syscall: add jail support to ForkExec on FreeBSD

Introduce a new SysProcAttr member called Jail on FreeBSD. This allows
supplying an existing jail's ID to which the child process is attached
before calling the exec system call.

Fixes #46259

Change-Id: Ie282e5b83429131f9a9e1e27cfcb3bcc995d1d4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/458335
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Goutnik <dgoutnik@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Samuel Karp <samuelkarp@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

api/next/46259.txt

@@ -0,0 +1,10 @@
+pkg syscall (freebsd-386), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-386-cgo), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-amd64), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-amd64-cgo), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-arm), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-arm-cgo), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-arm64), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-arm64-cgo), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-riscv64), type SysProcAttr struct, Jail int #46259
+pkg syscall (freebsd-riscv64-cgo), type SysProcAttr struct, Jail int #46259

src/syscall/exec_freebsd.go

@@ -32,6 +32,7 @@ type SysProcAttr struct {
 	Foreground bool
 	Pgid       int    // Child's process group ID if Setpgid.
 	Pdeathsig  Signal // Signal that the process will get when its parent dies (Linux and FreeBSD only)
+	Jail       int    // Jail to which the child process is attached (FreeBSD only).
 }
 
 const (
@@ -103,6 +104,15 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	// Fork succeeded, now in child.
 
+	// Attach to the given jail, if any. The system call also changes the
+	// process' root and working directories to the jail's path directory.
+	if sys.Jail > 0 {
+		_, _, err1 = RawSyscall(SYS_JAIL_ATTACH, uintptr(sys.Jail), 0, 0)
+		if err1 != 0 {
+			goto childerror
+		}
+	}
+
 	// Enable tracing if requested.
 	if sys.Ptrace {
 		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)

src/syscall/exec_freebsd_test.go

@@ -0,0 +1,98 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build freebsd
+
+package syscall_test
+
+import (
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+const (
+	flagJailCreate = uintptr(0x1)
+)
+
+func prepareJail(t *testing.T) (int, string) {
+	t.Helper()
+
+	root := t.TempDir()
+	paramPath := []byte("path\x00")
+	conf := make([]syscall.Iovec, 4)
+	conf[0].Base = &paramPath[0]
+	conf[0].SetLen(len(paramPath))
+	p, err := syscall.BytePtrFromString(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf[1].Base = p
+	conf[1].SetLen(len(root) + 1)
+
+	paramPersist := []byte("persist\x00")
+	conf[2].Base = &paramPersist[0]
+	conf[2].SetLen(len(paramPersist))
+	conf[3].Base = nil
+	conf[3].SetLen(0)
+
+	id, _, err1 := syscall.Syscall(syscall.SYS_JAIL_SET,
+		uintptr(unsafe.Pointer(&conf[0])), uintptr(len(conf)), flagJailCreate)
+	if err1 != 0 {
+		t.Fatalf("jail_set: %v", err1)
+	}
+	t.Cleanup(func() {
+		_, _, err1 := syscall.Syscall(syscall.SYS_JAIL_REMOVE, id, 0, 0)
+		if err1 != 0 {
+			t.Errorf("failed to cleanup jail: %v", err)
+		}
+	})
+
+	return int(id), root
+}
+
+func TestJailAttach(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		jailed, err := syscall.SysctlUint32("security.jail.jailed")
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(2)
+		}
+		if jailed != 1 {
+			t.Fatalf("jailed = %d, want 1", jailed)
+		}
+		return
+	}
+
+	testenv.MustHaveGoBuild(t)
+	// Make sure we are running as root, so we have permissions to create
+	// and remove jails.
+	if os.Getuid() != 0 {
+		t.Skip("kernel prohibits jail system calls in unprivileged process")
+	}
+
+	jid, root := prepareJail(t)
+
+	// Since jail attach does an implicit chroot to the jail's path,
+	// we need the binary there, and it must be statically linked.
+	x := filepath.Join(root, "syscall.test")
+	cmd := exec.Command(testenv.GoToolPath(t), "test", "-c", "-o", x, "syscall")
+	cmd.Env = append(os.Environ(), "CGO_ENABLED=0")
+	if o, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("Build of syscall in jail root failed, output %v, err %v", o, err)
+	}
+
+	cmd = exec.Command("/syscall.test", "-test.run=TestJailAttach", "/")
+	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+	cmd.SysProcAttr = &syscall.SysProcAttr{Jail: jid}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+}