Re: [PATCH] staging: Fix atomicity violation in get_serial_info()

[Johan Hovold]

On Mon, Sep 30, 2024 at 06:14:03PM +0800, Qiu-ji Chen wrote:
> Atomicity violation occurs during consecutive reads of the members of 
> gb_tty. Consider a scenario where, because the consecutive reads of gb_tty
> members are not protected by a lock, the value of gb_tty may still be 
> changing during the read process. 
> 
> gb_tty->port.close_delay and gb_tty->port.closing_wait are updated
> together, such as in the set_serial_info() function. If during the
> read process, gb_tty->port.close_delay and gb_tty->port.closing_wait
> are still being updated, it is possible that gb_tty->port.close_delay
> is updated while gb_tty->port.closing_wait is not. In this case,
> the code first reads gb_tty->port.close_delay and then
> gb_tty->port.closing_wait. A new gb_tty->port.close_delay and an
> old gb_tty->port.closing_wait could be read. Such values, whether
> before or after the update, should not coexist as they represent an
> intermediate state.
> 
> This could result in a mismatch of the values read for gb_tty->minor, 

No, gb_tty minor is only set at probe().

> gb_tty->port.close_delay, and gb_tty->port.closing_wait, which in turn 
> could cause ss->close_delay and ss->closing_wait to be mismatched.

Sure, but that's a pretty minor issue as Dan already pointed out.

> To address this issue, we have enclosed all sequential read operations of 
> the gb_tty variable within a lock. This ensures that the value of gb_tty 
> remains unchanged throughout the process, guaranteeing its validity.
> 
> This possible bug is found by an experimental static analysis tool
> developed by our team. This tool analyzes the locking APIs
> to extract function pairs that can be concurrently executed, and then
> analyzes the instructions in the paired functions to identify possible
> concurrency bugs including data races and atomicity violations.
> 
> Fixes: b71e571adaa5 ("staging: greybus: uart: fix TIOCSSERIAL jiffies conversions")

And this obviously isn't the correct commit to blame. Please be more
careful.

> Cc: stable@xxxxxxxxxxxxxxx

Since this is unlikely to cause any issues for a user, I don't think
stable backport is warranted either.

> Signed-off-by: Qiu-ji Chen <chenqiuji666@xxxxxxxxx>
> ---
>  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 cdf4ebb93b10..8cc18590d97b 100644
> --- a/drivers/staging/greybus/uart.c
> +++ b/drivers/staging/greybus/uart.c
> @@ -595,12 +595,14 @@ 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;

gb_tty is not protected by the port mutex.

>  	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;
>  }

Johan