// SPDX-License-Identifier: Apache-2.0
/*
* Copyright (C) 2020 Aleksa Sarai <[email protected]>
* Copyright (C) 2020 SUSE LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package devices
import (
"bufio"
"fmt"
"io"
"sort"
"strconv"
"strings"
"github.com/opencontainers/runc/libcontainer/devices"
)
// deviceMeta is a Rule without the Allow or Permissions fields, and no
// wildcard-type support. It's effectively the "match" portion of a metadata
// rule, for the purposes of our emulation.
type deviceMeta struct {
node devices.Type
major int64
minor int64
}
// deviceRule is effectively the tuple (deviceMeta, Permissions).
type deviceRule struct {
meta deviceMeta
perms devices.Permissions
}
// deviceRules is a mapping of device metadata rules to the associated
// permissions in the ruleset.
type deviceRules map[deviceMeta]devices.Permissions
func (r deviceRules) orderedEntries() []deviceRule {
var rules []deviceRule
for meta, perms := range r {
rules = append(rules, deviceRule{meta: meta, perms: perms})
}
sort.Slice(rules, func(i, j int) bool {
// Sort by (major, minor, type).
a, b := rules[i].meta, rules[j].meta
return a.major < b.major ||
(a.major == b.major && a.minor < b.minor) ||
(a.major == b.major && a.minor == b.minor && a.node < b.node)
})
return rules
}
type Emulator struct {
defaultAllow bool
rules deviceRules
}
func (e *Emulator) IsBlacklist() bool {
return e.defaultAllow
}
func (e *Emulator) IsAllowAll() bool {
return e.IsBlacklist() && len(e.rules) == 0
}
func parseLine(line string) (*deviceRule, error) {
// Input: node major:minor perms.
fields := strings.FieldsFunc(line, func(r rune) bool {
return r == ' ' || r == ':'
})
if len(fields) != 4 {
return nil, fmt.Errorf("malformed devices.list rule %s", line)
}
var (
rule deviceRule
node = fields[0]
major = fields[1]
minor = fields[2]
perms = fields[3]
)
// Parse the node type.
switch node {
case "a":
// Super-special case -- "a" always means every device with every
// access mode. In fact, for devices.list this actually indicates that
// the cgroup is in black-list mode.
// TODO: Double-check that the entire file is "a *:* rwm".
return nil, nil
case "b":
rule.meta.node = devices.BlockDevice
case "c":
rule.meta.node = devices.CharDevice
default:
return nil, fmt.Errorf("unknown device type %q", node)
}
// Parse the major number.
if major == "*" {
rule.meta.major = devices.Wildcard
} else {
val, err := strconv.ParseUint(major, 10, 32)
if err != nil {
return nil, fmt.Errorf("invalid major number: %w", err)
}
rule.meta.major = int64(val)
}
// Parse the minor number.
if minor == "*" {
rule.meta.minor = devices.Wildcard
} else {
val, err := strconv.ParseUint(minor, 10, 32)
if err != nil {
return nil, fmt.Errorf("invalid minor number: %w", err)
}
rule.meta.minor = int64(val)
}
// Parse the access permissions.
rule.perms = devices.Permissions(perms)
if !rule.perms.IsValid() || rule.perms.IsEmpty() {
return nil, fmt.Errorf("parse access mode: contained unknown modes or is empty: %q", perms)
}
return &rule, nil
}
func (e *Emulator) addRule(rule deviceRule) error { //nolint:unparam
if e.rules == nil {
e.rules = make(map[deviceMeta]devices.Permissions)
}
// Merge with any pre-existing permissions.
oldPerms := e.rules[rule.meta]
newPerms := rule.perms.Union(oldPerms)
e.rules[rule.meta] = newPerms
return nil
}
func (e *Emulator) rmRule(rule deviceRule) error {
// Give an error if any of the permissions requested to be removed are
// present in a partially-matching wildcard rule, because such rules will
// be ignored by cgroupv1.
//
// This is a diversion from cgroupv1, but is necessary to avoid leading
// users into a false sense of security. cgroupv1 will silently(!) ignore
// requests to remove partial exceptions, but we really shouldn't do that.
//
// It may seem like we could just "split" wildcard rules which hit this
// issue, but unfortunately there are 2^32 possible major and minor
// numbers, which would exhaust kernel memory quickly if we did this. Not
// to mention it'd be really slow (the kernel side is implemented as a
// linked-list of exceptions).
for _, partialMeta := range []deviceMeta{
{node: rule.meta.node, major: devices.Wildcard, minor: rule.meta.minor},
{node: rule.meta.node, major: rule.meta.major, minor: devices.Wildcard},
{node: rule.meta.node, major: devices.Wildcard, minor: devices.Wildcard},
} {
// This wildcard rule is equivalent to the requested rule, so skip it.
if rule.meta == partialMeta {
continue
}
// Only give an error if the set of permissions overlap.
partialPerms := e.rules[partialMeta]
if !partialPerms.Intersection(rule.perms).IsEmpty() {
return fmt.Errorf("requested rule [%v %v] not supported by devices cgroupv1 (cannot punch hole in existing wildcard rule [%v %v])", rule.meta, rule.perms, partialMeta, partialPerms)
}
}
// Subtract all of the permissions listed from the full match rule. If the
// rule didn't exist, all of this is a no-op.
newPerms := e.rules[rule.meta].Difference(rule.perms)
if newPerms.IsEmpty() {
delete(e.rules, rule.meta)
} else {
e.rules[rule.meta] = newPerms
}
// TODO: The actual cgroup code doesn't care if an exception didn't exist
// during removal, so not erroring out here is /accurate/ but quite
// worrying. Maybe we should do additional validation, but again we
// have to worry about backwards-compatibility.
return nil
}
func (e *Emulator) allow(rule *deviceRule) error {
// This cgroup is configured as a black-list. Reset the entire emulator,
// and put is into black-list mode.
if rule == nil || rule.meta.node == devices.WildcardDevice {
*e = Emulator{
defaultAllow: true,
rules: nil,
}
return nil
}
var err error
if e.defaultAllow {
err = wrapErr(e.rmRule(*rule), "unable to remove 'deny' exception")
} else {
err = wrapErr(e.addRule(*rule), "unable to add 'allow' exception")
}
return err
}
func (e *Emulator) deny(rule *deviceRule) error {
// This cgroup is configured as a white-list. Reset the entire emulator,
// and put is into white-list mode.
if rule == nil || rule.meta.node == devices.WildcardDevice {
*e = Emulator{
defaultAllow: false,
rules: nil,
}
return nil
}
var err error
if e.defaultAllow {
err = wrapErr(e.addRule(*rule), "unable to add 'deny' exception")
} else {
err = wrapErr(e.rmRule(*rule), "unable to remove 'allow' exception")
}
return err
}
func (e *Emulator) Apply(rule devices.Rule) error {
if !rule.Type.CanCgroup() {
return fmt.Errorf("cannot add rule [%#v] with non-cgroup type %q", rule, rule.Type)
}
innerRule := &deviceRule{
meta: deviceMeta{
node: rule.Type,
major: rule.Major,
minor: rule.Minor,
},
perms: rule.Permissions,
}
if innerRule.meta.node == devices.WildcardDevice {
innerRule = nil
}
if rule.Allow {
return e.allow(innerRule)
}
return e.deny(innerRule)
}
// EmulatorFromList takes a reader to a "devices.list"-like source, and returns
// a new Emulator that represents the state of the devices cgroup. Note that
// black-list devices cgroups cannot be fully reconstructed, due to limitations
// in the devices cgroup API. Instead, such cgroups are always treated as
// "allow all" cgroups.
func EmulatorFromList(list io.Reader) (*Emulator, error) {
// Normally cgroups are in black-list mode by default, but the way we
// figure out the current mode is whether or not devices.list has an
// allow-all rule. So we default to a white-list, and the existence of an
// "a *:* rwm" entry will tell us otherwise.
e := &Emulator{
defaultAllow: false,
}
// Parse the "devices.list".
s := bufio.NewScanner(list)
for s.Scan() {
line := s.Text()
deviceRule, err := parseLine(line)
if err != nil {
return nil, fmt.Errorf("error parsing line %q: %w", line, err)
}
// "devices.list" is an allow list. Note that this means that in
// black-list mode, we have no idea what rules are in play. As a
// result, we need to be very careful in Transition().
if err := e.allow(deviceRule); err != nil {
return nil, fmt.Errorf("error adding devices.list rule: %w", err)
}
}
if err := s.Err(); err != nil {
return nil, fmt.Errorf("error reading devices.list lines: %w", err)
}
return e, nil
}
// Transition calculates what is the minimally-disruptive set of rules need to
// be applied to a devices cgroup in order to transition to the given target.
// This means that any already-existing rules will not be applied, and
// disruptive rules (like denying all device access) will only be applied if
// necessary.
//
// This function is the sole reason for all of Emulator -- to allow us
// to figure out how to update a containers' cgroups without causing spurious
// device errors (if possible).
func (source *Emulator) Transition(target *Emulator) ([]*devices.Rule, error) {
var transitionRules []*devices.Rule
oldRules := source.rules
// If the default policy doesn't match, we need to include a "disruptive"
// rule (either allow-all or deny-all) in order to switch the cgroup to the
// correct default policy.
//
// However, due to a limitation in "devices.list" we cannot be sure what
// deny rules are in place in a black-list cgroup. Thus if the source is a
// black-list we also have to include a disruptive rule.
if source.IsBlacklist() || source.defaultAllow != target.defaultAllow {
transitionRules = append(transitionRules, &devices.Rule{
Type: 'a',
Major: -1,
Minor: -1,
Permissions: devices.Permissions("rwm"),
Allow: target.defaultAllow,
})
// The old rules are only relevant if we aren't starting out with a
// disruptive rule.
oldRules = nil
}
// NOTE: We traverse through the rules in a sorted order so we always write
// the same set of rules (this is to aid testing).
// First, we create inverse rules for any old rules not in the new set.
// This includes partial-inverse rules for specific permissions. This is a
// no-op if we added a disruptive rule, since oldRules will be empty.
for _, rule := range oldRules.orderedEntries() {
meta, oldPerms := rule.meta, rule.perms
newPerms := target.rules[meta]
droppedPerms := oldPerms.Difference(newPerms)
if !droppedPerms.IsEmpty() {
transitionRules = append(transitionRules, &devices.Rule{
Type: meta.node,
Major: meta.major,
Minor: meta.minor,
Permissions: droppedPerms,
Allow: target.defaultAllow,
})
}
}
// Add any additional rules which weren't in the old set. We happen to
// filter out rules which are present in both sets, though this isn't
// strictly necessary.
for _, rule := range target.rules.orderedEntries() {
meta, newPerms := rule.meta, rule.perms
oldPerms := oldRules[meta]
gainedPerms := newPerms.Difference(oldPerms)
if !gainedPerms.IsEmpty() {
transitionRules = append(transitionRules, &devices.Rule{
Type: meta.node,
Major: meta.major,
Minor: meta.minor,
Permissions: gainedPerms,
Allow: !target.defaultAllow,
})
}
}
return transitionRules, nil
}
// Rules returns the minimum set of rules necessary to convert a *deny-all*
// cgroup to the emulated filter state (note that this is not the same as a
// default cgroupv1 cgroup -- which is allow-all). This is effectively just a
// wrapper around Transition() with the source emulator being an empty cgroup.
func (e *Emulator) Rules() ([]*devices.Rule, error) {
defaultCgroup := &Emulator{defaultAllow: false}
return defaultCgroup.Transition(e)
}
func wrapErr(err error, text string) error {
if err == nil {
return nil
}
return fmt.Errorf(text+": %w", err)
}
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