!963 [sync] PR-937: tcp: restrict net.ipv4.tcp_app_win

Merge Pull Request from: @openeuler-sync-bot 
 

Origin pull request: 
https://gitee.com/openeuler/kernel/pulls/937 
 
PR sync from:  YueHaibing <yuehaibing@huawei.com>
 https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/thread/IQ4SJZTGXXAAT4OQ72ZLUTPEDFFVDQQX/ 
 
 
Link:https://gitee.com/openeuler/kernel/pulls/963 

Reviewed-by: Jialin Zhang <zhangjialin11@huawei.com> 
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com> 

Documentation/networking/ip-sysctl.rst

@@ -272,6 +272,8 @@ tcp_app_win - INTEGER
 	Reserve max(window/2^tcp_app_win, mss) of window for application
 	buffer. Value 0 is special, it means that nothing is reserved.
 
+	Possible values are [0, 31], inclusive.
+
 	Default: 31
 
 tcp_autocorking - BOOLEAN

net/ipv4/sysctl_net_ipv4.c

@@ -37,6 +37,7 @@ static int ip_local_port_range_min[] = { 1, 1 };
 static int ip_local_port_range_max[] = { 65535, 65535 };
 static int tcp_adv_win_scale_min = -31;
 static int tcp_adv_win_scale_max = 31;
+static int tcp_app_win_max = 31;
 static int tcp_min_snd_mss_min = TCP_MIN_SND_MSS;
 static int tcp_min_snd_mss_max = 65535;
 static int ip_privileged_port_min;
@@ -1156,7 +1157,9 @@ static struct ctl_table ipv4_net_table[] = {
 		.data		= &init_net.ipv4.sysctl_tcp_app_win,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= &tcp_app_win_max,
 	},
 	{
 		.procname	= "tcp_adv_win_scale",