tcp: restrict net.ipv4.tcp_app_win

UBSAN: shift-out-of-bounds in net/ipv4/tcp_input.c:555:23
shift exponent 255 is too large for 32-bit type 'int'
CPU: 1 PID: 7907 Comm: ssh Not tainted 6.3.0-rc4-00161-g62bad54b26db-dirty torvalds#206
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x136/0x150
 __ubsan_handle_shift_out_of_bounds+0x21f/0x5a0
 tcp_init_transfer.cold+0x3a/0xb9
 tcp_finish_connect+0x1d0/0x620
 tcp_rcv_state_process+0xd78/0x4d60
 tcp_v4_do_rcv+0x33d/0x9d0
 __release_sock+0x133/0x3b0
 release_sock+0x58/0x1b0

'maxwin' is int, shifting int for 32 or more bits is undefined behaviour.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -340,6 +340,8 @@ 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

@@ -25,6 +25,7 @@ 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;
@@ -1198,6 +1199,8 @@ static struct ctl_table ipv4_net_table[] = {
 		.maxlen		= sizeof(u8),
 		.mode		= 0644,
 		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= &tcp_app_win_max,
 	},
 	{
 		.procname	= "tcp_adv_win_scale",