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package shim
import (
"context"
"encoding/gob"
"errors"
"os"
"os/exec"
"strconv"
"strings"
"time"
"git.gensokyo.uk/security/fortify/helper/proc"
"git.gensokyo.uk/security/fortify/internal"
"git.gensokyo.uk/security/fortify/internal/fmsg"
"git.gensokyo.uk/security/fortify/internal/sandbox"
)
// used by the parent process
type Shim 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
// bwrap --sync-fd value
sync *uintptr
}
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) WaitFallback() chan error {
return s.killFallback
}
func (s *Shim) Start(
// string representation of application id
aid string,
// string representation of supplementary group ids
supp []string,
// bwrap --sync-fd
syncFd *os.File,
) (*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{
Env + "=" + 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 = "/"
// pass sync fd if set
if syncFd != nil {
fd := proc.ExtraFile(s.cmd, syncFd)
s.sync = &fd
}
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 *Shim) Serve(ctx context.Context, payload *Payload) 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() }()
payload.Sync = s.sync
encodeErr := make(chan error)
go func() { encodeErr <- s.encoder.Encode(payload) }()
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(errors.New("shim setup canceled"),
"shim setup canceled")
}
if errors.Is(err, context.DeadlineExceeded) {
return fmsg.WrapError(errors.New("deadline exceeded waiting for shim"),
"deadline exceeded waiting for shim")
}
// unreachable
return err
}
}
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