netfilter: ip6t_NPT: Fix adjustment calculation

Cast __wsum from/to __sum16 is wrong.  Instead, apply appropriate
conversion function: csum_unfold() or csum_fold().

[ The original patch has been modified to undo the final ~ that
  csum_fold returns. We only need to fold the 32-bit word that
  results from the checksum calculation into a 16-bit to ensure
  that the original subnet is restored appropriately. Spotted by
  Ulrich Weber. ]

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/ipv6/netfilter/ip6t_NPT.c b/net/ipv6/netfilter/ip6t_NPT.c
index 7302b0b7b642..68788c84aee7 100644
--- a/net/ipv6/netfilter/ip6t_NPT.c
+++ b/net/ipv6/netfilter/ip6t_NPT.c
@@ -30,7 +30,7 @@ static int ip6t_npt_checkentry(const struct xt_tgchk_param *par)
                                 (__force __wsum)npt->dst_pfx.in6.s6_addr16[i]);
         }
 
-        npt->adjustment = (__force __sum16) csum_sub(src_sum, dst_sum);
+        npt->adjustment = ~csum_fold(csum_sub(src_sum, dst_sum));
         return 0;
 }
 
@@ -66,8 +66,8 @@ static bool ip6t_npt_map_pfx(const struct ip6t_npt_tginfo *npt,
                         return false;
         }
 
-        sum = (__force __sum16) csum_add((__force __wsum)addr->s6_addr16[idx],
-                         npt->adjustment);
+        sum = ~csum_fold(csum_add(csum_unfold((__force __sum16)addr->s6_addr16[idx]),
+                                  csum_unfold(npt->adjustment)));
         if (sum == CSUM_MANGLED_0)
                 sum = 0;
         *(__force __sum16 *)&addr->s6_addr16[idx] = sum;