netdev - [PATCH net-next] net: fix recent csum changes

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Date:   Fri,  3 Dec 2021 10:52:38 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        David Laight <David.Laight@...LAB.COM>,
        David Lebrun <dlebrun@...gle.com>
Subject: [PATCH net-next] net: fix recent csum changes

From: Eric Dumazet <edumazet@...gle.com>

Vladimir reported csum issues after my recent change in skb_postpull_rcsum()

Issue here is the following:

initial skb->csum is the csum of

[part to be pulled][rest of packet]

Old code:
 skb->csum = csum_sub(skb->csum, csum_partial(pull, pull_length, 0));

New code:
 skb->csum = ~csum_partial(pull, pull_length, ~skb->csum);

This is broken if the csum of [pulled part]
happens to be equal to skb->csum, because end
result of skb->csum is 0 in new code, instead
of being 0xffffffff

David Laight suggested to use

skb->csum = -csum_partial(pull, pull_length, -skb->csum);

I based my patches on existing code present in include/net/seg6.h,
update_csum_diff4() and update_csum_diff16() which might need
a similar fix.

I guess that my tests, mostly pulling 40 bytes of IPv6 header
were not providing enough entropy to hit this bug.

Fixes: 29c3002644bd ("net: optimize skb_postpull_rcsum()")
Fixes: 0bd28476f636 ("gro: optimize skb_gro_postpull_rcsum()")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Vladimir Oltean <vladimir.oltean@....com>
Suggested-by: David Laight <David.Laight@...LAB.COM>
Cc: David Lebrun <dlebrun@...gle.com>
---
 include/linux/skbuff.h | 2 +-
 include/net/gro.h      | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index eae4bd3237a41cc1b60b44c91fbfe21dfdd8f117..2bbcdaf99ed3df739ddfd2de4be747262226d4b9 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3486,7 +3486,7 @@ static inline void skb_postpull_rcsum(struct sk_buff *skb,
 				      const void *start, unsigned int len)
 {
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
-		skb->csum = ~csum_partial(start, len, ~skb->csum);
+		skb->csum = -csum_partial(start, len, -skb->csum);
 	else if (skb->ip_summed == CHECKSUM_PARTIAL &&
 		 skb_checksum_start_offset(skb) < 0)
 		skb->ip_summed = CHECKSUM_NONE;
diff --git a/include/net/gro.h b/include/net/gro.h
index b1139fca7c435ca0c353c4ed17628dd7f3df4401..4529c4c6f3ca4ac23da569b0bc0e00e3c9dcd765 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -173,8 +173,8 @@ static inline void skb_gro_postpull_rcsum(struct sk_buff *skb,
 					const void *start, unsigned int len)
 {
 	if (NAPI_GRO_CB(skb)->csum_valid)
-		NAPI_GRO_CB(skb)->csum = ~csum_partial(start, len,
-						       ~NAPI_GRO_CB(skb)->csum);
+		NAPI_GRO_CB(skb)->csum = -csum_partial(start, len,
+						       -NAPI_GRO_CB(skb)->csum);
 }
 
 /* GRO checksum functions. These are logical equivalents of the normal
-- 
2.34.1.400.ga245620fadb-goog