2023-06-26 09:00:51

by Tuo Li

Subject: [PATCH] staging: greybus: fix a possible data-inconsistency due to data race in get_serial_info()

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.
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 <[email protected]>
Signed-off-by: Tuo Li <[email protected]>
---
drivers/staging/greybus/uart.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 20a34599859f..b8875517ea6a 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -596,12 +596,14 @@ static int get_serial_info(struct tty_struct *tty,
{
struct gb_tty *gb_tty = tty->driver_data;

+ mutex_lock(&gb_tty->port.mutex);
ss->line = gb_tty->minor;
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;
+ mutex_unlock(&gb_tty->port.mutex);

return 0;
}
--
2.34.1

2023-07-03 13:32:16

by Greg Kroah-Hartman

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 <[email protected]>

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

thanks,

greg k-h