github.com/kubiko/snapd@v0.0.0-20201013125620-d4f3094d9ddf/cmd/snap-update-ns/bootstrap.c

/*
 * Copyright (C) 2017 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

// IMPORTANT: all the code in this file may be run with elevated privileges
// when invoking snap-update-ns from the setuid snap-confine.
//
// This file is a preprocessor for snap-update-ns' main() function. It will
// perform input validation and clear the environment so that snap-update-ns'
// go code runs with safe inputs when called by the setuid() snap-confine.

#include "bootstrap.h"

#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <limits.h>
#include <sched.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/capability.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

// bootstrap_errno contains a copy of errno if a system call fails.
int bootstrap_errno = 0;
// bootstrap_msg contains a static string if something fails.
const char *bootstrap_msg = NULL;

// setns_into_snap switches mount namespace into that of a given snap.
static int setns_into_snap(const char *snap_name)
{
	// Construct the name of the .mnt file to open.
	char buf[PATH_MAX] = {
		0,
	};
	int n = snprintf(buf, sizeof buf, "/run/snapd/ns/%s.mnt", snap_name);
	if (n >= sizeof buf || n < 0) {
		bootstrap_errno = 0;
		bootstrap_msg = "cannot format mount namespace file name";
		return -1;
	}
	// Open the mount namespace file.
	int fd = open(buf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
	if (fd < 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot open mount namespace file";
		return -1;
	}
	// Switch to the mount namespace of the given snap.
	int err = setns(fd, CLONE_NEWNS);
	if (err < 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot switch mount namespace";
	};

	close(fd);
	return err;
}

// switch_to_privileged_user drops to the real user ID while retaining
// CAP_SYS_ADMIN, for operations such as mount().
static int switch_to_privileged_user()
{
	uid_t real_uid;
	gid_t real_gid;

	real_uid = getuid();
	if (real_uid == 0) {
		// We're running as root: no need to switch IDs
		return 0;
	}
	real_gid = getgid();

	// _LINUX_CAPABILITY_VERSION_3 valid for kernel >= 2.6.26. See
	// https://github.com/torvalds/linux/blob/master/kernel/capability.c
	struct __user_cap_header_struct hdr =
	    { _LINUX_CAPABILITY_VERSION_3, 0 };
	struct __user_cap_data_struct data[2] = { {0} };

	data[0].effective = (CAP_TO_MASK(CAP_SYS_ADMIN) |
			     CAP_TO_MASK(CAP_SETUID) | CAP_TO_MASK(CAP_SETGID));
	data[0].permitted = data[0].effective;
	data[0].inheritable = 0;
	data[1].effective = 0;
	data[1].permitted = 0;
	data[1].inheritable = 0;

	if (capset(&hdr, data) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot set permitted capabilities mask";
		return -1;
	}

	if (prctl(PR_SET_KEEPCAPS, 1, 0, 0, 0) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg =
		    "cannot tell kernel to keep capabilities over setuid";
		return -1;
	}

	if (setgroups(1, &real_gid) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot drop supplementary groups";
		return -1;
	}

	if (setgid(real_gid) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot switch to real group ID";
		return -1;
	}

	if (setuid(real_uid) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg = "cannot switch to real user ID";
		return -1;
	}
	// After changing uid, our effective capabilities were dropped.
	// Reacquire CAP_SYS_ADMIN, and discard CAP_SETUID/CAP_SETGID.
	data[0].effective = CAP_TO_MASK(CAP_SYS_ADMIN);
	data[0].permitted = data[0].effective;
	if (capset(&hdr, data) != 0) {
		bootstrap_errno = errno;
		bootstrap_msg =
		    "cannot enable capabilities after switching to real user";
		return -1;
	}

	return 0;
}

// TODO: reuse the code from snap-confine, if possible.
static int skip_lowercase_letters(const char **p)
{
	int skipped = 0;
	const char *c;
	for (c = *p; *c >= 'a' && *c <= 'z'; ++c) {
		skipped += 1;
	}
	*p = (*p) + skipped;
	return skipped;
}

// TODO: reuse the code from snap-confine, if possible.
static int skip_digits(const char **p)
{
	int skipped = 0;
	const char *c;
	for (c = *p; *c >= '0' && *c <= '9'; ++c) {
		skipped += 1;
	}
	*p = (*p) + skipped;
	return skipped;
}

// TODO: reuse the code from snap-confine, if possible.
static int skip_one_char(const char **p, char c)
{
	if (**p == c) {
		*p += 1;
		return 1;
	}
	return 0;
}

// validate_snap_name performs full validation of the given name.
int validate_snap_name(const char *snap_name)
{
	// NOTE: This function should be synchronized with the two other
	// implementations: sc_snap_name_validate and snap.ValidateName.

	// Ensure that name is not NULL
	if (snap_name == NULL) {
		bootstrap_msg = "snap name cannot be NULL";
		return -1;
	}
	// This is a regexp-free routine hand-codes the following pattern:
	//
	// "^([a-z0-9]+-?)*[a-z](-?[a-z0-9])*$"
	//
	// The only motivation for not using regular expressions is so that we
	// don't run untrusted input against a potentially complex regular
	// expression engine.
	const char *p = snap_name;
	if (skip_one_char(&p, '-')) {
		bootstrap_msg = "snap name cannot start with a dash";
		return -1;
	}
	bool got_letter = false;
	int n = 0, m;
	for (; *p != '\0';) {
		if ((m = skip_lowercase_letters(&p)) > 0) {
			n += m;
			got_letter = true;
			continue;
		}
		if ((m = skip_digits(&p)) > 0) {
			n += m;
			continue;
		}
		if (skip_one_char(&p, '-') > 0) {
			n++;
			if (*p == '\0') {
				bootstrap_msg =
				    "snap name cannot end with a dash";
				return -1;
			}
			if (skip_one_char(&p, '-') > 0) {
				bootstrap_msg =
				    "snap name cannot contain two consecutive dashes";
				return -1;
			}
			continue;
		}
		bootstrap_msg =
		    "snap name must use lower case letters, digits or dashes";
		return -1;
	}
	if (!got_letter) {
		bootstrap_msg = "snap name must contain at least one letter";
		return -1;
	}
	if (n < 2) {
		bootstrap_msg = "snap name must be longer than 1 character";
		return -1;
	}
	if (n > 40) {
		bootstrap_msg = "snap name must be shorter than 40 characters";
		return -1;
	}

	bootstrap_msg = NULL;
	return 0;
}

static int instance_key_validate(const char *instance_key)
{
	// NOTE: see snap.ValidateInstanceName for reference of a valid instance key
	// format

	// Ensure that name is not NULL
	if (instance_key == NULL) {
		bootstrap_msg = "instance key cannot be NULL";
		return -1;
	}
	// This is a regexp-free routine hand-coding the following pattern:
	//
	// "^[a-z]{1,10}$"
	//
	// The only motivation for not using regular expressions is so that we don't
	// run untrusted input against a potentially complex regular expression
	// engine.
	int i = 0;
	for (i = 0; instance_key[i] != '\0'; i++) {
		char c = instance_key[i];
		/* NOTE: We are reimplementing islower() and isdigit()
		 * here. For context see
		 * https://github.com/golang/go/issues/29689 */
		if ((c >= 'a' && c <= 'z') || (c >= '0' && c <= '9')) {
			continue;
		}
		bootstrap_msg =
		    "instance key must use lower case letters or digits";
		return -1;
	}

	if (i == 0) {
		bootstrap_msg =
		    "instance key must contain at least one letter or digit";
		return -1;
	} else if (i > 10) {
		bootstrap_msg =
		    "instance key must be shorter than 10 characters";
		return -1;
	}
	return 0;
}

// validate_instance_name performs full validation of the given snap instance name.
int validate_instance_name(const char *instance_name)
{
	// NOTE: This function should be synchronized with the two other
	// implementations: sc_instance_name_validate and snap.ValidateInstanceName.

	if (instance_name == NULL) {
		bootstrap_msg = "snap instance name cannot be NULL";
		return -1;
	}
	// 40 char snap_name + '_' + 10 char instance_key + 1 extra overflow + 1
	// NULL
	char s[53] = { 0 };
	strncpy(s, instance_name, sizeof(s) - 1);

	char *t = s;
	const char *snap_name = strsep(&t, "_");
	const char *instance_key = strsep(&t, "_");
	const char *third_separator = strsep(&t, "_");
	if (third_separator != NULL) {
		bootstrap_msg =
		    "snap instance name can contain only one underscore";
		return -1;
	}

	if (validate_snap_name(snap_name) < 0) {
		return -1;
	}
	// When the instance_name is a normal snap name, instance_key will be
	// NULL, so only validate instance_key when we found one.
	if (instance_key != NULL && instance_key_validate(instance_key) < 0) {
		return -1;
	}

	return 0;
}

// parse the -u argument, returns -1 on failure or 0 on success.
static int parse_arg_u(int argc, char *const *argv, int *optind,
		       unsigned long *uid_out)
{
	if (*optind + 1 == argc || argv[*optind + 1] == NULL) {
		bootstrap_msg = "-u requires an argument";
		bootstrap_errno = 0;
		return -1;
	}
	const char *uid_text = argv[*optind + 1];
	errno = 0;
	char *uid_text_end = NULL;
	unsigned long parsed_uid = strtoul(uid_text, &uid_text_end, 10);
	int saved_errno = errno;
	char c = *uid_text;
	if (
		   /* Reject overflow in parsed representation */
		   (parsed_uid == ULONG_MAX && errno != 0)
		   /* Reject leading whitespace allowed by strtoul. */
		   /* NOTE: We are reimplementing isspace() here.
		    * For context see
		    * https://github.com/golang/go/issues/29689 */
		   || c == ' ' || c == '\t' || c == '\v' || c == '\r'
		   || c == '\n'
		   /* Reject empty string. */
		   || (*uid_text == '\0')
		   /* Reject partially parsed strings. */
		   || (*uid_text != '\0' && uid_text_end != NULL
		       && *uid_text_end != '\0')) {
		bootstrap_msg = "cannot parse user id";
		bootstrap_errno = saved_errno;
		return -1;
	}
	if ((long)parsed_uid < 0) {
		bootstrap_msg = "user id cannot be negative";
		bootstrap_errno = 0;
		return -1;
	}
	if (uid_out != NULL) {
		*uid_out = parsed_uid;
	}
	*optind += 1;		// Account for the argument to -u.
	return 0;
}

// process_arguments parses given a command line
// argc and argv are defined as for the main() function
void process_arguments(int argc, char *const *argv, const char **snap_name_out,
		       bool *should_setns_out, bool *process_user_fstab,
		       unsigned long *uid_out)
{
	// Find the name of the called program. If it is ending with ".test" then do nothing.
	// NOTE: This lets us use cgo/go to write tests without running the bulk
	// of the code automatically.
	//
	if (argv == NULL || argc < 1) {
		bootstrap_errno = 0;
		bootstrap_msg = "argv0 is corrupted";
		return;
	}
	const char *argv0 = argv[0];
	const char *argv0_suffix_maybe = strstr(argv0, ".test");
	if (argv0_suffix_maybe != NULL
	    && argv0_suffix_maybe[strlen(".test")] == '\0') {
		bootstrap_errno = 0;
		bootstrap_msg = "bootstrap is not enabled while testing";
		return;
	}

	bool should_setns = true;
	bool user_fstab = false;
	const char *snap_name = NULL;

	// Sanity check the command line arguments.  The go parts will
	// scan this too.
	int i;
	for (i = 1; i < argc; i++) {
		const char *arg = argv[i];
		if (arg[0] == '-') {
			/* We have an option */
			if (!strcmp(arg, "--from-snap-confine")) {
				// When we are running under "--from-snap-confine"
				// option skip the setns call as snap-confine has
				// already placed us in the right namespace.
				should_setns = false;
			} else if (!strcmp(arg, "--user-mounts")) {
				user_fstab = true;
				// Processing the user-fstab file implies we're being
				// called from snap-confine.
				should_setns = false;
			} else if (!strcmp(arg, "-u")) {
				if (parse_arg_u(argc, argv, &i, uid_out) < 0) {
					return;
				}
				// Providing an user identifier implies we are performing an
				// update of a specific user mount namespace and that we are
				// invoked from snapd and we should setns ourselves. When
				// invoked from snap-confine we are only called with
				// --from-snap-confine and with --user-mounts.
				should_setns = true;
				user_fstab = true;
			} else {
				bootstrap_errno = 0;
				bootstrap_msg = "unsupported option";
				return;
			}
		} else {
			// We expect a single positional argument: the snap name
			if (snap_name != NULL) {
				bootstrap_errno = 0;
				bootstrap_msg = "too many positional arguments";
				return;
			}
			snap_name = arg;
		}
	}

	// If there's no snap name given, just bail out.
	if (snap_name == NULL) {
		bootstrap_errno = 0;
		bootstrap_msg = "snap name not provided";
		return;
	}
	// Ensure that the snap instance name is valid so that we don't blindly setns into
	// something that is controlled by a potential attacker.
	if (validate_instance_name(snap_name) < 0) {
		bootstrap_errno = 0;
		// bootstap_msg is set by validate_instance_name;
		return;
	}
	// We have a valid snap name now so let's store it.
	if (snap_name_out != NULL) {
		*snap_name_out = snap_name;
	}
	if (should_setns_out != NULL) {
		*should_setns_out = should_setns;
	}
	if (process_user_fstab != NULL) {
		*process_user_fstab = user_fstab;
	}
	bootstrap_errno = 0;
	bootstrap_msg = NULL;
}

// bootstrap prepares snap-update-ns to work in the namespace of the snap given
// on command line.
void bootstrap(int argc, char **argv, char **envp)
{
	// We may have been started via a setuid-root snap-confine. In order to
	// prevent environment-based attacks we start by erasing all environment
	// variables.
	char *snapd_debug = getenv("SNAPD_DEBUG");
	if (clearenv() != 0) {
		bootstrap_errno = 0;
		bootstrap_msg = "bootstrap could not clear the environment";
		return;
	}
	if (snapd_debug != NULL) {
		setenv("SNAPD_DEBUG", snapd_debug, 0);
	}
	// Analyze the read process cmdline to find the snap name and decide if we
	// should use setns to jump into the mount namespace of a particular snap.
	// This is spread out for easier testability.
	const char *snap_name = NULL;
	bool should_setns = false;
	bool process_user_fstab = false;
	unsigned long uid = 0;
	process_arguments(argc, argv, &snap_name, &should_setns,
			  &process_user_fstab, &uid);
	if (process_user_fstab) {
		switch_to_privileged_user();
		// switch_to_privileged_user sets bootstrap_{errno,msg}
	} else if (snap_name != NULL && should_setns) {
		setns_into_snap(snap_name);
		// setns_into_snap sets bootstrap_{errno,msg}
	}
}