github.com/anonymouse64/snapd@v0.0.0-20210824153203-04c4c42d842d/cmd/snap-confine/udev-support.c

/*
 * Copyright (C) 2015-2020 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/>.
 *
 */
#include "config.h"

#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
#include <sys/types.h>
#include <unistd.h>
#include <limits.h>
#include <stdint.h>

#include <libudev.h>

#include "../libsnap-confine-private/cgroup-support.h"
#include "../libsnap-confine-private/cleanup-funcs.h"
#include "../libsnap-confine-private/device-cgroup-support.h"
#include "../libsnap-confine-private/snap.h"
#include "../libsnap-confine-private/string-utils.h"
#include "../libsnap-confine-private/utils.h"
#include "udev-support.h"

/* Allow access to common devices. */
static void sc_udev_allow_common(sc_device_cgroup * cgroup)
{
	/* The devices we add here have static number allocation.
	 * https://www.kernel.org/doc/html/v4.11/admin-guide/devices.html */
	sc_device_cgroup_allow(cgroup, S_IFCHR, 1, 3);	// /dev/null
	sc_device_cgroup_allow(cgroup, S_IFCHR, 1, 5);	// /dev/zero
	sc_device_cgroup_allow(cgroup, S_IFCHR, 1, 7);	// /dev/full
	sc_device_cgroup_allow(cgroup, S_IFCHR, 1, 8);	// /dev/random
	sc_device_cgroup_allow(cgroup, S_IFCHR, 1, 9);	// /dev/urandom
	sc_device_cgroup_allow(cgroup, S_IFCHR, 5, 0);	// /dev/tty
	sc_device_cgroup_allow(cgroup, S_IFCHR, 5, 1);	// /dev/console
	sc_device_cgroup_allow(cgroup, S_IFCHR, 5, 2);	// /dev/ptmx
}

/** Allow access to current and future PTY slaves.
 *
 * We unconditionally add them since we use a devpts newinstance. Unix98 PTY
 * slaves major are 136-143.
 *
 * See also:
 * https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
 **/
static void sc_udev_allow_pty_slaves(sc_device_cgroup * cgroup)
{
	for (unsigned pty_major = 136; pty_major <= 143; pty_major++) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, pty_major,
				       SC_DEVICE_MINOR_ANY);
	}
}

/** Allow access to Nvidia devices.
 *
 * Nvidia modules are proprietary and therefore aren't in sysfs and can't be
 * udev tagged. For now, just add existing nvidia devices to the cgroup
 * unconditionally (AppArmor will still mediate the access).  We'll want to
 * rethink this if snapd needs to mediate access to other proprietary devices.
 *
 * Device major and minor numbers are described in (though nvidia-uvm currently
 * isn't listed):
 *
 * https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
 **/
static void sc_udev_allow_nvidia(sc_device_cgroup * cgroup)
{
	struct stat sbuf;

	/* Allow access to /dev/nvidia0 through /dev/nvidia254 */
	for (unsigned nv_minor = 0; nv_minor < 255; nv_minor++) {
		char nv_path[15] = { 0 };	// /dev/nvidiaXXX
		sc_must_snprintf(nv_path, sizeof(nv_path), "/dev/nvidia%u",
				 nv_minor);

		/* Stop trying to find devices after one is not found. In this manner,
		 * we'll add /dev/nvidia0 and /dev/nvidia1 but stop trying to find
		 * nvidia3 - nvidia254 if nvidia2 is not found. */
		if (stat(nv_path, &sbuf) < 0) {
			break;
		}
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}

	if (stat("/dev/nvidiactl", &sbuf) == 0) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}
	if (stat("/dev/nvidia-uvm", &sbuf) == 0) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}
	if (stat("/dev/nvidia-modeset", &sbuf) == 0) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}
}

/**
 * Allow access to /dev/uhid.
 *
 * Currently /dev/uhid isn't represented in sysfs, so add it to the device
 * cgroup if it exists and let AppArmor handle the mediation.
 **/
static void sc_udev_allow_uhid(sc_device_cgroup * cgroup)
{
	struct stat sbuf;

	if (stat("/dev/uhid", &sbuf) == 0) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}
}

/**
 * Allow access to /dev/net/tun
 *
 * When CONFIG_TUN=m, /dev/net/tun will exist but using it doesn't
 * autoload the tun module but also /dev/net/tun isn't udev tagged
 * until it is loaded. To work around this, if /dev/net/tun exists, add
 * it unconditionally to the cgroup and rely on AppArmor to mediate the
 * access. LP: #1859084
 **/
static void sc_udev_allow_dev_net_tun(sc_device_cgroup * cgroup)
{
	struct stat sbuf;

	if (stat("/dev/net/tun", &sbuf) == 0) {
		sc_device_cgroup_allow(cgroup, S_IFCHR, major(sbuf.st_rdev),
				       minor(sbuf.st_rdev));
	}
}

/**
 * Allow access to assigned devices.
 *
 * The snapd udev security backend uses udev rules to tag matching devices with
 * tags corresponding to snap applications. Here we interrogate udev and allow
 * access to all assigned devices.
 **/
static void sc_udev_allow_assigned_device(sc_device_cgroup * cgroup,
					  struct udev_device *device)
{
	const char *path = udev_device_get_syspath(device);
	dev_t devnum = udev_device_get_devnum(device);
	unsigned int major = major(devnum);
	unsigned int minor = minor(devnum);
	/* The manual page of udev_device_get_devnum says:
	 * > On success, udev_device_get_devnum() returns the device type of
	 * > the passed device. On failure, a device type with minor and major
	 * > number set to 0 is returned. */
	if (major == 0 && minor == 0) {
		debug("cannot get major/minor numbers for syspath %s", path);
		return;
	}
	/* devnode is bound to the lifetime of the device and we cannot release
	 * it separately. */
	const char *devnode = udev_device_get_devnode(device);
	if (devnode == NULL) {
		debug("cannot find /dev node from udev device");
		return;
	}
	debug("inspecting type of device: %s", devnode);
	struct stat file_info;
	if (stat(devnode, &file_info) < 0) {
		debug("cannot stat %s", devnode);
		return;
	}
	int devtype = file_info.st_mode & S_IFMT;
	if (devtype == S_IFBLK || devtype == S_IFCHR) {
		sc_device_cgroup_allow(cgroup, devtype, major, minor);
	}
}

static void sc_udev_setup_acls_common(sc_device_cgroup * cgroup)
{

	/* Allow access to various devices. */
	sc_udev_allow_common(cgroup);
	sc_udev_allow_pty_slaves(cgroup);
	sc_udev_allow_nvidia(cgroup);
	sc_udev_allow_uhid(cgroup);
	sc_udev_allow_dev_net_tun(cgroup);
}

static char *sc_security_to_udev_tag(const char *security_tag)
{
	char *udev_tag = sc_strdup(security_tag);
	for (char *c = strchr(udev_tag, '.'); c != NULL; c = strchr(c, '.')) {
		*c = '_';
	}
	return udev_tag;
}

static void sc_cleanup_udev(struct udev **udev)
{
	if (udev != NULL && *udev != NULL) {
		udev_unref(*udev);
		*udev = NULL;
	}
}

static void sc_cleanup_udev_enumerate(struct udev_enumerate **enumerate)
{
	if (enumerate != NULL && *enumerate != NULL) {
		udev_enumerate_unref(*enumerate);
		*enumerate = NULL;
	}
}

void sc_setup_device_cgroup(const char *security_tag)
{
	debug("setting up device cgroup");

	/* Derive the udev tag from the snap security tag.
	 *
	 * Because udev does not allow for dots in tag names, those are replaced by
	 * underscores in snapd. We just match that behavior. */
	char *udev_tag SC_CLEANUP(sc_cleanup_string) = NULL;
	udev_tag = sc_security_to_udev_tag(security_tag);

	/* Use udev APIs to talk to udev-the-daemon to determine the list of
	 * "devices" with that tag assigned. The list may be empty, in which case
	 * there's no udev tagging in effect and we must refrain from constructing
	 * the cgroup as it would interfere with the execution of a program. */
	struct udev SC_CLEANUP(sc_cleanup_udev) * udev = NULL;
	udev = udev_new();
	if (udev == NULL) {
		die("cannot connect to udev");
	}
	struct udev_enumerate SC_CLEANUP(sc_cleanup_udev_enumerate) * devices =
	    NULL;
	devices = udev_enumerate_new(udev);
	if (devices == NULL) {
		die("cannot create udev device enumeration");
	}
	if (udev_enumerate_add_match_tag(devices, udev_tag) < 0) {
		die("cannot add tag match to udev device enumeration");
	}
	if (udev_enumerate_scan_devices(devices) < 0) {
		die("cannot enumerate udev devices");
	}
	/* NOTE: udev_list_entry is bound to life-cycle of the used udev_enumerate */
	struct udev_list_entry *assigned;
	assigned = udev_enumerate_get_list_entry(devices);
	if (assigned == NULL) {
		/* NOTE: Nothing is assigned, don't create or use the device cgroup. */
		debug("no devices tagged with %s, skipping device cgroup setup",
		      udev_tag);
		return;
	}

	/* Note that -1 is the neutral value for a file descriptor.
	 * The cleanup function associated with this variable closes
	 * descriptors other than -1. */
	sc_device_cgroup *cgroup SC_CLEANUP(sc_device_cgroup_cleanup) =
	    sc_device_cgroup_new(security_tag, 0);
	/* Setup the device group access control list */
	sc_udev_setup_acls_common(cgroup);
	for (struct udev_list_entry * entry = assigned; entry != NULL;
	     entry = udev_list_entry_get_next(entry)) {
		const char *path = udev_list_entry_get_name(entry);
		if (path == NULL) {
			die("udev_list_entry_get_name failed");
		}
		struct udev_device *device =
		    udev_device_new_from_syspath(udev, path);
		/** This is a non-fatal error as devices can disappear asynchronously
		 * and on slow devices we may indeed observe a device that no longer
		 * exists.
		 *
		 * Similar debug + continue pattern repeats in all the udev calls in
		 * this function. Related to LP: #1881209 */
		if (device == NULL) {
			debug("cannot find device from syspath %s", path);
			continue;
		}

		sc_udev_allow_assigned_device(cgroup, device);
		udev_device_unref(device);
	}
	/* Move ourselves to the device cgroup */
	sc_device_cgroup_attach_pid(cgroup, getpid());
	debug("associated snap application process %i with device cgroup %s",
	      getpid(), security_tag);
}