linux-kernel - Re: [PATCH] staging: greybus: fix a possible data-inconsistency due to data race in get_serial

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Date:   Mon, 3 Jul 2023 15:23:53 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tuo Li <islituo@...il.com>
Cc:     dtwlin@...il.com, johan@...nel.org, elder@...nel.org,
        greybus-dev@...ts.linaro.org, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org, baijiaju1990@...look.com,
        BassCheck <bass@...a.edu.cn>
Subject: Re: [PATCH] staging: greybus: fix a possible data-inconsistency due
 to data race in get_serial_info()

On Mon, Jun 26, 2023 at 04:43:39PM +0800, Tuo Li wrote:
> The variables gb_tty->port.close_delay and gb_tty->port.closing_wait are
> ofter accessed together while holding the lock gb_tty->port.mutex. Here is 
> an example in set_serial_info():
> 
>   mutex_lock(&gb_tty->port.mutex);
>   ...
>   gb_tty->port.close_delay = close_delay;
>   gb_tty->port.closing_wait = closing_wait;
>   ...
>   mutex_unlock(&gb_tty->port.mutex);
> 
> However, they are accessed without holding the lock gb_tty->port.mutex when
> are accessed in get_serial_info():
> 
>   ss->close_delay = jiffies_to_msecs(gb_tty->port.close_delay) / 10;
>   ss->closing_wait =
>     gb_tty->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
> 	ASYNC_CLOSING_WAIT_NONE :
> 	jiffies_to_msecs(gb_tty->port.closing_wait) / 10;
> 
> In my opinion, this may be a harmful race, because ss->close_delay can be
> inconsistent with ss->closing_wait if gb_tty->port.close_delay and 
> gb_tty->port.closing_wait are updated by another thread after the 
> assignment to ss->close_delay.

And how can that happen?

Also you have trailing whitespace in your changelog text :(

> Besides, the select operator may return wrong value if 
> gb_tty->port.closing_wait is updated right after the condition is 
> calculated.
> 
> To fix this possible data-inconsistency caused by data race, a lock and 
> unlock pair is added when accessing different fields of gb_tty->port.
> 
> Reported-by: BassCheck <bass@...a.edu.cn>

As per the documentation for research tools like this, you need to
explain how this was tested.

thanks,

greg k-h