os: don't follow symlinks on Windows when O_CREATE|O_EXCL

Match standard Unix behavior: Symlinks are not followed when
O_CREATE|O_EXCL is passed to open.

Thanks to Junyoung Park and Dong-uk Kim of KAIST Hacking Lab
for discovering this issue.

Fixes #73702
Fixes CVE-2025-0913

Change-Id: Ieb46a6780c5e9a6090b09cd34290f04a8e3b0ca5
Reviewed-on: https://go-review.googlesource.com/c/go/+/672396
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/internal/syscall/windows/at_windows.go

@@ -94,6 +94,7 @@ func Openat(dirfd syscall.Handle, name string, flag uint64, perm uint32) (_ sysc
 	switch {
 	case flag&(syscall.O_CREAT|syscall.O_EXCL) == (syscall.O_CREAT | syscall.O_EXCL):
 		disposition = FILE_CREATE
+		options |= FILE_OPEN_REPARSE_POINT // don't follow symlinks
 	case flag&syscall.O_CREAT == syscall.O_CREAT:
 		disposition = FILE_OPEN_IF
 	default:

src/os/os_test.go

@@ -2299,6 +2299,31 @@ func TestFilePermissions(t *testing.T) {
 
 }
 
+func TestOpenFileCreateExclDanglingSymlink(t *testing.T) {
+	testMaybeRooted(t, func(t *testing.T, r *Root) {
+		const link = "link"
+		if err := Symlink("does_not_exist", link); err != nil {
+			t.Fatal(err)
+		}
+		var f *File
+		var err error
+		if r == nil {
+			f, err = OpenFile(link, O_WRONLY|O_CREATE|O_EXCL, 0o666)
+		} else {
+			f, err = r.OpenFile(link, O_WRONLY|O_CREATE|O_EXCL, 0o666)
+		}
+		if err == nil {
+			f.Close()
+		}
+		if !errors.Is(err, ErrExist) {
+			t.Errorf("OpenFile of a dangling symlink with O_CREATE|O_EXCL = %v, want ErrExist", err)
+		}
+		if _, err := Stat(link); err == nil {
+			t.Errorf("OpenFile of a dangling symlink with O_CREATE|O_EXCL created a file")
+		}
+	})
+}
+
 // TestFileRDWRFlags tests the O_RDONLY, O_WRONLY, and O_RDWR flags.
 func TestFileRDWRFlags(t *testing.T) {
 	for _, test := range []struct {

src/syscall/syscall_windows.go

@@ -403,15 +403,16 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 	//
 	// Instead, we ftruncate the file after opening when O_TRUNC is set.
 	var createmode uint32
+	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
 	switch {
 	case flag&(O_CREAT|O_EXCL) == (O_CREAT | O_EXCL):
 		createmode = CREATE_NEW
+		attrs |= FILE_FLAG_OPEN_REPARSE_POINT // don't follow symlinks
 	case flag&O_CREAT == O_CREAT:
 		createmode = OPEN_ALWAYS
 	default:
 		createmode = OPEN_EXISTING
 	}
-	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
 	if perm&S_IWRITE == 0 {
 		attrs = FILE_ATTRIBUTE_READONLY
 	}