lkml.org 
[lkml]   [2020]   [Jan]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] b43: Fix possible a data race in b43_op_tx()
On Sun, 12 Jan 2020 00:14:55 +0800
Jia-Ju Bai <baijiaju1990@gmail.com> wrote:

> The functions b43_op_tx() and b43_tx_work() may be concurrently executed.
>
> In b43_tx_work(), the variable wl->tx_queue_stopped[queue_num] is
> accessed with holding a mutex lock wl->mutex. But in b43_op_tx(), the
> identical variable wl->tx_queue_stopped[skb->queue_mapping] is accessed
> without holding this mutex lock. Thus, a possible data race may occur.
>
> To fix this data race, in b43_op_tx(), the variable
> wl->tx_queue_stopped[skb->queue_mapping] is accessed with holding the
> mutex lock wl->mutex.
>
> This data race is found by the runtime testing of our tool DILP-2.
>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
> drivers/net/wireless/broadcom/b43/main.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/wireless/broadcom/b43/main.c b/drivers/net/wireless/broadcom/b43/main.c
> index 39da1a4c30ac..adedb38f50f2 100644
> --- a/drivers/net/wireless/broadcom/b43/main.c
> +++ b/drivers/net/wireless/broadcom/b43/main.c
> @@ -3625,6 +3625,11 @@ static void b43_op_tx(struct ieee80211_hw *hw,
> struct sk_buff *skb)
> {
> struct b43_wl *wl = hw_to_b43_wl(hw);
> + bool stopped;
> +
> + mutex_lock(&wl->mutex);
> + stopped = wl->tx_queue_stopped[skb->queue_mapping];
> + mutex_unlock(&wl->mutex);
>
> if (unlikely(skb->len < 2 + 2 + 6)) {
> /* Too short, this can't be a valid frame. */
> @@ -3634,7 +3639,7 @@ static void b43_op_tx(struct ieee80211_hw *hw,
> B43_WARN_ON(skb_shinfo(skb)->nr_frags);
>
> skb_queue_tail(&wl->tx_queue[skb->queue_mapping], skb);
> - if (!wl->tx_queue_stopped[skb->queue_mapping]) {
> + if (!stopped) {
> ieee80211_queue_work(wl->hw, &wl->tx_work);
> } else {
> ieee80211_stop_queue(wl->hw, skb->queue_mapping);

Hi,

thanks for your patch.

Unfortunately it is not possible to acquire a sleeping mutex in the tx
op:

/**
* struct ieee80211_ops - callbacks from mac80211 to the driver
*
* @tx: Handler that 802.11 module calls for each transmitted frame.
* skb contains the buffer starting from the IEEE 802.11 header.
* The low-level driver should send the frame out based on
* configuration in the TX control data. This handler should,
* preferably, never fail and stop queues appropriately.
* Must be atomic.
^^^^^^^^^^^^^^

I also don't think that the change really fixes any race.
The variable tx_queue_stopped is a boolean. Reading that under mutex
and then doing the actual action based on a copy does not really change
anything. The other end may just set it to false after mutex_unlock,
but before the queue_work. Thus this change does probably even increase
the race window size.

The other thing to consider is:
What can actually go wrong, if the race happens?
I currently don't see any fatal behavior.
A packet might still make it into the queue, although it has already
been stopped. But that's not fatal.

--
Michael
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2020-01-11 18:51    [W:0.038 / U:21.224 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site