runtime: implement SUID/SGID protections

On Unix platforms, the runtime previously did nothing special when a
program was run with either the SUID or SGID bits set. This can be
dangerous in certain cases, such as when dumping memory state, or
assuming the status of standard i/o file descriptors.

Taking cues from glibc, this change implements a set of protections when
a binary is run with SUID or SGID bits set (or is SUID/SGID-like). On
Linux, whether to enable these protections is determined by whether the
AT_SECURE flag is passed in the auxiliary vector. On platforms which
have the issetugid syscall (the BSDs, darwin, and Solaris/Illumos), that
is used. On the remaining platforms (currently only AIX) we check
!(getuid() == geteuid() && getgid == getegid()).

Currently when we determine a binary is "tainted" (using the glibc
terminology), we implement two specific protections:
  1. we check if the file descriptors 0, 1, and 2 are open, and if they
     are not, we open them, pointing at /dev/null (or fail).
  2. we force GOTRACKBACK=none, and generally prevent dumping of
     trackbacks and registers when a program panics/aborts.

In the future we may add additional protections.

This change requires implementing issetugid on the platforms which
support it, and implementing getuid, geteuid, getgid, and getegid on
AIX.

Thanks to Vincent Dehors from Synacktiv for reporting this issue.

Fixes #60272
Fixes CVE-2023-29403

Change-Id: I73fc93f2b7a8933c192ce3eabbf1db359db7d5fa
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1878434
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/501223
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/extern.go

@@ -233,6 +233,25 @@ the set of Go environment variables. They influence the building of Go programs
 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by
 constants or functions in this package, but they do not influence the execution
 of the run-time system.
+
+# Security
+
+On Unix platforms, Go's runtime system behaves slightly differently when a
+binary is setuid/setgid or executed with setuid/setgid-like properties, in order
+to prevent dangerous behaviors. On Linux this is determined by checking for the
+AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is
+determined by checking the issetugid syscall, and on AIX it is determined by
+checking if the uid/gid match the effective uid/gid.
+
+When the runtime determines the binary is setuid/setgid-like, it does three main
+things:
+  - The standard input/output file descriptors (0, 1, 2) are checked to be open.
+    If any of them are closed, they are opened pointing at /dev/null.
+  - The value of the GOTRACEBACK environment variable is set to 'none'.
+  - When a signal is received that terminates the program, or the program
+    encounters an unrecoverable panic that would otherwise override the value
+    of GOTRACEBACK, the goroutine stack, registers, and other memory related
+    information are omitted.
 */
 package runtime
 

src/runtime/os2_aix.go

@@ -55,6 +55,10 @@ var (
 //go:cgo_import_dynamic libc_sysconf sysconf "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_usleep usleep "libc.a/shr_64.o"
 //go:cgo_import_dynamic libc_write write "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_getuid getuid "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_geteuid geteuid "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_getgid getgid "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_getegid getegid "libc.a/shr_64.o"
 
 //go:cgo_import_dynamic libpthread___pth_init __pth_init "libpthread.a/shr_xpg5_64.o"
 //go:cgo_import_dynamic libpthread_attr_destroy pthread_attr_destroy "libpthread.a/shr_xpg5_64.o"
@@ -95,6 +99,10 @@ var (
 //go:linkname libc_sysconf libc_sysconf
 //go:linkname libc_usleep libc_usleep
 //go:linkname libc_write libc_write
+//go:linkname libc_getuid libc_getuid
+//go:linkname libc_geteuid libc_geteuid
+//go:linkname libc_getgid libc_getgid
+//go:linkname libc_getegid libc_getegid
 
 //go:linkname libpthread___pth_init libpthread___pth_init
 //go:linkname libpthread_attr_destroy libpthread_attr_destroy
@@ -137,6 +145,10 @@ var (
 	libc_sysconf,
 	libc_usleep,
 	libc_write,
+	libc_getuid,
+	libc_geteuid,
+	libc_getgid,
+	libc_getegid,
 	//libpthread
 	libpthread___pth_init,
 	libpthread_attr_destroy,

src/runtime/os_aix.go

@@ -373,3 +373,43 @@ const sigPerThreadSyscall = 1 << 31
 func runPerThreadSyscall() {
 	throw("runPerThreadSyscall only valid on linux")
 }
+
+//go:nosplit
+func getuid() int32 {
+	r, errno := syscall0(&libc_getuid)
+	if errno != 0 {
+		print("getuid failed ", errno)
+		throw("getuid")
+	}
+	return int32(r)
+}
+
+//go:nosplit
+func geteuid() int32 {
+	r, errno := syscall0(&libc_geteuid)
+	if errno != 0 {
+		print("geteuid failed ", errno)
+		throw("geteuid")
+	}
+	return int32(r)
+}
+
+//go:nosplit
+func getgid() int32 {
+	r, errno := syscall0(&libc_getgid)
+	if errno != 0 {
+		print("getgid failed ", errno)
+		throw("getgid")
+	}
+	return int32(r)
+}
+
+//go:nosplit
+func getegid() int32 {
+	r, errno := syscall0(&libc_getegid)
+	if errno != 0 {
+		print("getegid failed ", errno)
+		throw("getegid")
+	}
+	return int32(r)
+}

src/runtime/os_dragonfly.go

@@ -65,6 +65,8 @@ func kevent(kq int32, ch *keventt, nch int32, ev *keventt, nev int32, ts *timesp
 func pipe2(flags int32) (r, w int32, errno int32)
 func fcntl(fd, cmd, arg int32) (ret int32, errno int32)
 
+func issetugid() int32
+
 // From DragonFly's <sys/sysctl.h>
 const (
 	_CTL_HW      = 6

src/runtime/os_freebsd.go

@@ -50,6 +50,8 @@ func kevent(kq int32, ch *keventt, nch int32, ev *keventt, nev int32, ts *timesp
 func pipe2(flags int32) (r, w int32, errno int32)
 func fcntl(fd, cmd, arg int32) (ret int32, errno int32)
 
+func issetugid() int32
+
 // From FreeBSD's <sys/sysctl.h>
 const (
 	_CTL_HW      = 6

src/runtime/os_linux.go

@@ -216,6 +216,7 @@ const (
 	_AT_NULL   = 0  // End of vector
 	_AT_PAGESZ = 6  // System physical page size
 	_AT_HWCAP  = 16 // hardware capability bit vector
+	_AT_SECURE = 23 // secure mode boolean
 	_AT_RANDOM = 25 // introduced in 2.6.29
 	_AT_HWCAP2 = 26 // hardware capability bit vector 2
 )
@@ -290,6 +291,9 @@ func sysargs(argc int32, argv **byte) {
 // the ELF AT_RANDOM auxiliary vector.
 var startupRandomData []byte
 
+// secureMode holds the value of AT_SECURE passed in the auxiliary vector.
+var secureMode bool
+
 func sysauxv(auxv []uintptr) (pairs int) {
 	var i int
 	for ; auxv[i] != _AT_NULL; i += 2 {
@@ -302,6 +306,9 @@ func sysauxv(auxv []uintptr) (pairs int) {
 
 		case _AT_PAGESZ:
 			physPageSize = val
+
+		case _AT_SECURE:
+			secureMode = val == 1
 		}
 
 		archauxv(tag, val)

src/runtime/os_netbsd.go

@@ -81,6 +81,8 @@ func kevent(kq int32, ch *keventt, nch int32, ev *keventt, nev int32, ts *timesp
 func pipe2(flags int32) (r, w int32, errno int32)
 func fcntl(fd, cmd, arg int32) (ret int32, errno int32)
 
+func issetugid() int32
+
 const (
 	_ESRCH     = 3
 	_ETIMEDOUT = 60

src/runtime/os_openbsd_syscall2.go

@@ -98,3 +98,5 @@ func sigaltstack(new, old *stackt)
 func fcntl(fd, cmd, arg int32) (ret int32, errno int32)
 
 func walltime() (sec int64, nsec int32)
+
+func issetugid() int32

src/runtime/os_solaris.go

@@ -267,3 +267,7 @@ func sysvicall6(fn *libcFunc, a1, a2, a3, a4, a5, a6 uintptr) uintptr {
 	}
 	return libcall.r1
 }
+
+func issetugid() int32 {
+	return int32(sysvicall0(&libc_issetugid))
+}

src/runtime/panic.go

@@ -1148,6 +1148,10 @@ func fatalthrow(t throwType) {
 	// Switch to the system stack to avoid any stack growth, which may make
 	// things worse if the runtime is in a bad state.
 	systemstack(func() {
+		if isSecureMode() {
+			exit(2)
+		}
+
 		startpanic_m()
 
 		if dopanic_m(gp, pc, sp) {

src/runtime/proc.go

@@ -740,6 +740,7 @@ func schedinit() {
 
 	goargs()
 	goenvs()
+	secure()
 	parsedebugvars()
 	gcinit()
 

src/runtime/security_aix.go

@@ -0,0 +1,17 @@
+// 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.
+
+package runtime
+
+// secureMode is only ever mutated in schedinit, so we don't need to worry about
+// synchronization primitives.
+var secureMode bool
+
+func initSecureMode() {
+	secureMode = !(getuid() == geteuid() && getgid() == getegid())
+}
+
+func isSecureMode() bool {
+	return secureMode
+}

src/runtime/security_issetugid.go

@@ -0,0 +1,19 @@
+// 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 darwin || dragonfly || freebsd || illumos || netbsd || openbsd || solaris
+
+package runtime
+
+// secureMode is only ever mutated in schedinit, so we don't need to worry about
+// synchronization primitives.
+var secureMode bool
+
+func initSecureMode() {
+	secureMode = issetugid() == 1
+}
+
+func isSecureMode() bool {
+	return secureMode
+}

src/runtime/security_linux.go

@@ -0,0 +1,15 @@
+// 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.
+
+package runtime
+
+import _ "unsafe"
+
+func initSecureMode() {
+	// We have already initialized the secureMode bool in sysauxv.
+}
+
+func isSecureMode() bool {
+	return secureMode
+}

src/runtime/security_nonunix.go

@@ -0,0 +1,13 @@
+// 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 !unix
+
+package runtime
+
+func isSecureMode() bool {
+	return false
+}
+
+func secure() {}

src/runtime/security_test.go

@@ -0,0 +1,143 @@
+// 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 unix
+
+package runtime_test
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+func privesc(command string, args ...string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
+	defer cancel()
+	var cmd *exec.Cmd
+	if runtime.GOOS == "darwin" {
+		cmd = exec.CommandContext(ctx, "sudo", append([]string{"-n", command}, args...)...)
+	} else {
+		cmd = exec.CommandContext(ctx, "su", highPrivUser, "-c", fmt.Sprintf("%s %s", command, strings.Join(args, " ")))
+	}
+	_, err := cmd.CombinedOutput()
+	return err
+}
+
+const highPrivUser = "root"
+
+func setSetuid(t *testing.T, user, bin string) {
+	t.Helper()
+	// We escalate privileges here even if we are root, because for some reason on some builders
+	// (at least freebsd-amd64-13_0) the default PATH doesn't include /usr/sbin, which is where
+	// chown lives, but using 'su root -c' gives us the correct PATH.
+
+	// buildTestProg uses os.MkdirTemp which creates directories with 0700, which prevents
+	// setuid binaries from executing because of the missing g+rx, so we need to set the parent
+	// directory to better permissions before anything else. We created this directory, so we
+	// shouldn't need to do any privilege trickery.
+	if err := privesc("chmod", "0777", filepath.Dir(bin)); err != nil {
+		t.Skipf("unable to set permissions on %q, likely no passwordless sudo/su: %s", filepath.Dir(bin), err)
+	}
+
+	if err := privesc("chown", user, bin); err != nil {
+		t.Skipf("unable to set permissions on test binary, likely no passwordless sudo/su: %s", err)
+	}
+	if err := privesc("chmod", "u+s", bin); err != nil {
+		t.Skipf("unable to set permissions on test binary, likely no passwordless sudo/su: %s", err)
+	}
+}
+
+func TestSUID(t *testing.T) {
+	// This test is relatively simple, we build a test program which opens a
+	// file passed via the TEST_OUTPUT envvar, prints the value of the
+	// GOTRACEBACK envvar to stdout, and prints "hello" to stderr. We then chown
+	// the program to "nobody" and set u+s on it. We execute the program, only
+	// passing it two files, for stdin and stdout, and passing
+	// GOTRACEBACK=system in the env.
+	//
+	// We expect that the program will trigger the SUID protections, resetting
+	// the value of GOTRACEBACK, and opening the missing stderr descriptor, such
+	// that the program prints "GOTRACEBACK=none" to stdout, and nothing gets
+	// written to the file pointed at by TEST_OUTPUT.
+
+	if *flagQuick {
+		t.Skip("-quick")
+	}
+
+	testenv.MustHaveGoBuild(t)
+
+	helloBin, err := buildTestProg(t, "testsuid")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	f, err := os.CreateTemp(t.TempDir(), "suid-output")
+	if err != nil {
+		t.Fatal(err)
+	}
+	tempfilePath := f.Name()
+	f.Close()
+
+	lowPrivUser := "nobody"
+	setSetuid(t, lowPrivUser, helloBin)
+
+	b := bytes.NewBuffer(nil)
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	proc, err := os.StartProcess(helloBin, []string{helloBin}, &os.ProcAttr{
+		Env:   []string{"GOTRACEBACK=system", "TEST_OUTPUT=" + tempfilePath},
+		Files: []*os.File{os.Stdin, pw},
+	})
+	if err != nil {
+		if os.IsPermission(err) {
+			t.Skip("don't have execute permission on setuid binary, possibly directory permission issue?")
+		}
+		t.Fatal(err)
+	}
+	done := make(chan bool, 1)
+	go func() {
+		io.Copy(b, pr)
+		pr.Close()
+		done <- true
+	}()
+	ps, err := proc.Wait()
+	if err != nil {
+		t.Fatal(err)
+	}
+	pw.Close()
+	<-done
+	output := b.String()
+
+	if ps.ExitCode() == 99 {
+		t.Skip("binary wasn't setuid (uid == euid), unable to effectively test")
+	}
+
+	expected := "GOTRACEBACK=none\n"
+	if output != expected {
+		t.Errorf("unexpected output, got: %q, want %q", output, expected)
+	}
+
+	fc, err := os.ReadFile(tempfilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(fc) != "" {
+		t.Errorf("unexpected file content, got: %q", string(fc))
+	}
+
+	// TODO: check the registers aren't leaked?
+}

src/runtime/security_unix.go

@@ -0,0 +1,72 @@
+// 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 unix
+
+package runtime
+
+func secure() {
+	initSecureMode()
+
+	if !isSecureMode() {
+		return
+	}
+
+	// When secure mode is enabled, we do two things:
+	//   1. ensure the file descriptors 0, 1, and 2 are open, and if not open them,
+	//      pointing at /dev/null (or fail)
+	//   2. enforce specific environment variable values (currently we only force
+	//		GOTRACEBACK=none)
+	//
+	// Other packages may also disable specific functionality when secure mode
+	// is enabled (determined by using linkname to call isSecureMode).
+	//
+	// NOTE: we may eventually want to enforce (1) regardless of whether secure
+	// mode is enabled or not.
+
+	secureFDs()
+	secureEnv()
+}
+
+func secureEnv() {
+	var hasTraceback bool
+	for i := 0; i < len(envs); i++ {
+		if hasPrefix(envs[i], "GOTRACEBACK=") {
+			hasTraceback = true
+			envs[i] = "GOTRACEBACK=none"
+		}
+	}
+	if !hasTraceback {
+		envs = append(envs, "GOTRACEBACK=none")
+	}
+}
+
+func secureFDs() {
+	const (
+		// F_GETFD and EBADF are standard across all unixes, define
+		// them here rather than in each of the OS specific files
+		F_GETFD = 0x01
+		EBADF   = 0x09
+	)
+
+	devNull := []byte("/dev/null\x00")
+	for i := 0; i < 3; i++ {
+		ret, errno := fcntl(int32(i), F_GETFD, 0)
+		if ret >= 0 {
+			continue
+		}
+		if errno != EBADF {
+			print("runtime: unexpected error while checking standard file descriptor ", i, ", errno=", errno, "\n")
+			throw("cannot secure fds")
+		}
+
+		if ret := open(&devNull[0], 2 /* O_RDWR */, 0); ret < 0 {
+			print("runtime: standard file descriptor ", i, " closed, unable to open /dev/null, errno=", errno, "\n")
+			throw("cannot secure fds")
+		} else if ret != int32(i) {
+			print("runtime: opened unexpected file descriptor ", ret, " when attempting to open ", i, "\n")
+			throw("cannot secure fds")
+		}
+	}
+}

src/runtime/signal_unix.go

@@ -783,6 +783,10 @@ func fatalsignal(sig uint32, c *sigctxt, gp *g, mp *m) *g {
 		print("Signal ", sig, "\n")
 	}
 
+	if isSecureMode() {
+		exit(2)
+	}
+
 	print("PC=", hex(c.sigpc()), " m=", mp.id, " sigcode=", c.sigcode(), "\n")
 	if mp.incgo && gp == mp.g0 && mp.curg != nil {
 		print("signal arrived during cgo execution\n")

src/runtime/sys_darwin.go

@@ -544,6 +544,11 @@ func setNonblock(fd int32) {
 	}
 }
 
+func issetugid() int32 {
+	return libcCall(unsafe.Pointer(abi.FuncPCABI0(issetugid_trampoline)), nil)
+}
+func issetugid_trampoline()
+
 // Tell the linker that the libc_* functions are to be found
 // in a system library, with the libc_ prefix missing.
 
@@ -594,3 +599,5 @@ func setNonblock(fd int32) {
 
 //go:cgo_import_dynamic libc_notify_is_valid_token notify_is_valid_token "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_xpc_date_create_from_current xpc_date_create_from_current "/usr/lib/libSystem.B.dylib"
+
+//go:cgo_import_dynamic libc_issetugid issetugid "/usr/lib/libSystem.B.dylib"

src/runtime/sys_darwin_amd64.s

@@ -792,3 +792,7 @@ TEXT runtime·syscall_x509(SB),NOSPLIT,$16
 
 	XORL	AX, AX        // no error (it's ignored anyway)
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	CALL	libc_issetugid(SB)
+	RET

src/runtime/sys_darwin_arm64.s

@@ -763,3 +763,7 @@ TEXT runtime·syscall_x509(SB),NOSPLIT,$0
 	ADD	$16, RSP
 	MOVD	R0, 56(R2)	// save r1
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	BL	libc_issetugid(SB)
+	RET

src/runtime/sys_dragonfly_amd64.s

@@ -400,3 +400,13 @@ noerr:
 	MOVL	AX, ret+16(FP)
 	MOVL	$0, errno+20(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVQ	$0, DI
+	MOVQ	$0, SI
+	MOVQ	$0, DX
+	MOVL	$253, AX
+	SYSCALL
+	MOVL	AX, ret+0(FP)
+	RET

src/runtime/sys_freebsd_386.s

@@ -29,6 +29,7 @@
 #define SYS___sysctl		202
 #define SYS_clock_gettime	232
 #define SYS_nanosleep		240
+#define SYS_issetugid		253
 #define SYS_sched_yield		331
 #define SYS_sigprocmask		340
 #define SYS_kqueue		362
@@ -472,3 +473,10 @@ TEXT runtime·cpuset_getaffinity(SB), NOSPLIT, $0-28
 	RET
 
 GLOBL runtime·tlsoffset(SB),NOPTR,$4
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVL	$SYS_issetugid, AX
+	INT	$0x80
+	MOVL	AX, ret+0(FP)
+	RET

src/runtime/sys_freebsd_amd64.s

@@ -31,6 +31,7 @@
 #define SYS___sysctl		202
 #define SYS_clock_gettime	232
 #define SYS_nanosleep		240
+#define SYS_issetugid		253
 #define SYS_sched_yield		331
 #define SYS_sigprocmask		340
 #define SYS_kqueue		362
@@ -575,3 +576,13 @@ TEXT runtime·cpuset_getaffinity(SB), NOSPLIT, $0-44
 	NEGQ	AX
 	MOVL	AX, ret+40(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVQ	$0, DI
+	MOVQ	$0, SI
+	MOVQ	$0, DX
+	MOVL	$SYS_issetugid, AX
+	SYSCALL
+	MOVL	AX, ret+0(FP)
+	RET

src/runtime/sys_freebsd_arm.s

@@ -27,6 +27,7 @@
 #define SYS_fcntl (SYS_BASE + 92)
 #define SYS___sysctl (SYS_BASE + 202)
 #define SYS_nanosleep (SYS_BASE + 240)
+#define SYS_issetugid (SYS_BASE + 253)
 #define SYS_clock_gettime (SYS_BASE + 232)
 #define SYS_sched_yield (SYS_BASE + 331)
 #define SYS_sigprocmask (SYS_BASE + 340)
@@ -446,3 +447,10 @@ TEXT runtime·getCntxct(SB),NOSPLIT|NOFRAME,$0-8
 
 	MOVW	R0, ret+4(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVW $SYS_issetugid, R7
+	SWI $0
+	MOVW	R0, ret+0(FP)
+	RET

src/runtime/sys_freebsd_arm64.s

@@ -29,6 +29,7 @@
 #define SYS_fcntl		92
 #define SYS___sysctl		202
 #define SYS_nanosleep		240
+#define SYS_issetugid		253
 #define SYS_clock_gettime	232
 #define SYS_sched_yield		331
 #define SYS_sigprocmask		340
@@ -466,3 +467,10 @@ TEXT runtime·getCntxct(SB),NOSPLIT,$0
 
 	MOVW	R0, ret+8(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT|NOFRAME,$0
+	MOVD $SYS_issetugid, R8
+	SVC
+	MOVW	R0, ret+0(FP)
+	RET

src/runtime/sys_freebsd_riscv64.s

@@ -28,6 +28,7 @@
 #define SYS_fcntl		92
 #define SYS___sysctl		202
 #define SYS_nanosleep		240
+#define SYS_issetugid		253
 #define SYS_clock_gettime	232
 #define SYS_sched_yield		331
 #define SYS_sigprocmask		340
@@ -437,3 +438,11 @@ TEXT runtime·getCntxct(SB),NOSPLIT|NOFRAME,$0
 	RDTIME	A0
 	MOVW	A0, ret+0(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT|NOFRAME,$0
+	MOV $SYS_issetugid, T0
+	ECALL
+	MOVW	A0, ret+0(FP)
+	RET
+

src/runtime/sys_netbsd_386.s

@@ -27,6 +27,7 @@
 #define SYS___sysctl			202
 #define SYS___sigaltstack14		281
 #define SYS___sigprocmask14		293
+#define SYS_issetugid			305
 #define SYS_getcontext			307
 #define SYS_setcontext			308
 #define SYS__lwp_create			309
@@ -467,3 +468,10 @@ noerr:
 	MOVL	AX, ret+12(FP)
 	MOVL	$0, errno+16(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVL	$SYS_issetugid, AX
+	INT	$0x80
+	MOVL	AX, ret+0(FP)
+	RET

src/runtime/sys_netbsd_amd64.s

@@ -28,6 +28,7 @@
 #define SYS___sysctl			202
 #define SYS___sigaltstack14		281
 #define SYS___sigprocmask14		293
+#define SYS_issetugid			305
 #define SYS_getcontext			307
 #define SYS_setcontext			308
 #define SYS__lwp_create			309
@@ -445,3 +446,13 @@ noerr:
 	MOVL	AX, ret+16(FP)
 	MOVL	$0, errno+20(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVQ	$0, DI
+	MOVQ	$0, SI
+	MOVQ	$0, DX
+	MOVL	$SYS_issetugid, AX
+	SYSCALL
+	MOVL	AX, ret+0(FP)
+	RET

src/runtime/sys_netbsd_arm.s

@@ -28,6 +28,7 @@
 #define SYS___sysctl			SWI_OS_NETBSD | 202
 #define SYS___sigaltstack14		SWI_OS_NETBSD | 281
 #define SYS___sigprocmask14		SWI_OS_NETBSD | 293
+#define SYS_issetugid			SWI_OS_NETBSD | 305
 #define SYS_getcontext			SWI_OS_NETBSD | 307
 #define SYS_setcontext			SWI_OS_NETBSD | 308
 #define SYS__lwp_create			SWI_OS_NETBSD | 309
@@ -418,3 +419,9 @@ TEXT runtime·read_tls_fallback(SB),NOSPLIT|NOFRAME,$0
 	SWI $SYS__lwp_getprivate
 	MOVM.IAW    (R13), [R1, R2, R3, R12]
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	SWI $SYS_issetugid
+	MOVW	R0, ret+0(FP)
+	RET

src/runtime/sys_netbsd_arm64.s

@@ -28,6 +28,7 @@
 #define SYS___sysctl			202
 #define SYS___sigaltstack14		281
 #define SYS___sigprocmask14		293
+#define SYS_issetugid			305
 #define SYS_getcontext			307
 #define SYS_setcontext			308
 #define SYS__lwp_create			309
@@ -426,3 +427,9 @@ noerr:
 	MOVW	R0, ret+16(FP)
 	MOVW	$0, errno+20(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT|NOFRAME,$0
+	SVC $SYS_issetugid
+	MOVW	R0, ret+0(FP)
+	RET

src/runtime/sys_openbsd2.go

@@ -258,6 +258,14 @@ func exitThread(wait *atomic.Uint32) {
 	throw("exitThread")
 }
 
+//go:nosplit
+//go:cgo_unsafe_args
+func issetugid() (ret int32) {
+	libcCall(unsafe.Pointer(abi.FuncPCABI0(issetugid_trampoline)), unsafe.Pointer(&ret))
+	return
+}
+func issetugid_trampoline()
+
 // Tell the linker that the libc_* functions are to be found
 // in a system library, with the libc_ prefix missing.
 
@@ -290,4 +298,6 @@ func exitThread(wait *atomic.Uint32) {
 //go:cgo_import_dynamic libc_sigaction sigaction "libc.so"
 //go:cgo_import_dynamic libc_sigaltstack sigaltstack "libc.so"
 
+//go:cgo_import_dynamic libc_issetugid issetugid "libc.so"
+
 //go:cgo_import_dynamic _ _ "libc.so"

src/runtime/sys_openbsd_386.s

@@ -979,3 +979,12 @@ ok:
 	MOVL	BP, SP
 	POPL	BP
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	PUSHL	BP
+	CALL	libc_issetugid(SB)
+	NOP	SP			// tell vet SP changed - stop checking offsets
+	MOVL	8(SP), DX		// pointer to return value
+	MOVL	AX, 0(DX)
+	POPL	BP
+	RET

src/runtime/sys_openbsd_amd64.s

@@ -658,3 +658,9 @@ TEXT runtime·syscall10X(SB),NOSPLIT,$48
 ok:
 	XORL	AX, AX        // no error (it's ignored anyway)
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	MOVQ	DI, BX			// BX is caller-save
+	CALL	libc_issetugid(SB)
+	MOVL	AX, 0(BX)		// return value
+	RET

src/runtime/sys_openbsd_arm.s

@@ -816,3 +816,12 @@ ok:
 	MOVW	$0, R0		// no error (it's ignored anyway)
 	MOVW	R9, R13
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	MOVW	R13, R9
+	MOVW	R0, R8
+	BIC     $0x7, R13		// align for ELF ABI
+	BL	libc_issetugid(SB)
+	MOVW	R0, 0(R8)
+	MOVW	R9, R13
+	RET

src/runtime/sys_openbsd_arm64.s

@@ -644,3 +644,9 @@ TEXT runtime·syscall10X(SB),NOSPLIT,$0
 
 ok:
 	RET
+
+TEXT runtime·issetugid_trampoline(SB),NOSPLIT,$0
+	MOVD	R0, R19			// pointer to args
+	CALL	libc_issetugid(SB)
+	MOVW	R0, 0(R19)		// return value
+	RET

src/runtime/sys_openbsd_mips64.s

@@ -379,3 +379,10 @@ noerr:
 	MOVW	R2, ret+16(FP)
 	MOVW	R4, errno+20(FP)
 	RET
+
+// func issetugid() int32
+TEXT runtime·issetugid(SB),NOSPLIT,$0
+	MOVV	$253, R2	// sys_issetugid
+	SYSCALL
+	MOVW	R2, ret+0(FP)
+	RET

src/runtime/syscall2_solaris.go

@@ -23,6 +23,7 @@ import _ "unsafe" // for go:linkname
 //go:cgo_import_dynamic libc_setpgid setpgid "libc.so"
 //go:cgo_import_dynamic libc_syscall syscall "libc.so"
 //go:cgo_import_dynamic libc_wait4 wait4 "libc.so"
+//go:cgo_import_dynamic libc_issetugid issetugid "libc.so"
 
 //go:linkname libc_chdir libc_chdir
 //go:linkname libc_chroot libc_chroot
@@ -41,3 +42,4 @@ import _ "unsafe" // for go:linkname
 //go:linkname libc_setpgid libc_setpgid
 //go:linkname libc_syscall libc_syscall
 //go:linkname libc_wait4 libc_wait4
+//go:linkname libc_issetugid libc_issetugid

src/runtime/syscall_solaris.go

@@ -23,6 +23,7 @@ var (
 	libc_setuid,
 	libc_setpgid,
 	libc_syscall,
+	libc_issetugid,
 	libc_wait4 libcFunc
 )
 

src/runtime/testdata/testsuid/main.go

@@ -0,0 +1,25 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+)
+
+func main() {
+	if os.Geteuid() == os.Getuid() {
+		os.Exit(99)
+	}
+
+	fmt.Fprintf(os.Stdout, "GOTRACEBACK=%s\n", os.Getenv("GOTRACEBACK"))
+	f, err := os.OpenFile(os.Getenv("TEST_OUTPUT"), os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		log.Fatalf("os.Open failed: %s", err)
+	}
+	defer f.Close()
+	fmt.Fprintf(os.Stderr, "hello\n")
+}