passt/pasta.c

// SPDX-License-Identifier: AGPL-3.0-or-later

/* PASST - Plug A Simple Socket Transport
 *  for qemu/UNIX domain socket mode
 *
 * PASTA - Pack A Subtle Tap Abstraction
 *  for network namespace/tap device mode
 *
 * pasta.c - pasta (namespace) specific implementations
 *
 * Copyright (c) 2020-2021 Red Hat GmbH
 * Author: Stefano Brivio <sbrivio@redhat.com>
 *
 * #syscalls:pasta clone waitid exit exit_group rt_sigprocmask
 * #syscalls:pasta rt_sigreturn|sigreturn ppc64:sigreturn s390x:sigreturn
 */

#include <sched.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <syslog.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <signal.h>
#include <dirent.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <net/ethernet.h>
#include <sys/syscall.h>

#include "util.h"
#include "passt.h"
#include "netlink.h"

/* PID of child, in case we created a namespace */
static int pasta_child_pid;

/**
 * pasta_child_handler() - Exit once shell exits (if we started it), reap clones
 * @signal:	Unused, handler deals with SIGCHLD only
 */
void pasta_child_handler(int signal)
{
	siginfo_t infop;

	(void)signal;

	if (signal != SIGCHLD)
		return;

	if (pasta_child_pid &&
	    !waitid(P_PID, pasta_child_pid, &infop, WEXITED | WNOHANG)) {
		if (infop.si_pid == pasta_child_pid)
			exit(EXIT_SUCCESS);
			/* Nothing to do, detached PID namespace going away */
	}

	waitid(P_ALL, 0, NULL, WEXITED | WNOHANG);
	waitid(P_ALL, 0, NULL, WEXITED | WNOHANG);
}

/**
 * pasta_wait_for_ns() - Busy loop until we can enter the target namespace
 * @arg:	Execution context
 *
 * Return: 0
 */
static int pasta_wait_for_ns(void *arg)
{
	struct ctx *c = (struct ctx *)arg;
	char ns[PATH_MAX];

	if (c->netns_only)
		goto netns;

	snprintf(ns, PATH_MAX, "/proc/%i/ns/user", pasta_child_pid);
	do
		while ((c->pasta_userns_fd = open(ns, O_RDONLY)) < 0);
	while (setns(c->pasta_userns_fd, CLONE_NEWUSER) &&
	       !close(c->pasta_userns_fd));

netns:
	snprintf(ns, PATH_MAX, "/proc/%i/ns/net", pasta_child_pid);
	do
		while ((c->pasta_netns_fd = open(ns, O_RDONLY)) < 0);
	while (setns(c->pasta_netns_fd, CLONE_NEWNET) &&
	       !close(c->pasta_netns_fd));

	return 0;
}

/**
 * struct pasta_setup_ns_arg - Argument for pasta_setup_ns()
 * @c:		Execution context
 * @euid:	Effective UID of caller
 */
struct pasta_setup_ns_arg {
	struct ctx *c;
	int euid;
};

/**
 * pasta_setup_ns() - Map credentials, enable access to ping sockets, run shell
 * @arg:	See @pasta_setup_ns_arg
 *
 * Return: this function never returns
 */
static int pasta_setup_ns(void *arg)
{
	struct pasta_setup_ns_arg *a = (struct pasta_setup_ns_arg *)arg;
	char *shell;
	int fd;

	if (!a->c->netns_only) {
		char buf[BUFSIZ];

		snprintf(buf, BUFSIZ, "%i %i %i", 0, a->euid, 1);

		fd = open("/proc/self/uid_map", O_WRONLY);
		if (write(fd, buf, strlen(buf)) < 0)
			warn("Cannot set uid_map in namespace");
		close(fd);

		fd = open("/proc/self/setgroups", O_WRONLY);
		if (write(fd, "deny", sizeof("deny")) < 0)
			warn("Cannot write to setgroups in namespace");
		close(fd);

		fd = open("/proc/self/gid_map", O_WRONLY);
		if (write(fd, buf, strlen(buf)) < 0)
			warn("Cannot set gid_map in namespace");
		close(fd);
	}

	fd = open("/proc/sys/net/ipv4/ping_group_range", O_WRONLY);
	if (write(fd, "0 0", strlen("0 0")) < 0)
		warn("Cannot set ping_group_range, ICMP requests might fail");
	close(fd);

	shell = getenv("SHELL") ? getenv("SHELL") : "/bin/sh";
	if (strstr(shell, "/bash"))
		execve(shell, ((char *[]) { shell, "-l", NULL }), environ);
	else
		execve(shell, ((char *[]) { shell, NULL }), environ);

	perror("execve");
	exit(EXIT_FAILURE);
}

/**
 * pasta_start_ns() - Fork shell in new namespace if target ns is not given
 * @c:		Execution context
 */
void pasta_start_ns(struct ctx *c)
{
	struct pasta_setup_ns_arg arg = { .c = c, .euid = geteuid() };
	char ns_fn_stack[NS_FN_STACK_SIZE];

	c->foreground = 1;
	if (!c->debug)
		c->quiet = 1;

	pasta_child_pid = clone(pasta_setup_ns,
				ns_fn_stack + sizeof(ns_fn_stack) / 2,
				(c->netns_only ? 0 : CLONE_NEWNET) |
				CLONE_NEWIPC | CLONE_NEWPID | CLONE_NEWUSER |
				CLONE_NEWUTS,
				(void *)&arg);

	if (pasta_child_pid == -1) {
		perror("clone");
		exit(EXIT_FAILURE);
	}

	drop_caps();

	if (pasta_child_pid) {
		NS_CALL(pasta_wait_for_ns, c);
		return;
	}
}

/**
 * pasta_ns_conf() - Set up loopback and tap interfaces in namespace as needed
 * @c:		Execution context
 */
void pasta_ns_conf(struct ctx *c)
{
	nl_link(1, 1 /* lo */, MAC_ZERO, 1, 0);

	if (c->pasta_conf_ns) {
		int prefix_len;

		nl_link(1, c->pasta_ifi, c->mac_guest, 1, c->mtu);

		if (c->v4) {
			prefix_len = __builtin_popcount(c->mask4);
			nl_addr(1, c->pasta_ifi, AF_INET, &c->addr4,
				&prefix_len, NULL);
			nl_route(1, c->pasta_ifi, AF_INET, &c->gw4);
		}

		if (c->v6) {
			prefix_len = 64;
			nl_addr(1, c->pasta_ifi, AF_INET6, &c->addr6,
				&prefix_len, NULL);
			nl_route(1, c->pasta_ifi, AF_INET6, &c->gw6);
		}
	} else {
		nl_link(1, c->pasta_ifi, c->mac_guest, 0, 0);
	}

	proto_update_l2_buf(c->mac_guest, NULL, NULL);
}