Re: [PATCH net] udp: fix dst races with multicast early demux

[David Miller]

From: Eric Dumazet 
Date: Sat, 01 Aug 2015 12:14:33 +0200

> From: Eric Dumazet 
> 
> Multicast dst are not cached. They carry DST_NOCACHE.
> 
> As mentioned in commit f8864972126899 ("ipv4: fix dst race in
> sk_dst_get()"), these dst need special care before caching them
> into a socket.
> 
> Caching them is allowed only if their refcnt was not 0, ie we
> must use atomic_inc_not_zero()
> 
> Also, we must use READ_ONCE() to fetch sk->sk_rx_dst, as mentioned
> in commit d0c294c53a771 ("tcp: prevent fetching dst twice in early demux
> code")
> 
> Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
> Signed-off-by: Eric Dumazet 
> Reported-by: Gregory Hoggarth 
> Reported-by: Alex Gartrell 
> Cc: Michal Kubeček 
> ---
> David : I will be on vacation for following 7 days, no internet access.
> Please wait for tests done by Gregory & Alex before merging this ?
> Thanks !

Now that this has been tested by Gregory, applied and queued up for
-stable thanks!
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RE: [PATCH net] udp: fix dst races with multicast early demux

[Gregory Hoggarth]

Hi,

I have included this patch into my code and re-run our tests overnight, out of 
644 iterations we did not see the kernel crash. Previous reproduction rate we 
would have expected 4-6 crashes in this time.

So I think this fixes the issue we are seeing.

Thanks,
Greg


From: netdev-ow...@vger.kernel.org  on behalf of 
Eric Dumazet 
Sent: Saturday, 1 August 2015 10:14 p.m.
To: Gregory Hoggarth
Cc: Shawn Bohrer; netdev@vger.kernel.org; alexgartr...@gmail.com; Michal Kubeček
Subject: [PATCH net] udp: fix dst races with multicast early demux

From: Eric Dumazet 

Multicast dst are not cached. They carry DST_NOCACHE.

As mentioned in commit f8864972126899 ("ipv4: fix dst race in
sk_dst_get()"), these dst need special care before caching them
into a socket.

Caching them is allowed only if their refcnt was not 0, ie we
must use atomic_inc_not_zero()

Also, we must use READ_ONCE() to fetch sk->sk_rx_dst, as mentioned
in commit d0c294c53a771 ("tcp: prevent fetching dst twice in early demux
code")

Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet 
Reported-by: Gregory Hoggarth 
Reported-by: Alex Gartrell 
Cc: Michal Kubeček 
---
David : I will be on vacation for following 7 days, no internet access.
Please wait for tests done by Gregory & Alex before merging this ?
Thanks !

 net/ipv4/udp.c |   13 ++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 83aa604f9273..1b8c5ba7d5f7 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1995,12 +1995,19 @@ 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);
-   if (dst)
-   skb_dst_set_noref(skb, dst);
+   if (dst) {
+   /* DST_NOCACHE can not be used without taking a reference */
+   if (dst->flags & DST_NOCACHE) {
+   if (likely(atomic_inc_not_zero(&dst->__refcnt)))
+   skb_dst_set(skb, dst);
+   } else {
+   skb_dst_set_noref(skb, dst);
+   }
+   }
 }

 int udp_rcv(struct sk_buff *skb)


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