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package app
import (
"context"
"encoding/gob"
"errors"
"log"
"os"
"os/exec"
"os/signal"
"strconv"
"strings"
"syscall"
"time"
"git.gensokyo.uk/security/fortify/internal"
"git.gensokyo.uk/security/fortify/internal/fmsg"
"git.gensokyo.uk/security/fortify/sandbox"
)
const shimEnv = "FORTIFY_SHIM"
type shimParams struct {
// finalised container params
Container *sandbox.Params
// path to outer home directory
Home string
// verbosity pass through
Verbose bool
}
// ShimMain is the main function of the shim process and runs as the unconstrained target user.
func ShimMain() {
fmsg.Prepare("shim")
if err := sandbox.SetDumpable(sandbox.SUID_DUMP_DISABLE); err != nil {
log.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
}
var (
params shimParams
closeSetup func() error
)
if f, err := sandbox.Receive(shimEnv, ¶ms, nil); err != nil {
if errors.Is(err, sandbox.ErrInvalid) {
log.Fatal("invalid config descriptor")
}
if errors.Is(err, sandbox.ErrNotSet) {
log.Fatal("FORTIFY_SHIM not set")
}
log.Fatalf("cannot receive shim setup params: %v", err)
} else {
internal.InstallFmsg(params.Verbose)
closeSetup = f
}
if params.Container == nil || params.Container.Ops == nil {
log.Fatal("invalid container params")
}
// close setup socket
if err := closeSetup(); err != nil {
log.Printf("cannot close setup pipe: %v", err)
// not fatal
}
// ensure home directory as target user
if s, err := os.Stat(params.Home); err != nil {
if os.IsNotExist(err) {
if err = os.Mkdir(params.Home, 0700); err != nil {
log.Fatalf("cannot create home directory: %v", err)
}
} else {
log.Fatalf("cannot access home directory: %v", err)
}
// home directory is created, proceed
} else if !s.IsDir() {
log.Fatalf("path %q is not a directory", params.Home)
}
var name string
if len(params.Container.Args) > 0 {
name = params.Container.Args[0]
}
ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
defer stop() // unreachable
container := sandbox.New(ctx, name)
container.Params = *params.Container
container.Stdin, container.Stdout, container.Stderr = os.Stdin, os.Stdout, os.Stderr
container.Cancel = func(cmd *exec.Cmd) error { return cmd.Process.Signal(os.Interrupt) }
container.WaitDelay = 2 * time.Second
if err := container.Start(); err != nil {
fmsg.PrintBaseError(err, "cannot start container:")
os.Exit(1)
}
if err := container.Serve(); err != nil {
fmsg.PrintBaseError(err, "cannot configure container:")
}
if err := container.Wait(); err != nil {
var exitError *exec.ExitError
if !errors.As(err, &exitError) {
if errors.Is(err, context.Canceled) {
os.Exit(2)
}
log.Printf("wait: %v", err)
os.Exit(127)
}
os.Exit(exitError.ExitCode())
}
}
type shimProcess struct {
// user switcher process
cmd *exec.Cmd
// fallback exit notifier with error returned killing the process
killFallback chan error
// monitor to shim encoder
encoder *gob.Encoder
}
func (s *shimProcess) Unwrap() *exec.Cmd { return s.cmd }
func (s *shimProcess) Fallback() chan error { return s.killFallback }
func (s *shimProcess) String() string {
if s.cmd == nil {
return "(unused shim manager)"
}
return s.cmd.String()
}
func (s *shimProcess) Start(
aid string,
supp []string,
) (*time.Time, error) {
// prepare user switcher invocation
fsuPath := internal.MustFsuPath()
s.cmd = exec.Command(fsuPath)
// pass shim setup pipe
if fd, e, err := sandbox.Setup(&s.cmd.ExtraFiles); err != nil {
return nil, fmsg.WrapErrorSuffix(err,
"cannot create shim setup pipe:")
} else {
s.encoder = e
s.cmd.Env = []string{
shimEnv + "=" + strconv.Itoa(fd),
"FORTIFY_APP_ID=" + aid,
}
}
// format fsu supplementary groups
if len(supp) > 0 {
fmsg.Verbosef("attaching supplementary group ids %s", supp)
s.cmd.Env = append(s.cmd.Env, "FORTIFY_GROUPS="+strings.Join(supp, " "))
}
s.cmd.Stdin, s.cmd.Stdout, s.cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
s.cmd.Dir = "/"
fmsg.Verbose("starting shim via fsu:", s.cmd)
// withhold messages to stderr
fmsg.Suspend()
if err := s.cmd.Start(); err != nil {
return nil, fmsg.WrapErrorSuffix(err,
"cannot start fsu:")
}
startTime := time.Now().UTC()
return &startTime, nil
}
func (s *shimProcess) Serve(ctx context.Context, params *shimParams) error {
// kill shim if something goes wrong and an error is returned
s.killFallback = make(chan error, 1)
killShim := func() {
if err := s.cmd.Process.Signal(os.Interrupt); err != nil {
s.killFallback <- err
}
}
defer func() { killShim() }()
encodeErr := make(chan error)
go func() { encodeErr <- s.encoder.Encode(params) }()
select {
// encode return indicates setup completion
case err := <-encodeErr:
if err != nil {
return fmsg.WrapErrorSuffix(err,
"cannot transmit shim config:")
}
killShim = func() {}
return nil
// setup canceled before payload was accepted
case <-ctx.Done():
err := ctx.Err()
if errors.Is(err, context.Canceled) {
return fmsg.WrapError(syscall.ECANCELED,
"shim setup canceled")
}
if errors.Is(err, context.DeadlineExceeded) {
return fmsg.WrapError(syscall.ETIMEDOUT,
"deadline exceeded waiting for shim")
}
// unreachable
return err
}
}
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