Allow strongSwan to be spawned as non-root user

This patch allows for giving strongSwan only the runtime capabilities it
needs, rather than full root privileges.

Adds preprocessor directives which allow strongSwan to be configured to
 1) start up as a non-root user
 2) avoid modprobe()'ing IPsec kernel modules into the kernel, which
    would normally require root or CAP_SYS_MODULE

Additionally, some small mods to charon/libstrongswan ensure that charon
fully supports starting as a non-root user.

Tested with strongSwan 5.5.3.

src/charon/charon.c

@@ -231,9 +231,15 @@ static bool check_pidfile()
 			DBG1(DBG_LIB, "setting FD_CLOEXEC for '"PID_FILE"' failed: %s",
 				 strerror(errno));
 		}
-		ignore_result(fchown(fd,
-							 lib->caps->get_uid(lib->caps),
-							 lib->caps->get_gid(lib->caps)));
+		/* Only fchown() the pidfile if we have CAP_CHOWN. Otherwise,
+		 * directory permissions should allow pidfile to be accessed
+		 * by the UID/GID under which the charon deamon will run. */
+		if (lib->caps->check(lib->caps, CAP_CHOWN))
+		{
+			ignore_result(fchown(fd,
+								 lib->caps->get_uid(lib->caps),
+								 lib->caps->get_gid(lib->caps)));
+		}
 		fprintf(pidfile, "%d\n", getpid());
 		fflush(pidfile);
 		return FALSE;

src/libstrongswan/networking/streams/stream_service_unix.c

@@ -39,8 +39,9 @@ stream_service_t *stream_service_create_unix(char *uri, int backlog)
 	}
 	if (!lib->caps->check(lib->caps, CAP_CHOWN))
 	{	/* required to chown(2) service socket */
-		DBG1(DBG_NET, "socket '%s' requires CAP_CHOWN capability", uri);
-		return NULL;
+		DBG1(DBG_NET, "cannot change ownership of socket '%s' without "
+			 "CAP_CHOWN capability. socket directory should be accessible to "
+			 "UID/GID under which the deamon will run", uri);
 	}
 	fd = socket(AF_UNIX, SOCK_STREAM, 0);
 	if (fd == -1)
@@ -58,7 +59,9 @@ stream_service_t *stream_service_create_unix(char *uri, int backlog)
 		return NULL;
 	}
 	umask(old);
-	if (chown(addr.sun_path, lib->caps->get_uid(lib->caps),
+	/* only attempt to chown() socket if we have CAP_CHOWN */
+	if (lib->caps->check(lib->caps, CAP_CHOWN) &&
+		chown(addr.sun_path, lib->caps->get_uid(lib->caps),
 			  lib->caps->get_gid(lib->caps)) != 0)
 	{
 		DBG1(DBG_NET, "changing socket permissions for '%s' failed: %s",

src/libstrongswan/utils/capabilities.c

@@ -422,7 +422,10 @@ METHOD(capabilities_t, drop, bool,
 {
 #ifndef WIN32
 #ifdef HAVE_PRCTL
-	prctl(PR_SET_KEEPCAPS, 1, 0, 0, 0);
+	if (has_capability(this, CAP_SETPCAP, NULL))
+	{
+		prctl(PR_SET_KEEPCAPS, 1, 0, 0, 0);
+	}
 #endif
 
 	if (this->uid && !init_supplementary_groups(this))

src/libstrongswan/utils/capabilities.h

@@ -47,6 +47,9 @@ typedef struct capabilities_t capabilities_t;
 #ifndef CAP_DAC_OVERRIDE
 # define CAP_DAC_OVERRIDE 1
 #endif
+#ifndef CAP_SETPCAP
+# define CAP_SETPCAP 8
+#endif
 
 /**
  * POSIX capability dropping abstraction layer.

src/starter/starter.c

@@ -477,6 +477,7 @@ int main (int argc, char **argv)
 		}
 	}
 
+#ifndef STARTER_ALLOW_NON_ROOT
 	/* verify that we can start */
 	if (getuid() != 0)
 	{
@@ -484,6 +485,7 @@ int main (int argc, char **argv)
 		cleanup();
 		exit(LSB_RC_NOT_ALLOWED);
 	}
+#endif
 
 	if (check_pid(pid_file))
 	{
@@ -520,6 +522,7 @@ int main (int argc, char **argv)
 		exit(LSB_RC_INVALID_ARGUMENT);
 	}
 
+#ifndef SKIP_KERNEL_IPSEC_MODPROBES
 	/* determine if we have a native netkey IPsec stack */
 	if (!starter_netkey_init())
 	{
@@ -530,6 +533,7 @@ int main (int argc, char **argv)
 			DBG1(DBG_APP, "no known IPsec stack detected, ignoring!");
 		}
 	}
+#endif
 
 	last_reload = time_monotonic(NULL);