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package shim
import (
"errors"
"net"
"os"
"os/exec"
"sync"
"sync/atomic"
"syscall"
"time"
"git.ophivana.moe/security/fortify/acl"
"git.ophivana.moe/security/fortify/internal/fmsg"
)
// used by the parent process
type Shim struct {
// user switcher process
cmd *exec.Cmd
// uid of shim target user
uid uint32
// whether to check shim pid
checkPid bool
// user switcher executable path
executable string
// path to setup socket
socket string
// shim setup abort reason and completion
abort chan error
abortErr atomic.Pointer[error]
abortOnce sync.Once
// wayland mediation, nil if disabled
wl *Wayland
// shim setup payload
payload *Payload
}
func New(executable string, uid uint32, socket string, wl *Wayland, payload *Payload, checkPid bool) *Shim {
return &Shim{uid: uid, executable: executable, socket: socket, wl: wl, payload: payload, checkPid: checkPid}
}
func (s *Shim) String() string {
if s.cmd == nil {
return "(unused shim manager)"
}
return s.cmd.String()
}
func (s *Shim) Unwrap() *exec.Cmd {
return s.cmd
}
func (s *Shim) Abort(err error) {
s.abortOnce.Do(func() {
s.abortErr.Store(&err)
// s.abort is buffered so this will never block
s.abort <- err
})
}
func (s *Shim) AbortWait(err error) {
s.Abort(err)
<-s.abort
}
type CommandBuilder func(shimEnv string) (args []string)
func (s *Shim) Start(f CommandBuilder) (*time.Time, error) {
var (
cf chan *net.UnixConn
accept func()
)
// listen on setup socket
if c, a, err := s.serve(); err != nil {
return nil, fmsg.WrapErrorSuffix(err,
"cannot listen on shim setup socket:")
} else {
// accepts a connection after each call to accept
// connections are sent to the channel cf
cf, accept = c, a
}
// start user switcher process and save time
s.cmd = exec.Command(s.executable, f(EnvShim+"="+s.socket)...)
s.cmd.Env = []string{}
s.cmd.Stdin, s.cmd.Stdout, s.cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
s.cmd.Dir = "/"
fmsg.VPrintln("starting shim via user switcher:", s.cmd)
fmsg.Withhold() // withhold messages to stderr
if err := s.cmd.Start(); err != nil {
return nil, fmsg.WrapErrorSuffix(err,
"cannot start user switcher:")
}
startTime := time.Now().UTC()
// kill shim if something goes wrong and an error is returned
killShim := func() {
if err := s.cmd.Process.Signal(os.Interrupt); err != nil {
fmsg.Println("cannot terminate shim on faulted setup:", err)
}
}
defer func() { killShim() }()
accept()
conn := <-cf
if conn == nil {
return &startTime, fmsg.WrapErrorSuffix(*s.abortErr.Load(), "cannot accept call on setup socket:")
}
// authenticate against called provided uid and shim pid
if cred, err := peerCred(conn); err != nil {
return &startTime, fmsg.WrapErrorSuffix(*s.abortErr.Load(), "cannot retrieve shim credentials:")
} else if cred.Uid != s.uid {
fmsg.Printf("process %d owned by user %d tried to connect, expecting %d",
cred.Pid, cred.Uid, s.uid)
err = errors.New("compromised fortify build")
s.Abort(err)
return &startTime, err
} else if s.checkPid && cred.Pid != int32(s.cmd.Process.Pid) {
fmsg.Printf("process %d tried to connect to shim setup socket, expecting shim %d",
cred.Pid, s.cmd.Process.Pid)
err = errors.New("compromised target user")
s.Abort(err)
return &startTime, err
}
// serve payload and wayland fd if enabled
// this also closes the connection
err := s.payload.serve(conn, s.wl)
if err == nil {
killShim = func() {}
}
s.Abort(err) // aborting with nil indicates success
return &startTime, err
}
func (s *Shim) serve() (chan *net.UnixConn, func(), error) {
if s.abort != nil {
panic("attempted to serve shim setup twice")
}
s.abort = make(chan error, 1)
cf := make(chan *net.UnixConn)
accept := make(chan struct{}, 1)
if l, err := net.ListenUnix("unix", &net.UnixAddr{Name: s.socket, Net: "unix"}); err != nil {
return nil, nil, err
} else {
l.SetUnlinkOnClose(true)
fmsg.VPrintf("listening on shim setup socket %q", s.socket)
if err = acl.UpdatePerm(s.socket, int(s.uid), acl.Read, acl.Write, acl.Execute); err != nil {
fmsg.Println("cannot append ACL entry to shim setup socket:", err)
s.Abort(err) // ensures setup socket cleanup
}
go func() {
for {
select {
case err = <-s.abort:
if err != nil {
fmsg.VPrintln("aborting shim setup, reason:", err)
}
if err = l.Close(); err != nil {
fmsg.Println("cannot close setup socket:", err)
}
close(s.abort)
close(cf)
return
case <-accept:
if conn, err0 := l.AcceptUnix(); err0 != nil {
s.Abort(err0) // does not block, breaks loop
cf <- nil // receiver sees nil value and loads err0 stored during abort
} else {
cf <- conn
}
}
}
}()
}
return cf, func() { accept <- struct{}{} }, nil
}
// peerCred fetches peer credentials of conn
func peerCred(conn *net.UnixConn) (ucred *syscall.Ucred, err error) {
var raw syscall.RawConn
if raw, err = conn.SyscallConn(); err != nil {
return
}
err0 := raw.Control(func(fd uintptr) {
ucred, err = syscall.GetsockoptUcred(int(fd), syscall.SOL_SOCKET, syscall.SO_PEERCRED)
})
err = errors.Join(err, err0)
return
}
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