netdev - Re: Panic with demuxed ipv4 multicast udp sockets on 4.0.4

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Date:	Thu, 30 Jul 2015 07:45:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Gregory Hoggarth <Gregory.Hoggarth@...iedtelesis.co.nz>
Cc:	Shawn Bohrer <sbohrer@...advisors.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"alexgartrell@...il.com" <alexgartrell@...il.com>
Subject: Re: Panic with demuxed ipv4 multicast udp sockets on 4.0.4

On Thu, 2015-07-30 at 07:42 +0200, Eric Dumazet wrote:
> On Thu, 2015-07-30 at 01:41 +0000, Gregory Hoggarth wrote:
> > Hi,
> > 
> > My company has also started having what appears to be the same problem, since we upgraded our embedded system to 
> > linux kernel 3.16.
> > 
> > I tried applying the suggested fix of READ_ONCE (and also had to add in the necessary code to compiler.h as 3.16
> > didn't have it) and unfortunately it did not fix the issue at all.
> > 
> > Unfortunately we do not have an easy reproduction method, and do not know precisely what is going on in the system
> > when the issue occurs. We know it is a multicast UDP packet but that is about it. For us, the crash happens during
> > a critical stage in our system initialisation, making additional debugging and instrumentation difficult. Our 
> > reproduction rate is approximately 1 out of 100 test runs; testing overnight we will usually see 3-5 instances of 
> > the crash happening. All our attempts to increase the reproduction rate, or reproduce the issue in a simpler/more 
> > controlled way have failed.
> > 
> > Because we have customised the linux kernel, in some places radically, we assumed this was just a problem only we 
> > were seeing, so we were trying to fix it ourselves. Now that this appears to be a generic problem upstream, we've 
> > simply disabled UDP early demux in our system (since it's a new optimisation that we have lived without up till 
> > now) and will wait for this issue to be fixed upstream instead.
> > 
> > 
> > So I'm sharing the debug patch I've written to help gather data on what is going on in the system, and some
> > of the output we've gotten from the debug, in case this is useful for anyone else who is seeing this problem or
> > would like to try and fix it.
> > 
> > Feel free to ask questions, I'm not sure how much help I can be but will do my best. We'll be happy to assist in
> > testing any proposed fixes. I also have some more examples of kernel oops and debug output if that could be useful, 
> > although the debug is from earlier iterations of the patch so that historical output is not as detailed as the 
> > output generated by the latest version of the patch attached here.
> > 
> > Thanks,
> > Greg Hoggarth
> 
> CC UDP early demux author : Shawn Bohrer 
> 
> I believe this is a race condition with a dst escaping RCU protected
> region.
> 
> I will send a patch.
> 

Please try following fixes :

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 83aa604f9273..02baaa6d97b3 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1778,9 +1778,10 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
-		if (unlikely(sk->sk_rx_dst != dst))
+		if (unlikely(sk->sk_rx_dst != dst)) {
+			skb_dst_force(skb);
 			udp_sk_rx_dst_set(sk, dst);
-
+		}
 		ret = udp_queue_rcv_skb(sk, skb);
 		sock_put(sk);
 		/* a return value > 0 means to resubmit the input, but
@@ -1995,7 +1996,7 @@ void udp_v4_early_demux(struct sk_buff *skb)
 
 	skb->sk = sk;
 	skb->destructor = sock_efree;
-	dst = sk->sk_rx_dst;
+	dst = READ_ONCE(sk->sk_rx_dst);
 
 	if (dst)
 		dst = dst_check(dst, 0);


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