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>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index e91911d7aae2..1b4ec21497a4 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -26,6 +26,8 @@ static int zero;
 static int tcp_retr1_max = 255;
 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;
 
 /* Update system visible IP port range */
 static void set_local_port_range(int range[2])
@@ -426,7 +428,9 @@ static struct ctl_table ipv4_table[] = {
                 .data           = &sysctl_tcp_adv_win_scale,
                 .maxlen         = sizeof(int),
                 .mode           = 0644,
-                .proc_handler   = proc_dointvec
+                .proc_handler   = proc_dointvec_minmax,
+                .extra1         = &tcp_adv_win_scale_min,
+                .extra2         = &tcp_adv_win_scale_max,
         },
         {
                 .procname       = "tcp_tw_reuse",