net: core: fix flow symmetric hash

__flow_hash_consistentify() wrongly swaps ipv4 addresses in few cases.
This function is indirectly used by __skb_get_hash_symmetric(), which is
used to fanout packets in AF_PACKET.
Intrusion detection systems may be impacted by this issue.

__flow_hash_consistentify() computes the addresses difference then swaps
them if the difference is negative. In few cases src - dst and dst - src
are both negative.

The following snippet mimics __flow_hash_consistentify():

```
 #include <stdio.h>
 #include <stdint.h>

 int main(int argc, char** argv) {

     int diffs_d, diffd_s;
     uint32_t dst  = 0xb225a8c0; /* 178.37.168.192 --> 192.168.37.178 */
     uint32_t src  = 0x3225a8c0; /*  50.37.168.192 --> 192.168.37.50  */
     uint32_t dst2 = 0x3325a8c0; /*  51.37.168.192 --> 192.168.37.51  */

     diffs_d = src - dst;
     diffd_s = dst - src;

     printf("src:%08x dst:%08x, diff(s-d)=%d(0x%x) diff(d-s)=%d(0x%x)\n",
             src, dst, diffs_d, diffs_d, diffd_s, diffd_s);

     diffs_d = src - dst2;
     diffd_s = dst2 - src;

     printf("src:%08x dst:%08x, diff(s-d)=%d(0x%x) diff(d-s)=%d(0x%x)\n",
             src, dst2, diffs_d, diffs_d, diffd_s, diffd_s);

     return 0;
 }
```

Results:

src:3225a8c0 dst:b225a8c0, \
    diff(s-d)=-2147483648(0x80000000) \
    diff(d-s)=-2147483648(0x80000000)

src:3225a8c0 dst:3325a8c0, \
    diff(s-d)=-16777216(0xff000000) \
    diff(d-s)=16777216(0x1000000)

In the first case the addresses differences are always < 0, therefore
__flow_hash_consistentify() always swaps, thus dst->src and src->dst
packets have differents hashes.

Fixes: 0fef8db84ff46 ("net: Add full IPv6 addresses to flow_keys")
Signed-off-by: Ludovic Cintrat <ludovic.cintrat@gatewatcher.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 764c4cb3fe8f41d3ff9c4f9a981563d7296b2c72..5dc3860e9fc7e8d77573e8f29dc52db175f929f8 100644 (file)
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1611,9 +1611,8 @@ static inline void __flow_hash_consistentify(struct flow_keys *keys)
 
        switch (keys->control.addr_type) {
        case FLOW_DISSECTOR_KEY_IPV4_ADDRS:
-               addr_diff = (__force u32)keys->addrs.v4addrs.dst -
-                           (__force u32)keys->addrs.v4addrs.src;
-               if (addr_diff < 0)
+               if ((__force u32)keys->addrs.v4addrs.dst <
+                   (__force u32)keys->addrs.v4addrs.src)
                        swap(keys->addrs.v4addrs.src, keys->addrs.v4addrs.dst);
 
                if ((__force u16)keys->ports.dst <