Re: [PATCH] cw1200: Revert unnecessary patches that fix unreal use-after-free bugs

Jia-Ju Bai
2021-05-24T01:36:11Z

Thanks for fixing my previous mistake.
The patch looks good.

On 2021/5/22 6:32, Hang Zhang wrote:
> A previous commit 4f68ef64cd7f ("cw1200: Fix concurrency
> use-after-free bugs in cw1200_hw_scan()") tried to fix a seemingly
> use-after-free bug between cw1200_bss_info_changed() and
> cw1200_hw_scan(), where the former frees a sk_buff pointed
> to by frame.skb, and the latter accesses the sk_buff
> pointed to by frame.skb. However, this issue should be a
> false alarm because:
>
> (1) "frame.skb" is not a shared variable between the above
> two functions, because "frame" is a local function variable,
> each of the two functions has its own local "frame" - they
> just happen to have the same variable name.
>
> (2) the sk_buff(s) pointed to by these two "frame.skb" are
> also two different object instances, they are individually
> allocated by different dev_alloc_skb() within the two above
> functions. To free one object instance will not invalidate
> the access of another different one.
>
> Based on these facts, the previous commit should be unnecessary.
> Moreover, it also introduced a missing unlock which was
> addressed in a subsequent commit 51c8d24101c7 ("cw1200: fix missing
> unlock on error in cw1200_hw_scan()"). Now that the
> original use-after-free is unreal, these two commits should
> be reverted. This patch performs the reversion.
>
> Fixes: 4f68ef64cd7f ("cw1200: Fix concurrency use-after-free bugs in cw1200_hw_scan()")
> Fixes: 51c8d24101c7 ("cw1200: fix missing unlock on error in cw1200_hw_scan()")
> Signed-off-by: Hang Zhang <zh.nvgt@gmail.com>
> ---
> drivers/net/wireless/st/cw1200/scan.c | 17 +++++++----------
> 1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/wireless/st/cw1200/scan.c b/drivers/net/wireless/st/cw1200/scan.c
> index 988581cc134b..1f856fbbc0ea 100644
> --- a/drivers/net/wireless/st/cw1200/scan.c
> +++ b/drivers/net/wireless/st/cw1200/scan.c
> @@ -75,30 +75,27 @@ int cw1200_hw_scan(struct ieee80211_hw *hw,
> if (req->n_ssids > WSM_SCAN_MAX_NUM_OF_SSIDS)
> return -EINVAL;
>
> - /* will be unlocked in cw1200_scan_work() */
> - down(&priv->scan.lock);
> - mutex_lock(&priv->conf_mutex);
> -
> frame.skb = ieee80211_probereq_get(hw, priv->vif->addr, NULL, 0,
> req->ie_len);
> - if (!frame.skb) {
> - mutex_unlock(&priv->conf_mutex);
> - up(&priv->scan.lock);
> + if (!frame.skb)
> return -ENOMEM;
> - }
>
> if (req->ie_len)
> skb_put_data(frame.skb, req->ie, req->ie_len);
>
> + /* will be unlocked in cw1200_scan_work() */
> + down(&priv->scan.lock);
> + mutex_lock(&priv->conf_mutex);
> +
> ret = wsm_set_template_frame(priv, &frame);
> if (!ret) {
> /* Host want to be the probe responder. */
> ret = wsm_set_probe_responder(priv, true);
> }
> if (ret) {
> - dev_kfree_skb(frame.skb);
> mutex_unlock(&priv->conf_mutex);
> up(&priv->scan.lock);
> + dev_kfree_skb(frame.skb);
> return ret;
> }
>
> @@ -120,8 +117,8 @@ int cw1200_hw_scan(struct ieee80211_hw *hw,
> ++priv->scan.n_ssids;
> }
>
> - dev_kfree_skb(frame.skb);
> mutex_unlock(&priv->conf_mutex);
> + dev_kfree_skb(frame.skb);
> queue_work(priv->workqueue, &priv->scan.work);
> return 0;
> }

Acked-by: Jia-Ju Bai <baijiaju1990@gmail.com>

Best wishes,
Jia-Ju Bai