tcp: protect sysctl_tcp_cookie_size reads

Make sure sysctl_tcp_cookie_size is read once in
tcp_cookie_size_check(), or we might return an illegal value to caller
if sysctl_tcp_cookie_size is changed by another cpu.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: William Allen Simpson <william.allen.simpson@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 12 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 0d4a3cebfb46..61c2463e2753 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -385,27 +385,30 @@ struct tcp_out_options {
  */
 static u8 tcp_cookie_size_check(u8 desired)
 {
-        if (desired > 0) {
+        int cookie_size;
+
+        if (desired > 0)
                 /* previously specified */
                 return desired;
-        }
-        if (sysctl_tcp_cookie_size <= 0) {
+
+        cookie_size = ACCESS_ONCE(sysctl_tcp_cookie_size);
+        if (cookie_size <= 0)
                 /* no default specified */
                 return 0;
-        }
-        if (sysctl_tcp_cookie_size <= TCP_COOKIE_MIN) {
+
+        if (cookie_size <= TCP_COOKIE_MIN)
                 /* value too small, specify minimum */
                 return TCP_COOKIE_MIN;
-        }
-        if (sysctl_tcp_cookie_size >= TCP_COOKIE_MAX) {
+
+        if (cookie_size >= TCP_COOKIE_MAX)
                 /* value too large, specify maximum */
                 return TCP_COOKIE_MAX;
-        }
-        if (0x1 & sysctl_tcp_cookie_size) {
+
+        if (cookie_size & 1)
                 /* 8-bit multiple, illegal, fix it */
-                return (u8)(sysctl_tcp_cookie_size + 0x1);
-        }
-        return (u8)sysctl_tcp_cookie_size;
+                cookie_size++;
+
+        return (u8)cookie_size;
 }
 
 /* Write previously computed TCP options to the packet.