From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next] udp: properly support MSG_PEEK with truncated
 buffers
Date: Mon, 04 Jan 2016 17:25:13 -0500 (EST)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 30 Dec 2015 08:51:12 -0500

> From: Eric Dumazet <edumazet@google.com>
> 
> Backport of this upstream commit into stable kernels :
> 89c22d8c3b27 ("net: Fix skb csum races when peeking")
> exposed a bug in udp stack vs MSG_PEEK support, when user provides
> a buffer smaller than skb payload.
>     
> In this case,
> skb_copy_and_csum_datagram_iovec(skb, sizeof(struct udphdr),
>                                  msg->msg_iov);
> returns -EFAULT.
>     
> This bug does not happen in upstream kernels since Al Viro did a great
> job to replace this into :
> skb_copy_and_csum_datagram_msg(skb, sizeof(struct udphdr), msg);
> This variant is safe vs short buffers.
>     
> For the time being, instead reverting Herbert Xu patch and add back
> skb->ip_summed invalid changes, simply store the result of
> udp_lib_checksum_complete() so that we avoid computing the checksum a
> second time, and avoid the problematic
> skb_copy_and_csum_datagram_iovec() call.
> 
> This patch can be applied on recent kernels as it avoids a double
> checksumming, then backported to stable kernels as a bug fix.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied, thanks Eric.