_request_room; ensures there are
>> +        * no pending memory accesses to the freed buffer
>> +        */
>> +       while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
>>                 tty_buffer_free(port, buf->head);
>>                 buf->head = next;
>>         }
>> --
>> 2.5.0.457.gab17608
>>



-- 
Dmitry Vyukov, Software Engineer, dvyukov@...gle.com
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