Re: [PATCH for v4.9 LTS 86/87] net: account for current skb length when deciding about UFO

[Michal Kubecek]

On Sat, Jul 15, 2017 at 01:26:27AM +0000, Levin, Alexander (Sasha Levin) wrote:
> From: Michal Kubeček <mkubecek@xxxxxxx>
> 
> [ Upstream commit a5cb659bbc1c8644efa0c3138a757a1e432a4880 ]
> 
> Our customer encountered stuck NFS writes for blocks starting at specific
> offsets w.r.t. page boundary caused by networking stack sending packets via
> UFO enabled device with wrong checksum. The problem can be reproduced by
> composing a long UDP datagram from multiple parts using MSG_MORE flag:
> 
>   sendto(sd, buff, 1000, MSG_MORE, ...);
>   sendto(sd, buff, 1000, MSG_MORE, ...);
>   sendto(sd, buff, 3000, 0, ...);
> 
> Assume this packet is to be routed via a device with MTU 1500 and
> NETIF_F_UFO enabled. When second sendto() gets into __ip_append_data(),
> this condition is tested (among others) to decide whether to call
> ip_ufo_append_data():
> 
>   ((length + fragheaderlen) > mtu) || (skb && skb_is_gso(skb))
> 
> At the moment, we already have skb with 1028 bytes of data which is not
> marked for GSO so that the test is false (fragheaderlen is usually 20).
> Thus we append second 1000 bytes to this skb without invoking UFO. Third
> sendto(), however, has sufficient length to trigger the UFO path so that we
> end up with non-UFO skb followed by a UFO one. Later on, udp_send_skb()
> uses udp_csum() to calculate the checksum but that assumes all fragments
> have correct checksum in skb->csum which is not true for UFO fragments.
> 
> When checking against MTU, we need to add skb->len to length of new segment
> if we already have a partially filled skb and fragheaderlen only if there
> isn't one.
> 
> In the IPv6 case, skb can only be null if this is the first segment so that
> we have to use headersize (length of the first IPv6 header) rather than
> fragheaderlen (length of IPv6 header of further fragments) for skb == NULL.
> 
> Fixes: e89e9cf539a2 ("[IPv4/IPv6]: UFO Scatter-gather approach")
> Fixes: e4c5e13aa45c ("ipv6: Should use consistent conditional judgement for
> 	ip6 fragment between __ip6_append_data and ip6_finish_output")
> Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
> Acked-by: Vlad Yasevich <vyasevic@xxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> [SL: Drop changes to net/ipv4/ip_output.c because 4.9 doesn't have
> e89e9cf539a2 ("[IPv4/IPv6]: UFO Scatter-gather approach")]

Commit e89e9cf539a2 is in mainline since v2.6.16.28 so that 4.9 does
have it. The conflict on cherry-pick is caused by missing commit
e4c5e13aa45c but that is the same as in the IPv6 part. I'm adding the
IPv4 part backported to 4.9 below

> Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
> ---
>  net/ipv6/ip6_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 5a4b8e7bcedd..9213eba601d0 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1376,7 +1376,7 @@ static int __ip6_append_data(struct sock *sk,
>  	 */
>  
>  	cork->length += length;
> -	if ((((length + fragheaderlen) > mtu) ||
> +	if ((((length + (skb ? skb->len : headersize)) > mtu) ||
>  	     (skb && skb_is_gso(skb))) &&
>  	    (sk->sk_protocol == IPPROTO_UDP) &&
>  	    (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len &&
> -- 
> 2.11.0

This is strange, it looks as if e4c5e13aa45c was already applied before
but I can't see that in stable-4.9.y branch.


diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e5c1dbef3626..06215ba88b93 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -936,7 +936,8 @@ static int __ip_append_data(struct sock *sk,
 		csummode = CHECKSUM_PARTIAL;
 
 	cork->length += length;
-	if (((length > mtu) || (skb && skb_is_gso(skb))) &&
+	if ((((length + (skb ? skb->len : fragheaderlen)) > mtu) ||
+	     (skb && skb_is_gso(skb))) &&
 	    (sk->sk_protocol == IPPROTO_UDP) &&
 	    (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len &&
 	    (sk->sk_type == SOCK_DGRAM) && !sk->sk_no_check_tx) {
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index fd649599620e..9213eba601d0 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1376,7 +1376,7 @@ static int __ip6_append_data(struct sock *sk,
 	 */
 
 	cork->length += length;
-	if (((length > mtu) ||
+	if ((((length + (skb ? skb->len : headersize)) > mtu) ||
 	     (skb && skb_is_gso(skb))) &&
 	    (sk->sk_protocol == IPPROTO_UDP) &&
 	    (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len &&