Re: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del_chan

On Sun, Aug 6, 2017 at 6:32 PM, Gao Feng [off-list ref] wrote:

I think the RCU should be supposed to avoid the race between del_chan and lookup_chan.

More precisely, it is callid_sock which is protected by RCU.

Unless I miss any other code path, pptp_exit_module() is
problematic too, I don't think it can just vfree() the whole thing.

The synchronize_rcu could make sure if there was one which calls lookup_chan in this period, it would be finished and the sock refcnt is increased if necessary.

So I think it is ok to invoke sock_put directly without SOCK_RCU_FREE, because the lookup_chan caller has already hold the sock refcnt,

If you mean the sock_hold() inside lookup_chan(), no,
it doesn't help because we already dereference the sock
before it.

Also, lookup_chan_dst() does not have a refcnt, I don't
find any code preventing it deref'ing other sock in callid_sock
than the calling one.