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Subject: [PATCH] binddynport.c honor ip_local_reserved_ports
From:   Otto Hollmann <ohollmann@suse.cz>
To:     libtirpc-devel@lists.sourceforge.net
Cc:     linux-nfs@vger.kernel.org
Date:   Tue, 14 Jun 2022 16:25:38 +0200
Organization: SUSE
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Read reserved ports from /proc/sys/net/ipv4/ip_local_reserved_ports,
store them into bit-wise array and before binding to random port check
if port is not reserved.


Currently, there is no way how to reserve ports so then will not be
used by rpcbind.

Random ports are opened by rpcbind because of rmtcalls. There is
compile-time flag for disabling them, but in some cases we can not
simply disable them.

One solution would be run time option --enable-rmtcalls as already
discussed, but it was rejected. So if we want to keep rmtcalls enabled
and also be able to reserve some ports, there is no other way than
filtering available ports. The easiest and clearest way seems to be
just respect kernel list of ip_reserved_ports.

Unfortunately there is one known disadvantage/side effect - it affects
probability of ports which are right after reserved ones. The bigger
reserved block is, the higher is probability of selecting following
unreserved port. But if there is no reserved port, impact of this patch
is minimal/none.

Signed-off-by: Otto Hollmann <otto.hollmann@suse.com>
---
 src/binddynport.c | 107 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 99 insertions(+), 8 deletions(-)

diff --git a/src/binddynport.c b/src/binddynport.c
index 062629a..6f78ebe 100644
--- a/src/binddynport.c
+++ b/src/binddynport.c
@@ -37,6 +37,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <string.h>
+#include <syslog.h>
 
 #include <rpc/rpc.h>
 
@@ -56,6 +57,84 @@ enum {
        NPORTS          = ENDPORT - LOWPORT + 1,
 };
 
+/*
+ * This function decodes information about given port from provided array and
+ * return if port is reserved or not.
+ *
+ * @reserved_ports an array of size at least "NPORTS / (8*sizeof(char)) + 1".
+ * @port port number within range LOWPORT and ENDPORT
+ *
+ * Returns 0 if port is not reserved, non-negative if port is reserved.
+ */
+int is_reserved(char *reserved_ports, int port) {
+       port -= LOWPORT;
+       if (port < 0 || port >= NPORTS)
+               return 0;
+       return reserved_ports[port/(8*sizeof(char))] & 1<<(port%(8*sizeof(char)));
+}
+
+/*
+ * This function encodes information about given *reserved* port into provided
+ * array. Don't call this function for ports which are not reserved.
+ *
+ * @reserved_ports an array of size at least "NPORTS / (8*sizeof(char)) + 1".
+ * @port port number within range LOWPORT and ENDPORT
+ *
+ */
+void set_reserved(char *reserved_ports, int port) {
+       port -= LOWPORT;
+       if (port < 0 || port >= NPORTS)
+               return;
+       reserved_ports[port/(8*sizeof(char))] |= 1<<(port%(8*sizeof(char)));
+}
+
+/*
+ * Parse local reserved ports obtained from
+ * /proc/sys/net/ipv4/ip_local_reserved_ports into bit array.
+ *
+ * @reserved_ports a zeroed array of size at least
+ * "NPORTS / (8*sizeof(char)) + 1". Will be used for bit-wise encoding of
+ * reserved ports.
+ *
+ * On each call, reserved ports are read from /proc and bit-wise stored into
+ * provided array
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+
+int parse_reserved_ports(char *reserved_ports) {
+       int from, to;
+       char delimiter = ',';
+       int res;
+       FILE * file_ptr = fopen("/proc/sys/net/ipv4/ip_local_reserved_ports","r");
+       if (file_ptr == NULL) {
+               (void) syslog(LOG_ERR,
+                       "Unable to open open /proc/sys/net/ipv4/ip_local_reserved_ports.");
+               return -1;
+       }
+       do {
+               if ((res = fscanf(file_ptr, "%d", &to)) != 1) {
+                       if (res == EOF) break;
+                       goto err;
+               }
+               if (delimiter != '-') {
+                       from = to;
+               }
+               for (int i = from; i <= to; ++i) {
+                       set_reserved(reserved_ports, i);
+               }
+       } while ((res = fscanf(file_ptr, "%c", &delimiter)) == 1);
+       if (res != EOF)
+               goto err;
+       fclose(file_ptr);
+       return 0;
+err:
+       (void) syslog(LOG_ERR,
+               "An error occurred while parsing ip_local_reserved_ports.");
+       fclose(file_ptr);
+       return -1;
+}
+
 /*
  * Bind a socket to a dynamically-assigned IP port.
  *
@@ -81,7 +160,8 @@ int __binddynport(int fd)
        in_port_t port, *portp;
        struct sockaddr *sap;
        socklen_t salen;
-       int i, res;
+       int i, res, array_size;
+       char *reserved_ports;
 
        if (__rpc_sockisbound(fd))
                return 0;
@@ -119,21 +199,32 @@ int __binddynport(int fd)
                gettimeofday(&tv, NULL);
                seed = tv.tv_usec * getpid();
        }
+       array_size = NPORTS / (8*sizeof(char)) + 1;
+       reserved_ports = malloc(array_size);
+       if (!reserved_ports) {
+               goto out;
+       }
+       memset(reserved_ports, 0, array_size);
+       parse_reserved_ports(reserved_ports);
+
        port = (rand_r(&seed) % NPORTS) + LOWPORT;
        for (i = 0; i < NPORTS; ++i) {
-               *portp = htons(port++);
-               res = bind(fd, sap, salen);
-               if (res >= 0) {
-                       res = 0;
-                       break;
+               *portp = htons(port);
+               if (!is_reserved(reserved_ports, port++)) {
+                       res = bind(fd, sap, salen);
+                       if (res >= 0) {
+                               res = 0;
+                               break;
+                       }
+                       if (errno != EADDRINUSE)
+                               break;
                }
-               if (errno != EADDRINUSE)
-                       break;
                if (port > ENDPORT)
                        port = LOWPORT;
        }
 
 out:
+       free(reserved_ports);
        mutex_unlock(&port_lock);
        return res;
 }
-- 
2.26.2