Re: [PATCH v2] net-fq: Add WARN_ON check for null flow.

On 06/07/2018 04:59 PM, Cong Wang wrote:

On Thu, Jun 7, 2018 at 4:48 PM,  [off-list ref] wrote:

quoted

diff --git a/include/net/fq_impl.h b/include/net/fq_impl.h
index be7c0fa..cb911f0 100644
--- a/include/net/fq_impl.h
+++ b/include/net/fq_impl.h

@@ -78,7 +78,10 @@ static struct sk_buff *fq_tin_dequeue(struct fq *fq,
                        return NULL;
        }

-       flow = list_first_entry(head, struct fq_flow, flowchain);
+       flow = list_first_entry_or_null(head, struct fq_flow, flowchain);
+
+       if (WARN_ON_ONCE(!flow))
+               return NULL;

This does not make sense either. list_first_entry_or_null()
returns NULL only when the list is empty, but we already check
list_empty() right before this code, and it is protected by fq->lock.

Hello Michal,

git blame shows you as the author of the fq_impl.h code.

I saw a crash when debugging funky ath10k firmware in a 4.16 + hacks kernel.  There was an apparent
mostly-null deref in the fq_tin_dequeue method.  According to gdb, it was within
1 line of the dereference of 'flow'.

My hack above is probably not that useful.  Cong thinks maybe the locking is bad.

If you get a chance, please review this thread and see if you have any ideas for
a better fix (or better debugging code).

As always, if you would like me to generate you a buggy firmware that will crash
in the tx path and cause all sorts of mayhem in the ath10k driver and wifi stack,
I will be happy to do so.

https://www.mail-archive.com/netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org/msg239738.html

Thanks,
Ben

-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com