syscall: correctly set up uid/gid mappings in user namespaces

Before this CL, uid/gid mapping was always set up from the parent
process, which is a privileged operation.

When using unprivileged user namespaces, a process can modify its
uid/gid mapping after the unshare(2) call (but setting the uid/gid
mapping from another process is NOT possible).

Fixes #29789

Change-Id: I8c96a03f5da23fe80bbb83ef051ad89cf185d750
Reviewed-on: https://go-review.googlesource.com/c/go/+/158298
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 122 insertions, 19 deletions

diff --git a/src/syscall/exec_linux.go b/src/syscall/exec_linux.go
index ec8f296bca..3493f4b32b 100644
--- a/src/syscall/exec_linux.go
+++ b/src/syscall/exec_linux.go
@@ -82,10 +82,13 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	if sys.UidMappings != nil || sys.GidMappings != nil {
 		Close(p[0])
-		err := writeUidGidMappings(pid, sys)
 		var err2 Errno
-		if err != nil {
-			err2 = err.(Errno)
+		// uid/gid mappings will be written after fork and unshare(2) for user
+		// namespaces.
+		if sys.Unshareflags&CLONE_NEWUSER == 0 {
+			if err := writeUidGidMappings(pid, sys); err != nil {
+				err2 = err.(Errno)
+			}
 		}
 		RawSyscall(SYS_WRITE, uintptr(p[1]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
 		Close(p[1])
@@ -142,12 +145,32 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 	// Declare all variables at top in case any
 	// declarations require heap allocation (e.g., err1).
 	var (
-		err2   Errno
-		nextfd int
-		i      int
-		caps   caps
+		err2                      Errno
+		nextfd                    int
+		i                         int
+		caps                      caps
+		fd1                       uintptr
+		puid, psetgroups, pgid    []byte
+		uidmap, setgroups, gidmap []byte
 	)
 
+	if sys.UidMappings != nil {
+		puid = []byte("/proc/self/uid_map\000")
+		uidmap = formatIDMappings(sys.UidMappings)
+	}
+
+	if sys.GidMappings != nil {
+		psetgroups = []byte("/proc/self/setgroups\000")
+		pgid = []byte("/proc/self/gid_map\000")
+
+		if sys.GidMappingsEnableSetgroups {
+			setgroups = []byte("allow\000")
+		} else {
+			setgroups = []byte("deny\000")
+		}
+		gidmap = formatIDMappings(sys.GidMappings)
+	}
+
 	// Record parent PID so child can test if it has died.
 	ppid, _ := rawSyscallNoError(SYS_GETPID, 0, 0, 0)
 
@@ -264,6 +287,46 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		if err1 != 0 {
 			goto childerror
 		}
+
+		if sys.Unshareflags&CLONE_NEWUSER != 0 && sys.GidMappings != nil {
+			dirfd := int(_AT_FDCWD)
+			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&psetgroups[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
+				goto childerror
+			}
+			r1, _, err1 = RawSyscall(SYS_WRITE, uintptr(fd1), uintptr(unsafe.Pointer(&setgroups[0])), uintptr(len(setgroups)))
+			if err1 != 0 {
+				goto childerror
+			}
+			if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(fd1), 0, 0); err1 != 0 {
+				goto childerror
+			}
+
+			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&pgid[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
+				goto childerror
+			}
+			r1, _, err1 = RawSyscall(SYS_WRITE, uintptr(fd1), uintptr(unsafe.Pointer(&gidmap[0])), uintptr(len(gidmap)))
+			if err1 != 0 {
+				goto childerror
+			}
+			if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(fd1), 0, 0); err1 != 0 {
+				goto childerror
+			}
+		}
+
+		if sys.Unshareflags&CLONE_NEWUSER != 0 && sys.UidMappings != nil {
+			dirfd := int(_AT_FDCWD)
+			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&puid[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
+				goto childerror
+			}
+			r1, _, err1 = RawSyscall(SYS_WRITE, uintptr(fd1), uintptr(unsafe.Pointer(&uidmap[0])), uintptr(len(uidmap)))
+			if err1 != 0 {
+				goto childerror
+			}
+			if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(fd1), 0, 0); err1 != 0 {
+				goto childerror
+			}
+		}
+
 		// The unshare system call in Linux doesn't unshare mount points
 		// mounted with --shared. Systemd mounts / with --shared. For a
 		// long discussion of the pros and cons of this see debian bug 739593.
@@ -480,25 +543,22 @@ func forkExecPipe(p []int) (err error) {
 	return
 }
 
-// writeIDMappings writes the user namespace User ID or Group ID mappings to the specified path.
-func writeIDMappings(path string, idMap []SysProcIDMap) error {
-	fd, err := Open(path, O_RDWR, 0)
-	if err != nil {
-		return err
-	}
-
-	data := ""
+func formatIDMappings(idMap []SysProcIDMap) []byte {
+	var data []byte
 	for _, im := range idMap {
-		data = data + itoa(im.ContainerID) + " " + itoa(im.HostID) + " " + itoa(im.Size) + "\n"
+		data = append(data, []byte(itoa(im.ContainerID)+" "+itoa(im.HostID)+" "+itoa(im.Size)+"\n")...)
 	}
+	return data
+}
 
-	bytes, err := ByteSliceFromString(data)
+// writeIDMappings writes the user namespace User ID or Group ID mappings to the specified path.
+func writeIDMappings(path string, idMap []SysProcIDMap) error {
+	fd, err := Open(path, O_RDWR, 0)
 	if err != nil {
-		Close(fd)
 		return err
 	}
 
-	if _, err := Write(fd, bytes); err != nil {
+	if _, err := Write(fd, formatIDMappings(idMap)); err != nil {
 		Close(fd)
 		return err
 	}
diff --git a/src/syscall/exec_linux_test.go b/src/syscall/exec_linux_test.go
index 826487b676..09ced3b0e0 100644
--- a/src/syscall/exec_linux_test.go
+++ b/src/syscall/exec_linux_test.go
@@ -434,6 +434,49 @@ func TestUnshareMountNameSpaceChroot(t *testing.T) {
 	}
 }
 
+func TestUnshareUidGidMappingHelper(*testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+	defer os.Exit(0)
+	if err := syscall.Chroot(os.TempDir()); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+}
+
+// Test for Issue 29789: unshare fails when uid/gid mapping is specified
+func TestUnshareUidGidMapping(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("test exercises unprivileged user namespace, fails with privileges")
+	}
+	checkUserNS(t)
+	cmd := exec.Command(os.Args[0], "-test.run=TestUnshareUidGidMappingHelper")
+	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Unshareflags:               syscall.CLONE_NEWNS | syscall.CLONE_NEWUSER,
+		GidMappingsEnableSetgroups: false,
+		UidMappings: []syscall.SysProcIDMap{
+			{
+				ContainerID: 0,
+				HostID:      syscall.Getuid(),
+				Size:        1,
+			},
+		},
+		GidMappings: []syscall.SysProcIDMap{
+			{
+				ContainerID: 0,
+				HostID:      syscall.Getgid(),
+				Size:        1,
+			},
+		},
+	}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+	}
+}
+
 type capHeader struct {
 	version uint32
 	pid     int32