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From:   Gleb Mazovetskiy <glex.spb@gmail.com>
To:     "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>
Cc:     Paul Cercueil <paul@crapouillou.net>,
        Gleb Mazovetskiy <glex.spb@gmail.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH 1/1] tcp: configurable source port perturb table size
Date:   Mon, 14 Nov 2022 22:56:16 +0000
Message-Id: <20221114225616.16715-1-glex.spb@gmail.com>
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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>
---
 net/ipv4/Kconfig           | 10 ++++++++++
 net/ipv4/inet_hashtables.c | 10 +++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index e983bb0c5012..2dfb12230f08 100644
--- 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 d3dc28156622..033bf3c2538f 100644
--- 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,
-- 
2.37.2