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

[Jakub Kicinski]

On Fri, 12 Aug 2022 21:57:31 +0530 Siddh Raman Pant wrote:
> 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.

Similarly to Greg, I'm not very familiar with the code base but one
sure way to move things forward would be to point out a commit which
broke things and put it in a Fixes tag. Much easier to validate a fix
by looking at where things went wrong.