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package app
import (
"bytes"
"encoding/base64"
"encoding/gob"
"fmt"
"os"
"os/exec"
"strings"
"syscall"
)
const shimPayload = "FORTIFY_SHIM_PAYLOAD"
func (a *app) shimPayloadEnv() string {
r := &bytes.Buffer{}
enc := base64.NewEncoder(base64.StdEncoding, r)
if err := gob.NewEncoder(enc).Encode(a.seal.command); err != nil {
// should be unreachable
panic(err)
}
_ = enc.Close()
return shimPayload + "=" + r.String()
}
// TryShim attempts the early hidden launcher shim path
func TryShim() {
// environment variable contains encoded argv
if r, ok := os.LookupEnv(shimPayload); ok {
// everything beyond this point runs as target user
// proceed with caution!
// parse base64 revealing underlying gob stream
dec := base64.NewDecoder(base64.StdEncoding, strings.NewReader(r))
// decode argv gob stream
var argv []string
if err := gob.NewDecoder(dec).Decode(&argv); err != nil {
fmt.Println("fortify-shim: cannot decode shim payload:", err)
os.Exit(1)
}
// remove payload variable since the child does not need to see it
if err := os.Unsetenv(shimPayload); err != nil {
fmt.Println("fortify-shim: cannot unset shim payload:", err)
// not fatal, do not fail
}
// look up argv0
var argv0 string
if len(argv) > 0 {
// look up program from $PATH
if p, err := exec.LookPath(argv[0]); err != nil {
fmt.Printf("%s not found: %s\n", argv[0], err)
os.Exit(1)
} else {
argv0 = p
}
} else {
// no argv, look up shell instead
if argv0, ok = os.LookupEnv("SHELL"); !ok {
fmt.Println("fortify-shim: no command was specified and $SHELL was unset")
os.Exit(1)
}
argv = []string{argv0}
}
// exec target process
if err := syscall.Exec(argv0, argv, os.Environ()); err != nil {
fmt.Println("fortify-shim: cannot execute shim payload:", err)
os.Exit(1)
}
// unreachable
os.Exit(1)
return
}
}
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