Subject: Re: [Patch net-next] net_sched: move the empty tp check from ->destroy() to ->delete()

From: John Fastabend <[email-masked]>
Date: Sun, 27 Nov 2016 18:57:38 -0800

On 16-11-23 05:58 PM, Cong Wang wrote:
> Roi reported we could have a race condition where in ->classify() path
> we dereference tp->root and meanwhile a parallel ->destroy() makes it
> a NULL.
> 
> This is possible because ->destroy() could be called when deleting
> a filter to check if we are the last one in tp, this tp is still
> linked and visible at that time.
> 
> The root cause of this problem is the semantic of ->destroy(), it
> does two things (for non-force case):
> 
> 1) check if tp is empty
> 2) if tp is empty we could really destroy it
> 
> and its caller, if cares, needs to check its return value to see if
> it is really destroyed. Therefore we can't unlink tp unless we know
> it is empty.
> 
> As suggested by Daniel, we could actually move the test logic to ->delete()
> so that we can safely unlink tp after ->delete() tells us the last one is
> just deleted and before ->destroy().
> 
> What's more, even we unlink it before ->destroy(), it could still have
> readers since we don't wait for a grace period here, we should not modify
> tp->root in ->destroy() either.
> 
> Fixes: 1e052be69d04 ("net_sched: destroy proto tp when all filters are gone")
> Reported-by: Roi Dayan <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: John Fastabend <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> ---

Hi Cong,

Thanks a lot for doing this. Can you rebase it on top of Daniel's patch
though,

 [PATCH net] net, sched: respect rcu grace period on cls destruction

And then push the NULL pointer work for the cls_fw and cls_route
classifiers into another patch.

Then I believe the last thing to make this correct is to convert the
call_rcu() paths to call_rcu_bh().

.John