linux-kernel - Re: [PATCH v2] wifi: cfg80211: Fix UAF in ieee80211_scan

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Message-ID: <18292e1dcd8.2359a549180213.8185874405406307019@siddh.me>
Date:   Fri, 12 Aug 2022 21:57:31 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     "Greg KH" <gregkh@...uxfoundation.org>
Cc:     "johannes berg" <johannes@...solutions.net>,
        "david s. miller" <davem@...emloft.net>,
        "eric dumazet" <edumazet@...gle.com>,
        "jakub kicinski" <kuba@...nel.org>,
        "paolo abeni" <pabeni@...hat.com>,
        "netdev" <netdev@...r.kernel.org>,
        "syzbot+6cb476b7c69916a0caca" 
        <syzbot+6cb476b7c69916a0caca@...kaller.appspotmail.com>,
        "linux-wireless" <linux-wireless@...r.kernel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "syzbot+f9acff9bf08a845f225d" 
        <syzbot+f9acff9bf08a845f225d@...kaller.appspotmail.com>,
        "syzbot+9250865a55539d384347" 
        <syzbot+9250865a55539d384347@...kaller.appspotmail.com>,
        "linux-kernel-mentees" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: Re: [PATCH v2] wifi: cfg80211: Fix UAF in ieee80211_scan_rx()

On Fri, 12 Aug 2022 20:57:58 +0530  Greg KH  wrote:
> rcu just delays freeing of an object, you might just be delaying the
> race condition.  Just moving a single object to be freed with rcu feels
> very odd if you don't have another reference somewhere.

As mentioned in patch message, in net/mac80211/scan.c, we have:
        void ieee80211_scan_rx(struct ieee80211_local *local, struct sk_buff *skb)
        {
                ...
                scan_req = rcu_dereference(local->scan_req);
                sched_scan_req = rcu_dereference(local->sched_scan_req);

                if (scan_req)
                        scan_req_flags = scan_req->flags;
                ...
        }

So scan_req is probably supposed to be protected by RCU.

Also, in ieee80211_local's definition at net/mac80211/ieee80211_i.h, we have:
        struct cfg80211_scan_request __rcu *scan_req;

Thus, scan_req is indeed supposed to be protected by RCU, which this patch
addresses by adding a RCU head to the type's struct, and using kfree_rcu().

The above snippet is where the UAF happens (you can refer to syzkaller's log),
because __cfg80211_scan_done() is called and frees the pointer.

Thanks,
Siddh