tcp: restrict net.ipv4.tcp_adv_win_scale (#20312)

tcp_win_from_space() does the following:

      if (sysctl_tcp_adv_win_scale <= 0)
              return space >> (-sysctl_tcp_adv_win_scale);
      else
              return space - (space >> sysctl_tcp_adv_win_scale);

"space" is int.

As per C99 6.5.7 (3) shifting int for 32 or more bits is
undefined behaviour.

Indeed, if sysctl_tcp_adv_win_scale is exactly 32,
space >> 32 equals space and function returns 0.

Which means we busyloop in tcp_fixup_rcvbuf().

Restrict net.ipv4.tcp_adv_win_scale to [-31, 31].

Fix https://bugzilla.kernel.org/show_bug.cgi?id=20312

Steps to reproduce:

      echo 32 >/proc/sys/net/ipv4/tcp_adv_win_scale
      wget www.kernel.org
      [softlockup]

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -144,6 +144,7 @@ tcp_adv_win_scale - INTEGER
 	Count buffering overhead as bytes/2^tcp_adv_win_scale
 	(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
 	if it is <= 0.
+	Possible values are [-31, 31], inclusive.
 	Default: 2
 
 tcp_allowed_congestion_control - STRING