tcp: configurable source port perturb table size

On embedded systems with little memory and no relevant
security concerns, it is beneficial to reduce the size
of the table.

Reducing the size from 2^16 to 2^8 saves 255 KiB
of kernel RAM.

Makes the table size configurable as an expert option.

The size was previously increased from 2^8 to 2^16
in commit 4c2c8f03a5ab ("tcp: increase source port perturb table to
2^16").

Signed-off-by: Gleb Mazovetskiy <glex.spb@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index e983bb0c50127059c735c9e989747c9215596784..2dfb12230f08943e52c39e36e22616db794cd1f7 100644 (file)
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -402,6 +402,16 @@ config INET_IPCOMP
 
          If unsure, say Y.
 
+config INET_TABLE_PERTURB_ORDER
+       int "INET: Source port perturbation table size (as power of 2)" if EXPERT
+       default 16
+       help
+         Source port perturbation table size (as power of 2) for
+         RFC 6056 3.3.4.  Algorithm 4: Double-Hash Port Selection Algorithm.
+
+         The default is almost always what you want.
+         Only change this if you know what you are doing.
+
 config INET_XFRM_TUNNEL
        tristate
        select INET_TUNNEL

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index d3dc281566229479aa9027229a37973f8507d014..033bf3c2538fd2083127b73c7f40b6b032e59fc1 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -906,13 +906,13 @@ EXPORT_SYMBOL_GPL(inet_bhash2_update_saddr);
  * Note that we use 32bit integers (vs RFC 'short integers')
  * because 2^16 is not a multiple of num_ephemeral and this
  * property might be used by clever attacker.
+ *
  * RFC claims using TABLE_LENGTH=10 buckets gives an improvement, though
- * attacks were since demonstrated, thus we use 65536 instead to really
- * give more isolation and privacy, at the expense of 256kB of kernel
- * memory.
+ * attacks were since demonstrated, thus we use 65536 by default instead
+ * to really give more isolation and privacy, at the expense of 256kB
+ * of kernel memory.
  */
-#define INET_TABLE_PERTURB_SHIFT 16
-#define INET_TABLE_PERTURB_SIZE (1 << INET_TABLE_PERTURB_SHIFT)
+#define INET_TABLE_PERTURB_SIZE (1 << CONFIG_INET_TABLE_PERTURB_ORDER)
 static u32 *table_perturb;
 
 int __inet_hash_connect(struct inet_timewait_death_row *death_row,