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package fst
import (
"errors"
"fmt"
"io/fs"
"path"
"git.gensokyo.uk/security/fortify/dbus"
"git.gensokyo.uk/security/fortify/helper/bwrap"
"git.gensokyo.uk/security/fortify/internal/fmsg"
"git.gensokyo.uk/security/fortify/internal/linux"
)
// SandboxConfig describes resources made available to the sandbox.
type SandboxConfig struct {
// unix hostname within sandbox
Hostname string `json:"hostname,omitempty"`
// allow userns within sandbox
UserNS bool `json:"userns,omitempty"`
// share net namespace
Net bool `json:"net,omitempty"`
// share all devices
Dev bool `json:"dev,omitempty"`
// seccomp syscall filter policy
Syscall *bwrap.SyscallPolicy `json:"syscall"`
// do not run in new session
NoNewSession bool `json:"no_new_session,omitempty"`
// map target user uid to privileged user uid in the user namespace
MapRealUID bool `json:"map_real_uid"`
// direct access to wayland socket
DirectWayland bool `json:"direct_wayland,omitempty"`
// final environment variables
Env map[string]string `json:"env"`
// sandbox host filesystem access
Filesystem []*FilesystemConfig `json:"filesystem"`
// symlinks created inside the sandbox
Link [][2]string `json:"symlink"`
// read-only /etc directory
Etc string `json:"etc,omitempty"`
// automatically set up /etc symlinks
AutoEtc bool `json:"auto_etc"`
// paths to override by mounting tmpfs over them
Override []string `json:"override"`
}
// Bwrap returns the address of the corresponding bwrap.Config to s.
// Note that remaining tmpfs entries must be queued by the caller prior to launch.
func (s *SandboxConfig) Bwrap(os linux.System) (*bwrap.Config, error) {
if s == nil {
return nil, errors.New("nil sandbox config")
}
if s.Syscall == nil {
fmsg.VPrintln("syscall filter not configured, PROCEED WITH CAUTION")
}
var uid int
if !s.MapRealUID {
uid = 65534
} else {
uid = os.Geteuid()
}
conf := (&bwrap.Config{
Net: s.Net,
UserNS: s.UserNS,
Hostname: s.Hostname,
Clearenv: true,
SetEnv: s.Env,
/* this is only 4 KiB of memory on a 64-bit system,
permissive defaults on NixOS results in around 100 entries
so this capacity should eliminate copies for most setups */
Filesystem: make([]bwrap.FSBuilder, 0, 256),
Syscall: s.Syscall,
NewSession: !s.NoNewSession,
DieWithParent: true,
AsInit: true,
// initialise unconditionally as Once cannot be justified
// for saving such a miniscule amount of memory
Chmod: make(bwrap.ChmodConfig),
}).
SetUID(uid).SetGID(uid).
Procfs("/proc").
Tmpfs(Tmp, 4*1024)
if !s.Dev {
conf.DevTmpfs("/dev").Mqueue("/dev/mqueue")
} else {
conf.Bind("/dev", "/dev", false, true, true)
}
if !s.AutoEtc {
if s.Etc == "" {
conf.Dir("/etc")
} else {
conf.Bind(s.Etc, "/etc")
}
}
// retrieve paths and hide them if they're made available in the sandbox
var hidePaths []string
sc := os.Paths()
hidePaths = append(hidePaths, sc.RuntimePath, sc.SharePath)
_, systemBusAddr := dbus.Address()
if entries, err := dbus.Parse([]byte(systemBusAddr)); err != nil {
return nil, err
} else {
// there is usually only one, do not preallocate
for _, entry := range entries {
if entry.Method != "unix" {
continue
}
for _, pair := range entry.Values {
if pair[0] == "path" {
if path.IsAbs(pair[1]) {
// get parent dir of socket
dir := path.Dir(pair[1])
if dir == "." || dir == "/" {
fmsg.VPrintf("dbus socket %q is in an unusual location", pair[1])
}
hidePaths = append(hidePaths, dir)
} else {
fmsg.VPrintf("dbus socket %q is not absolute", pair[1])
}
}
}
}
}
hidePathMatch := make([]bool, len(hidePaths))
for i := range hidePaths {
if err := evalSymlinks(os, &hidePaths[i]); err != nil {
return nil, err
}
}
for _, c := range s.Filesystem {
if c == nil {
continue
}
if !path.IsAbs(c.Src) {
return nil, fmt.Errorf("src path %q is not absolute", c.Src)
}
dest := c.Dst
if c.Dst == "" {
dest = c.Src
} else if !path.IsAbs(dest) {
return nil, fmt.Errorf("dst path %q is not absolute", dest)
}
srcH := c.Src
if err := evalSymlinks(os, &srcH); err != nil {
return nil, err
}
for i := range hidePaths {
// skip matched entries
if hidePathMatch[i] {
continue
}
if ok, err := deepContainsH(srcH, hidePaths[i]); err != nil {
return nil, err
} else if ok {
hidePathMatch[i] = true
fmsg.VPrintf("hiding paths from %q", c.Src)
}
}
conf.Bind(c.Src, dest, !c.Must, c.Write, c.Device)
}
// hide marked paths before setting up shares
for i, ok := range hidePathMatch {
if ok {
conf.Tmpfs(hidePaths[i], 8192)
}
}
for _, l := range s.Link {
conf.Symlink(l[0], l[1])
}
if s.AutoEtc {
etc := s.Etc
if etc == "" {
etc = "/etc"
}
conf.Bind(etc, Tmp+"/etc")
// link host /etc contents to prevent passwd/group from being overwritten
if d, err := os.ReadDir(etc); err != nil {
return nil, err
} else {
for _, ent := range d {
name := ent.Name()
switch name {
case "passwd":
case "group":
case "mtab":
conf.Symlink("/proc/mounts", "/etc/"+name)
default:
conf.Symlink(Tmp+"/etc/"+name, "/etc/"+name)
}
}
}
}
return conf, nil
}
func evalSymlinks(os linux.System, v *string) error {
if p, err := os.EvalSymlinks(*v); err != nil {
if !errors.Is(err, fs.ErrNotExist) {
return err
}
fmsg.VPrintf("path %q does not yet exist", *v)
} else {
*v = p
}
return nil
}
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