linux-kernel - [PATCH v3 net 7/7] tcp: drop the hash_32() part from the index calculation

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Message-Id: <20220502084614.24123-8-w@1wt.eu>
Date:   Mon,  2 May 2022 10:46:14 +0200
From:   Willy Tarreau <w@....eu>
To:     netdev@...r.kernel.org
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Moshe Kol <moshe.kol@...l.huji.ac.il>,
        Yossi Gilad <yossi.gilad@...l.huji.ac.il>,
        Amit Klein <aksecurity@...il.com>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, Willy Tarreau <w@....eu>
Subject: [PATCH v3 net 7/7] tcp: drop the hash_32() part from the index calculation

In commit 190cc82489f4 ("tcp: change source port randomizarion at
connect() time"), the table_perturb[] array was introduced and an
index was taken from the port_offset via hash_32(). But it turns
out that hash_32() performs a multiplication while the input here
comes from the output of SipHash in secure_seq, that is well
distributed enough to avoid the need for yet another hash.

Suggested-by: Amit Klein <aksecurity@...il.com>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Willy Tarreau <w@....eu>
---
 net/ipv4/inet_hashtables.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index cc5f66328b47..a5d57fa679ca 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -778,7 +778,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 
 	net_get_random_once(table_perturb,
 			    INET_TABLE_PERTURB_SIZE * sizeof(*table_perturb));
-	index = hash_32(port_offset, INET_TABLE_PERTURB_SHIFT);
+	index = port_offset & (INET_TABLE_PERTURB_SIZE - 1);
 
 	offset = READ_ONCE(table_perturb[index]) + (port_offset >> 32);
 	offset %= remaining;
-- 
2.17.5