nbd-server: handle modern-style negotiation in a child process

Previously, the modern style negotiation was carried out in the root
server (listener) process before forking the actual client handler. This
made it possible for a malfunctioning or evil client to terminate the
root process simply by querying a non-existent export or aborting in the
middle of the negotation process (caused SIGPIPE in the server).

This commit moves the negotiation process to the child to keep the root
process up and running no matter what happens during the negotiation.

See http://sourceforge.net/mailarchive/message.php?msg_id=30410146

Signed-off-by: Tuomas Räsänen <tuomasjjrasanen@tjjr.fi>

nbd-server.c

@@ -2198,6 +2198,161 @@ void destroy_pid_t(gpointer data) {
 	g_free(data);
 }
 
+static pid_t
+spawn_child()
+{
+        pid_t pid;
+        sigset_t newset;
+        sigset_t oldset;
+
+        sigemptyset(&newset);
+        sigaddset(&newset, SIGCHLD);
+        sigaddset(&newset, SIGTERM);
+        sigprocmask(SIG_BLOCK, &newset, &oldset);
+        pid = fork();
+        if (pid < 0) {
+                msg(LOG_ERR, "Could not fork (%s)", strerror(errno));
+                goto out;
+        }
+        if (pid > 0) { /* Parent */
+                pid_t *pidp;
+
+                pidp = g_malloc(sizeof(pid_t));
+                *pidp = pid;
+                g_hash_table_insert(children, pidp, pidp);
+                goto out;
+        }
+        /* Child */
+        signal(SIGCHLD, SIG_DFL);
+        signal(SIGTERM, SIG_DFL);
+        signal(SIGHUP, SIG_DFL);
+out:
+        sigprocmask(SIG_SETMASK, &oldset, NULL);
+        return pid;
+}
+
+static int
+socket_accept(const int sock)
+{
+        struct sockaddr_storage addrin;
+        socklen_t addrinlen = sizeof(addrin);
+        int net;
+
+        net = accept(sock, (struct sockaddr *) &addrin, &addrinlen);
+        if (net < 0) {
+                err_nonfatal("Failed to accept socket connection: %m");
+        }
+
+        return net;
+}
+
+static void
+handle_modern_connection(GArray *const servers, const int sock)
+{
+        int net;
+        pid_t pid;
+        CLIENT *client = NULL;
+        int sock_flags_old;
+        int sock_flags_new;
+
+        net = socket_accept(sock);
+        if (net < 0)
+                return;
+
+        if (!dontfork) {
+                pid = spawn_child();
+                if (pid) {
+                        if (pid > 0)
+                                msg(LOG_INFO, "Spawned a child process");
+                        if (pid < 0)
+                                msg(LOG_ERR, "Failed to spawn a child process");
+                        close(net);
+                        return;
+                }
+                /* Child just continues. */
+        }
+
+        client = negotiate(net, NULL, servers, NEG_INIT | NEG_MODERN);
+        if (!client) {
+                msg(LOG_ERR, "Modern initial negotiation failed");
+                goto handler_err;
+        }
+
+        if (client->server->max_connections > 0 &&
+           g_hash_table_size(children) >= client->server->max_connections) {
+                msg(LOG_ERR, "Max connections (%d) reached",
+                    client->server->max_connections);
+                goto handler_err;
+        }
+
+        sock_flags_old = fcntl(net, F_GETFL, 0);
+        if (sock_flags_old == -1) {
+                msg(LOG_ERR, "Failed to get socket flags");
+                goto handler_err;
+        }
+
+        sock_flags_new = sock_flags_old & ~O_NONBLOCK;
+        if (sock_flags_new != sock_flags_old &&
+            fcntl(net, F_SETFL, sock_flags_new) == -1) {
+                msg(LOG_ERR, "Failed to set socket to blocking mode");
+                goto handler_err;
+        }
+
+        if (set_peername(net, client)) {
+                msg(LOG_ERR, "Failed to set peername");
+                goto handler_err;
+        }
+
+        if (!authorized_client(client)) {
+                msg(LOG_INFO, "Client '%s' is not authorized to access",
+                    client->clientname);
+                goto handler_err;
+        }
+
+        if (!dontfork) {
+                int i;
+
+                /* Free all root server resources here, because we are
+                 * currently in the child process serving one specific
+                 * connection. These are not simply needed anymore. */
+                g_hash_table_destroy(children);
+                children = NULL;
+                for (i = 0; i < modernsocks->len; i++) {
+                        close(g_array_index(modernsocks, int, i));
+                }
+                g_array_free(modernsocks, TRUE);
+
+                /* Now that we are in the child process after a
+                 * succesful negotiation, we do not need the list of
+                 * servers anymore, get rid of it.*/
+
+                for (i = 0; i < servers->len; i++) {
+                        const SERVER *const server = &g_array_index(servers, SERVER, i);
+                        close(server->socket);
+                }
+
+                /* FALSE does not free the
+                   actual data. This is required,
+                   because the client has a
+                   direct reference into that
+                   data, and otherwise we get a
+                   segfault... */
+                g_array_free(servers, FALSE);
+        }
+
+        msg(LOG_INFO, "Starting to serve");
+        serveconnection(client);
+        exit(EXIT_SUCCESS);
+
+handler_err:
+        g_free(client);
+        close(net);
+
+        if (!dontfork) {
+                exit(EXIT_FAILURE);
+        }
+}
+
 static void
 handle_connection(GArray *servers, int net, SERVER *serve, CLIENT *client)
 {
@@ -2425,28 +2580,18 @@ void serveloop(GArray* servers) {
 
 		memcpy(&rset, &mset, sizeof(fd_set));
 		if(select(max+1, &rset, NULL, NULL, NULL)>0) {
-			int net;
 
 			DEBUG("accept, ");
 			for(i=0; i < modernsocks->len; i++) {
 				int sock = g_array_index(modernsocks, int, i);
 				if(!FD_ISSET(sock, &rset)) {
 					continue;
 				}
-				CLIENT *client;
 
-				if((net=accept(sock, (struct sockaddr *) &addrin, &addrinlen)) < 0) {
-					err_nonfatal("accept: %m");
-					continue;
-				}
-				client = negotiate(net, NULL, servers, NEG_INIT | NEG_MODERN);
-				if(!client) {
-					close(net);
-					continue;
-				}
-				handle_connection(servers, net, client->server, client);
+				handle_modern_connection(servers, sock);
 			}
 			for(i=0; i < servers->len; i++) {
+				int net;
 				SERVER *serve;
 
 				serve=&(g_array_index(servers, SERVER, i));