Re: [PATCH v2] wifi: cfg80211: Fix UAF in ieee80211_scan_rx()

Siddh Raman Pant
2022-08-12T16:28:24Z

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