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|
package main
import (
"encoding/gob"
"errors"
"net"
"os"
"path"
"strconv"
"syscall"
init0 "git.ophivana.moe/security/fortify/cmd/finit/ipc"
shim "git.ophivana.moe/security/fortify/cmd/fshim/ipc"
"git.ophivana.moe/security/fortify/helper"
"git.ophivana.moe/security/fortify/internal"
"git.ophivana.moe/security/fortify/internal/fmsg"
)
// everything beyond this point runs as unconstrained target user
// proceed with caution!
func main() {
// sharing stdout with fortify
// USE WITH CAUTION
fmsg.SetPrefix("shim")
// setting this prevents ptrace
if err := internal.PR_SET_DUMPABLE__SUID_DUMP_DISABLE(); err != nil {
fmsg.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
panic("unreachable")
}
// re-exec
if len(os.Args) > 0 && (os.Args[0] != "fshim" || len(os.Args) != 1) && path.IsAbs(os.Args[0]) {
if err := syscall.Exec(os.Args[0], []string{"fshim"}, os.Environ()); err != nil {
fmsg.Println("cannot re-exec self:", err)
// continue anyway
}
}
// lookup socket path from environment
var socketPath string
if s, ok := os.LookupEnv(shim.Env); !ok {
fmsg.Fatal("FORTIFY_SHIM not set")
panic("unreachable")
} else {
socketPath = s
}
// check path to finit
var finitPath string
if p, ok := internal.Path(internal.Finit); !ok {
fmsg.Fatal("invalid finit path, this copy of fshim is not compiled correctly")
} else {
finitPath = p
}
// dial setup socket
var conn *net.UnixConn
if c, err := net.DialUnix("unix", nil, &net.UnixAddr{Name: socketPath, Net: "unix"}); err != nil {
fmsg.Fatal("cannot dial setup socket:", err)
panic("unreachable")
} else {
conn = c
}
// decode payload gob stream
var payload shim.Payload
if err := gob.NewDecoder(conn).Decode(&payload); err != nil {
fmsg.Fatal("cannot decode shim payload:", err)
} else {
fmsg.SetVerbose(payload.Verbose)
}
if payload.Bwrap == nil {
fmsg.Fatal("bwrap config not supplied")
}
// receive wayland fd over socket
wfd := -1
if payload.WL {
if fd, err := receiveWLfd(conn); err != nil {
fmsg.Fatal("cannot receive wayland fd:", err)
} else {
wfd = fd
}
}
// close setup socket
if err := conn.Close(); err != nil {
fmsg.Println("cannot close setup socket:", err)
// not fatal
}
var ic init0.Payload
// resolve argv0
ic.Argv = payload.Argv
if len(ic.Argv) > 0 {
// looked up from $PATH by parent
ic.Argv0 = payload.Exec[1]
} else {
// no argv, look up shell instead
var ok bool
if ic.Argv0, ok = os.LookupEnv("SHELL"); !ok {
fmsg.Fatal("no command was specified and $SHELL was unset")
}
ic.Argv = []string{ic.Argv0}
}
conf := payload.Bwrap
var extraFiles []*os.File
// pass wayland fd
if wfd != -1 {
if f := os.NewFile(uintptr(wfd), "wayland"); f != nil {
ic.WL = 3 + len(extraFiles)
extraFiles = append(extraFiles, f)
}
} else {
ic.WL = -1
}
// share config pipe
if r, w, err := os.Pipe(); err != nil {
fmsg.Fatal("cannot pipe:", err)
} else {
conf.SetEnv[init0.Env] = strconv.Itoa(3 + len(extraFiles))
extraFiles = append(extraFiles, r)
fmsg.VPrintln("transmitting config to init")
go func() {
// stream config to pipe
if err = gob.NewEncoder(w).Encode(&ic); err != nil {
fmsg.Fatal("cannot transmit init config:", err)
}
}()
}
helper.BubblewrapName = payload.Exec[0] // resolved bwrap path by parent
if b, err := helper.NewBwrap(conf, nil, finitPath,
func(int, int) []string { return make([]string, 0) }); err != nil {
fmsg.Fatal("malformed sandbox config:", err)
} else {
cmd := b.Unwrap()
cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
cmd.ExtraFiles = extraFiles
if fmsg.Verbose() {
fmsg.VPrintln("bwrap args:", conf.Args())
}
// run and pass through exit code
if err = b.Start(); err != nil {
fmsg.Fatal("cannot start target process:", err)
} else if err = b.Wait(); err != nil {
fmsg.VPrintln("wait:", err)
}
if b.Unwrap().ProcessState != nil {
fmsg.Exit(b.Unwrap().ProcessState.ExitCode())
} else {
fmsg.Exit(127)
}
}
}
func receiveWLfd(conn *net.UnixConn) (int, error) {
oob := make([]byte, syscall.CmsgSpace(4)) // single fd
if _, oobn, _, _, err := conn.ReadMsgUnix(nil, oob); err != nil {
return -1, err
} else if len(oob) != oobn {
return -1, errors.New("invalid message length")
}
var msg syscall.SocketControlMessage
if messages, err := syscall.ParseSocketControlMessage(oob); err != nil {
return -1, err
} else if len(messages) != 1 {
return -1, errors.New("unexpected message count")
} else {
msg = messages[0]
}
if fds, err := syscall.ParseUnixRights(&msg); err != nil {
return -1, err
} else if len(fds) != 1 {
return -1, errors.New("unexpected fd count")
} else {
return fds[0], nil
}
}
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