attach: handle /proc with hidepid={1,2} property

Receive fd for LSM security module before we set{g,u}id(). The reason is that
on set{g,u}id() the kernel will a) make us undumpable and b) we will change our
effective uid. This means our effective uid will be different from the
effective uid of the process that created us which means that this processs no
longer has capabilities in our namespace including CAP_SYS_PTRACE. This means
we will not be able to read and /proc/<pid> files for the process anymore when
/proc is mounted with hidepid={1,2}. So let's get the lsm label fd before the
set{g,u}id().

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/attach.c

@@ -820,7 +820,7 @@ struct attach_clone_payload {
 
 static int attach_child_main(struct attach_clone_payload *payload)
 {
-	int fd, ret;
+	int fd, lsm_fd, ret;
 	long flags;
 #if HAVE_SYS_PERSONALITY_H
 	long new_personality;
@@ -830,6 +830,9 @@ static int attach_child_main(struct attach_clone_payload *payload)
 	int ipc_socket = payload->ipc_socket;
 	lxc_attach_options_t* options = payload->options;
 	struct lxc_proc_context_info* init_ctx = payload->init_ctx;
+	bool needs_lsm = (options->namespaces & CLONE_NEWNS) &&
+			 (options->attach_flags & LXC_ATTACH_LSM) &&
+			 init_ctx->lsm_label;
 
 	/* A description of the purpose of this functionality is provided in the
 	 * lxc-attach(1) manual page. We have to remount here and not in the
@@ -883,6 +886,26 @@ static int attach_child_main(struct attach_clone_payload *payload)
 		rexit(-1);
 	}
 
+	/* This remark only affects fully unprivileged containers:
+	 * Receive fd for LSM security module before we set{g,u}id(). The reason
+	 * is that on set{g,u}id() the kernel will a) make us undumpable and b)
+	 * we will change our effective uid. This means our effective uid will
+	 * be different from the effective uid of the process that created us
+	 * which means that this processs no longer has capabilities in our
+	 * namespace including CAP_SYS_PTRACE. This means we will not be able to
+	 * read and /proc/<pid> files for the process anymore when /proc is
+	 * mounted with hidepid={1,2}. So let's get the lsm label fd before the
+	 * set{g,u}id().
+	 */
+	if (needs_lsm) {
+		ret = lxc_abstract_unix_recv_fds(ipc_socket, &lsm_fd, 1, NULL, 0);
+		if (ret <= 0) {
+			shutdown(ipc_socket, SHUT_RDWR);
+			rexit(-1);
+		}
+		TRACE("Received LSM label file descriptor %d from parent", lsm_fd);
+	}
+
 	/* Set {u,g}id. */
 	new_uid = 0;
 	new_gid = 0;
@@ -941,27 +964,19 @@ static int attach_child_main(struct attach_clone_payload *payload)
 		     "gainable privileges.");
 	}
 
-	if ((options->namespaces & CLONE_NEWNS) &&
-	    (options->attach_flags & LXC_ATTACH_LSM) && init_ctx->lsm_label) {
-		int lsm_labelfd, on_exec;
-
-		/* Receive fd for LSM security module. */
-		ret = lxc_abstract_unix_recv_fds(ipc_socket, &lsm_labelfd, 1, NULL, 0);
-		if (ret <= 0) {
-			shutdown(ipc_socket, SHUT_RDWR);
-			rexit(-1);
-		}
-		TRACE("Received LSM label file descriptor %d from parent", lsm_labelfd);
+	if (needs_lsm) {
+		int on_exec;
 
 		/* Change into our new LSM profile. */
 		on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? 1 : 0;
-		if (lsm_set_label_at(lsm_labelfd, on_exec, init_ctx->lsm_label) < 0) {
+		ret = lsm_set_label_at(lsm_fd, on_exec, init_ctx->lsm_label);
+		if (ret < 0) {
 			SYSERROR("Failed to set LSM label.");
 			shutdown(ipc_socket, SHUT_RDWR);
-			close(lsm_labelfd);
+			close(lsm_fd);
 			rexit(-1);
 		}
-		close(lsm_labelfd);
+		close(lsm_fd);
 	}
 
 	if (init_ctx->container && init_ctx->container->lxc_conf &&