We need to update the skb->csum after pulling the skb, otherwise
an unnecessary checksum (re)computation can ocure for IGMP/MLD packets
in the bridge code. Additionally this fixes the following splats for
network devices / bridge ports with support for and enabled RX checksum
offloading:

[...]
[   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: Álvaro Fernández Rojas <redacted>
Signed-off-by: Linus Lüssing <redacted>
---

v2:
* substituted the storing of the csum with an skb_push_rcsum() approach
 (did some more reading about CHECKSUM_PARTIAL, and it seems to me that the
  skb_push direction is actually the easier/"safer" one, so there should be
  no resetting to CHECKSUM_NONE necessary)
* Rewording of commit message, this bug should not cause any packet loss
  due to "automatic" checksum recomputations in software if skb->csum
  is bogus (in the CHECKSUM_COMPLETE case)

 include/linux/skbuff.h |   17 +++++++++++++++++
 net/core/skbuff.c      |   22 ++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 4ce9ff7..d66e007 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -2784,6 +2784,23 @@ static inline int skb_linearize_cow(struct sk_buff *skb)
 }
 
 /**
+ *	skb_postpush_rcsum - update checksum for received skb after push
+ *	@skb: buffer to update
+ *	@start: start of data before push
+ *	@len: length of data pushed
+ *
+ *	After doing a push on a received packet, you need to call this to
+ *	update the CHECKSUM_COMPLETE checksum.
+ */
+
+static inline void skb_postpush_rcsum(struct sk_buff *skb,
+				      const void *start, unsigned int len)
+{
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->csum = csum_add(skb->csum, csum_partial(start, len, 0));
+}
+
+/**
  *	skb_postpull_rcsum - update checksum for received skb after pull
  *	@skb: buffer to update
  *	@start: start of data before pull

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5bf88f5..8616d11 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -2948,6 +2948,24 @@ int skb_append_pagefrags(struct sk_buff *skb, struct page *page,
 EXPORT_SYMBOL_GPL(skb_append_pagefrags);
 
 /**
+ *	skb_push_rcsum - push skb and update receive checksum
+ *	@skb: buffer to update
+ *	@len: length of data pulled
+ *
+ *	This function performs an skb_push on the packet and updates
+ *	the CHECKSUM_COMPLETE checksum.  It should be used on
+ *	receive path processing instead of skb_push unless you know
+ *	that the checksum difference is zero (e.g., a valid IP header)
+ *	or you are setting ip_summed to CHECKSUM_NONE.
+ */
+static unsigned char *skb_push_rcsum(struct sk_buff *skb, unsigned len)
+{
+	skb_push(skb, len);
+	skb_postpush_rcsum(skb, skb->data, len);
+	return skb->data;
+}
+
+/**
  *	skb_pull_rcsum - pull skb and update receive checksum
  *	@skb: buffer to update
  *	@len: length of data pulled

@@ -4084,9 +4102,9 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
 	if (!pskb_may_pull(skb_chk, offset))
 		goto err;
 
-	__skb_pull(skb_chk, offset);
+	skb_pull_rcsum(skb_chk, offset);
 	ret = skb_chkf(skb_chk);
-	__skb_push(skb_chk, offset);
+	skb_push_rcsum(skb_chk, offset);
 
 	if (ret)
 		goto err;

-- 
1.7.10.4