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cgroups.go
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cgroups.go
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package main
import (
"strconv"
"io/ioutil"
"fmt"
"strings"
"bytes"
)
type deviceRule struct {
deviceType string
majorType int
minorType int
read bool
write bool
mknod bool
}
func (rule *deviceRule) String() string {
var major, minor string
if rule.majorType >= 0 {
major = strconv.Itoa(rule.majorType)
} else {
major = "*"
}
if rule.minorType >= 0 {
minor = strconv.Itoa(rule.minorType)
} else {
minor = "*"
}
types := fmt.Sprintf("%s:%s", major, minor)
access := ""
if rule.read {
access += "r"
}
if rule.write {
access += "w"
}
if rule.mknod {
access += "m"
}
return fmt.Sprintf("%s %s %s", rule.deviceType, types, access)
}
type DeviceRules struct {
rules []*deviceRule
}
func (rules *DeviceRules) String() string {
s := ""
for _, rule := range rules.rules {
s += rule.String() + "\n"
}
return s
}
func (rules *DeviceRules) CheckDevice(devType string, majorType int, minorType int, access string) bool {
for _, rule := range rules.rules {
if rule.deviceType != "a" && rule.deviceType != devType {
continue
}
if rule.majorType >= 0 && rule.majorType != majorType {
continue
}
if rule.majorType >= 0 && rule.minorType != minorType {
continue
}
if access == "r" && !rule.read {
continue
}
if access == "w" && !rule.write {
continue
}
if access == "m" && !rule.mknod {
continue
}
return true
}
return false
}
func (rules *DeviceRules) addRule(entry string) {
fields := strings.Fields(string(entry))
if len(fields) == 0 {
return
}
typeParts := strings.Split(fields[1], ":")
var major, minor int
if typeParts[0] == "*" {
major = -1
} else {
var err error
major, err = strconv.Atoi(typeParts[0])
if err != nil {
major = -2
}
}
if typeParts[1] == "*" {
minor = -1
} else {
var err error
minor, err = strconv.Atoi(typeParts[1])
if err != nil {
minor = -2
}
}
rule := &deviceRule{
deviceType: fields[0],
majorType: major,
minorType: minor,
read: strings.Contains(fields[2], "r"),
write: strings.Contains(fields[2], "w"),
mknod: strings.Contains(fields[2], "m"),
}
rules.rules = append(rules.rules, rule)
}
func NewDeviceRules() (*DeviceRules) {
return &DeviceRules{
rules: make([]*deviceRule, 0),
}
}
func ReadDevices(cgroupDevicesPath string) (*DeviceRules, error) {
ret := NewDeviceRules()
data, err := ioutil.ReadFile(cgroupDevicesPath + "/devices.list")
if err != nil {
return nil, fmt.Errorf("error reading devices.list: %s", err)
}
lines := bytes.Split(data, []byte("\n"))
for _, line := range lines {
ret.addRule(string(line))
}
return ret, nil
}
func getDangerousDevices(rules *DeviceRules) []*deviceRule {
/*
we're just going to assume anything in the docker default is safe and everything
else is scary and should be disallowed. Maybe we'll loosen this up later
c 1:5 rwm
c 1:3 rwm
c 1:9 rwm
c 1:8 rwm
c 5:0 rwm
c 5:1 rwm
c *:* m
b *:* m
c 1:7 rwm
c 136:* rwm
c 5:2 rwm
c 10:200 rwm
*/
ret := make([]*deviceRule, 0)
for _, rule := range rules.rules {
if rule.deviceType == "a" {
ret = append(ret, rule)
continue
} else if rule.deviceType == "c" {
if rule.majorType == 1 {
if rule.minorType == 3 || // /dev/null
rule.minorType == 5 || // /dev/zero
rule.minorType == 7 || // /dev/full
rule.minorType == 8 || // /dev/random
rule.minorType == 9 { // /dev/urandom
continue
}
} else if rule.majorType == 5 {
if rule.minorType == 0 || // /dev/tty
rule.minorType == 1 || // /dev/console
rule.minorType == 2 { // /dev/ptmx
continue
}
} else if rule.majorType == 10 && rule.minorType == 200 {
continue
} else if rule.majorType == 136 {
continue
}
}
if rule.mknod && !rule.read && !rule.write {
continue
}
ret = append(ret, rule)
}
return ret
}
func ScanCgroups(state *scanState) ([]*ScanResult, error) {
// Scan for capability related issues, store capabilities in state
results := make([]*ScanResult, 0)
if state.CgroupDeviceRules == nil {
return results, nil
}
// Scan for devices
allowedDevices := state.CgroupDeviceRules
dangerousDevices := getDangerousDevices(allowedDevices)
if len(dangerousDevices) > 0 {
devlist := ""
for _, device := range dangerousDevices {
devlist += device.String() + "\n"
}
desc := "The following cgroup device rules allow contianer users to access potentially dangerous devices:\n" + devlist
result := NewResult("Cgroup allows access to potentially dangerous devices", desc, SEV_MEDIUM)
results = append(results, result)
}
err := ioutil.WriteFile(state.CgroupPath+"/devices/devices.allow",
[]byte("a"), 0644)
if err == nil {
result := NewResult("Cgroups Device Settings Modifiable", "Processes in the conainer can modify the devices cgroup settings which would allow them to access potentially dangerous devices", SEV_HIGH)
results = append(results, result)
}
if state.CgroupCPUShares == 1024 {
result := NewResult("Cgroup Policy Does Not Restrict CPU Usage", "Processes in the conainer are capable of using excessive amounts of CPU time", SEV_INFO)
results = append(results, result)
}
if state.CgroupMaxMemory > 1024*1024*1024*8 {
result := NewResult("Cgroup Policy Allows for Excessive Memory Usage", "Processes in the conainer are capable of using excessive amounts (>8GB) of RAM", SEV_INFO)
results = append(results, result)
}
return results, nil
}