tcp: drop the hash_32() part from the index calculation

stable inclusion
from stable-v5.10.125
commit 7ccb026ecb997405b59d391140c25ee347891504
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6EY

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7ccb026ecb997405b59d391140c25ee347891504

--------------------------------

commit e8161345 upstream.

In commit 190cc824 ("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: NAmit Klein <aksecurity@gmail.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: NWei Li <liwei391@huawei.com>

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;