Linux Ethernet Bridging - Feb 2016 - [Bridge] [PATCH] net: fix bridge multicast packet checksum validation

We need to update the skb->csum after pulling the skb, otherwise
checksum validation will fail. This fixes multicast packet loss for
bridges and splats like the following:

[...]
[   43.986968] eth0: hw csum failure
[   43.990344] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.4.0 #2
[   43.996193] Hardware name: BCM2709
[   43.999647] [<800204e0>] (unwind_backtrace) from [<8001cf14>]
(show_stack+0x10/0x14)
[   44.007432] [<8001cf14>] (show_stack) from [<801ab614>]
(dump_stack+0x80/0x90)
[   44.014695] [<801ab614>] (dump_stack) from [<802e4548>]
(__skb_checksum_complete+0x6c/0xac)
[   44.023090] [<802e4548>] (__skb_checksum_complete) from
[<803a055c>] (ipv6_mc_validate_checksum+0x104/0x178)
[   44.032959] [<803a055c>] (ipv6_mc_validate_checksum) from
[<802e111c>] (skb_checksum_trimmed+0x130/0x188)
[   44.042565] [<802e111c>] (skb_checksum_trimmed) from [<803a06e8>]
(ipv6_mc_check_mld+0x118/0x338)
[   44.051501] [<803a06e8>] (ipv6_mc_check_mld) from [<803b2c98>]
(br_multicast_rcv+0x5dc/0xd00)
[   44.060077] [<803b2c98>] (br_multicast_rcv) from [<803aa510>]
(br_handle_frame_finish+0xac/0x51c)
[...]

Fixes: 9afd85c9e455 ("net: Export IGMP/MLD message validation code")
Reported-by: ?lvaro Fern?ndez Rojas <noltari at gmail.com>
Signed-off-by: Linus L?ssing <linus.luessing at c0d3.blue>
---

Steinar, can you check whether this fixes the bridge issues you reported on
bugzilla #99081? Not quite sure whether it is the same as yours as you
do not seem to have any such call traces.


I am not super happy with how this patch looks, but there is no
"skb_push_rcsum"
available and skb_pull_rcsum() seems non-reversible as is. Alternative
suggestions
always welcome.


 net/core/skbuff.c |   19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5bf88f5..6c34ef6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4076,6 +4076,11 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
 	struct sk_buff *skb_chk;
 	unsigned int offset = skb_transport_offset(skb);
 	__sum16 ret;
+	int ip_summed;
+	int csum_valid;
+	int csum_level;
+	int csum_bad;
+	__wsum csum;
 
 	skb_chk = skb_checksum_maybe_trim(skb, transport_len);
 	if (!skb_chk)
@@ -4084,10 +4089,22 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff
*skb,
 	if (!pskb_may_pull(skb_chk, offset))
 		goto err;
 
-	__skb_pull(skb_chk, offset);
+	ip_summed = skb->ip_summed;
+	csum_valid = skb->csum_valid;
+	csum_level = skb->csum_level;
+	csum_bad = skb->csum_bad;
+	csum = skb->csum;
+
+	skb_pull_rcsum(skb_chk, offset);
 	ret = skb_chkf(skb_chk);
 	__skb_push(skb_chk, offset);
 
+	skb->ip_summed = ip_summed;
+	skb->csum_valid = csum_valid;
+	skb->csum_level = csum_level;
+	skb->csum_bad = csum_bad;
+	skb->csum = csum;
+
 	if (ret)
 		goto err;
 
-- 
1.7.10.4

On Mon, Feb 15, 2016 at 03:07:06AM +0100, Linus L?ssing
wrote:
> Steinar, can you check whether this fixes the bridge issues you reported on
> bugzilla #99081? Not quite sure whether it is the same as yours as you
> do not seem to have any such call traces.
It doesn't immediately sound like the same problem; why would promisc change
anything if the problem is the checksumming?

I don't have any reboots scheduled for this machine right now, but I'll
see
what I can do wrt. testing.

/* Steinar */
-- 
Homepage: https://www.sesse.net/

On Thu, Feb 18, 2016 at 01:51:34PM +0100, Steinar H. Gunderson
wrote:
> On Mon, Feb 15, 2016 at 03:07:06AM +0100, Linus L?ssing wrote:
> > Steinar, can you check whether this fixes the bridge issues you
reported on
> > bugzilla #99081? Not quite sure whether it is the same as yours as you
> > do not seem to have any such call traces.
> 
> It doesn't immediately sound like the same problem; why would promisc
change
> anything if the problem is the checksumming?
The mdb you provided in the bugzilla ticket misses reports, so it
was unable to parse reports. Which could point to a checksumming
problem.

Enabling promisc probably did not fix the parsing for you, but instead
promisc forces to forward packets upstream on your interface independent
of the mdb. I would assume that even with promisc, your output
from "bridge mdb show" looks rather empty. Can you check?
> 
> I don't have any reboots scheduled for this machine right now, but
I'll see
> what I can do wrt. testing.
Thanks :).

From: Linus L?ssing <linus.luessing at c0d3.blue>
Date: Mon, 15 Feb 2016 03:07:06 +0100
> @@ -4084,10 +4089,22 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff
*skb,
>  	if (!pskb_may_pull(skb_chk, offset))
>  		goto err;
>  
> -	__skb_pull(skb_chk, offset);
> +	ip_summed = skb->ip_summed;
> +	csum_valid = skb->csum_valid;
> +	csum_level = skb->csum_level;
> +	csum_bad = skb->csum_bad;
> +	csum = skb->csum;
> +
> +	skb_pull_rcsum(skb_chk, offset);
>  	ret = skb_chkf(skb_chk);
>  	__skb_push(skb_chk, offset);
>  
> +	skb->ip_summed = ip_summed;
> +	skb->csum_valid = csum_valid;
> +	skb->csum_level = csum_level;
> +	skb->csum_bad = csum_bad;
> +	skb->csum = csum;
> +
There really has to be a better way to fix this :-/