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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 26 Jul 2022 12:07:16 +0200
Message-ID: <CANn89iLN27NWA7Stkr4ODp6V-Q-3em0dJ2JixDMNcNY7Ap5muA@mail.gmail.com>
Subject: Re: [PATCH] net: Fix UAF in ieee80211_scan_rx()
To:     Siddh Raman Pant <code@siddh.me>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
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On Fri, Jul 1, 2022 at 4:55 PM Siddh Raman Pant <code@siddh.me> wrote:
>
> ieee80211_scan_rx() tries to access scan_req->flags after a null check
> (see line 303 of mac80211/scan.c), but ___cfg80211_scan_done() uses
> kfree() on the scan_req (see line 991 of wireless/scan.c).
>
> This results in a UAF.
>
> ieee80211_scan_rx() is called inside a RCU read-critical section
> initiated by ieee80211_rx_napi() (see line 5043 of mac80211/rx.c).
>
> Thus, add an rcu_head to the scan_req struct so as to use kfree_rcu()
> instead of kfree() so that we don't free during the critical section.
>
> Bug report (3): https://syzkaller.appspot.com/bug?extid=f9acff9bf08a845f225d
> Reported-by: syzbot+f9acff9bf08a845f225d@syzkaller.appspotmail.com
> Reported-by: syzbot+6cb476b7c69916a0caca@syzkaller.appspotmail.com
> Reported-by: syzbot+9250865a55539d384347@syzkaller.appspotmail.com
>
> Signed-off-by: Siddh Raman Pant <code@siddh.me>
> ---
>  include/net/cfg80211.h | 2 ++
>  net/wireless/scan.c    | 2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/cfg80211.h b/include/net/cfg80211.h
> index 6d02e12e4702..ba4a49884de8 100644
> --- a/include/net/cfg80211.h
> +++ b/include/net/cfg80211.h
> @@ -2368,6 +2368,7 @@ struct cfg80211_scan_6ghz_params {
>   * @n_6ghz_params: number of 6 GHz params
>   * @scan_6ghz_params: 6 GHz params
>   * @bssid: BSSID to scan for (most commonly, the wildcard BSSID)
> + * @rcu_head: (internal) RCU head to use for freeing
>   */
>  struct cfg80211_scan_request {
>         struct cfg80211_ssid *ssids;
> @@ -2397,6 +2398,7 @@ struct cfg80211_scan_request {
>         bool scan_6ghz;
>         u32 n_6ghz_params;
>         struct cfg80211_scan_6ghz_params *scan_6ghz_params;
> +       struct rcu_head rcu_head;
>
>         /* keep last */
>         struct ieee80211_channel *channels[];
> diff --git a/net/wireless/scan.c b/net/wireless/scan.c
> index 6d82bd9eaf8c..638b2805222c 100644
> --- a/net/wireless/scan.c
> +++ b/net/wireless/scan.c
> @@ -988,7 +988,7 @@ void ___cfg80211_scan_done(struct cfg80211_registered_device *rdev,
>         kfree(rdev->int_scan_req);
>         rdev->int_scan_req = NULL;
>
> -       kfree(rdev->scan_req);
> +       kfree_rcu(rdev->scan_req, rcu_head);
>         rdev->scan_req = NULL;

Note: this is slightly racy.

You are supposed to follow this order in this situation.

1) Clear the pointer

Then:

2) wait an rcu grace period (synchronize_rcu()) or use call_rcu()/kfree_rcu().


>
>         if (!send_message)
> --
> 2.35.1
>
>