[Patch net] net: make pskb_trim_rcsum_slow() robust

[Cong Wang]

Most callers of pskb_trim_rcsum() simply drops the skb when
it fails, however, ip_check_defrag() still continues to pass
the skb up to stack. In that case, we should restore its previous
csum if __pskb_trim() fails.

Found this during code review.

Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Cc: Eric Dumazet <eduma...@google.com>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/core/skbuff.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 946de0e24c87..5decd6e6d2b6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1843,6 +1843,9 @@ EXPORT_SYMBOL(___pskb_trim);
  */
 int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
 {
+       __wsum old_csum = skb->csum;
+       int ret;
+
        if (skb->ip_summed == CHECKSUM_COMPLETE) {
                int delta = skb->len - len;
 
@@ -1850,7 +1853,10 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned 
int len)
                                           skb_checksum(skb, len, delta, 0),
                                           len);
        }
-       return __pskb_trim(skb, len);
+       ret = __pskb_trim(skb, len);
+       if (unlikely(ret))
+               skb->csum = old_csum;
+       return ret;
 }
 EXPORT_SYMBOL(pskb_trim_rcsum_slow);
 
-- 
2.16.4