all: avoid unsafe StringToUTF16Ptr on Windows

The syscall.StringToUTF16Ptr function panics when
provided with an input containing a NUL character.
Replace with syscall.UTF16PtrFromString.

Fixes potential panics in net.Dial, net.LookupPort,
and syscall.Readlink.

Fixes CVE-2026-39836
Fixes #79006

Change-Id: I2fd7bb750d27474047f199faca4061466a6a6964
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/4260
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/775320
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Cherry Mui <cherryyz@google.com>

src/internal/syscall/windows/registry/key.go

@@ -132,7 +132,11 @@ loopItems:
 func CreateKey(k Key, path string, access uint32) (newk Key, openedExisting bool, err error) {
 	var h syscall.Handle
 	var d uint32
-	err = regCreateKeyEx(syscall.Handle(k), syscall.StringToUTF16Ptr(path),
+	pathp, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, false, err
+	}
+	err = regCreateKeyEx(syscall.Handle(k), pathp,
 		0, nil, _REG_OPTION_NON_VOLATILE, access, nil, &h, &d)
 	if err != nil {
 		return 0, false, err
@@ -142,7 +146,11 @@ func CreateKey(k Key, path string, access uint32) (newk Key, openedExisting bool
 
 // DeleteKey deletes the subkey path of key k and its values.
 func DeleteKey(k Key, path string) error {
-	return regDeleteKey(syscall.Handle(k), syscall.StringToUTF16Ptr(path))
+	pathp, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return err
+	}
+	return regDeleteKey(syscall.Handle(k), pathp)
 }
 
 // A KeyInfo describes the statistics of a key. It is returned by Stat.

src/internal/syscall/windows/registry/value.go

@@ -333,7 +333,11 @@ func (k Key) SetBinaryValue(name string, value []byte) error {
 
 // DeleteValue removes a named value from the key k.
 func (k Key) DeleteValue(name string) error {
-	return regDeleteValue(syscall.Handle(k), syscall.StringToUTF16Ptr(name))
+	namep, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return err
+	}
+	return regDeleteValue(syscall.Handle(k), namep)
 }
 
 // ReadValueNames returns the value names of key k.

src/net/lookup_test.go

@@ -769,6 +769,7 @@ func TestLookupPort(t *testing.T) {
 		{"udp", "-1", 0, false},
 		{"udp", "65536", 0, false},
 		{"tcp", "123456789", 0, false},
+		{"tcp", "bad\x00port", 0, false},
 
 		// Issue 13610: LookupPort("tcp", "")
 		{"tcp", "", 0, true},

src/net/lookup_windows.go

@@ -215,8 +215,13 @@ func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int
 		hints.Family = syscall.AF_INET6
 	}
 
+	servicep, err := syscall.UTF16PtrFromString(service)
+	if err != nil {
+		return 0, newDNSError(err, network+"/"+service, "")
+	}
+
 	var result *syscall.AddrinfoW
-	e := syscall.GetAddrInfoW(nil, syscall.StringToUTF16Ptr(service), &hints, &result)
+	e := syscall.GetAddrInfoW(nil, servicep, &hints, &result)
 	if e != nil {
 		if port, err := lookupPortMap(network, service); err == nil {
 			return port, nil
@@ -269,7 +274,12 @@ func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error)
 	}
 	defer syscall.DnsRecordListFree(rec, 1)
 
-	resolved := resolveCNAME(syscall.StringToUTF16Ptr(name), rec)
+	namep, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return "", newDNSError(err, name, "")
+	}
+
+	resolved := resolveCNAME(namep, rec)
 	cname := windows.UTF16PtrToString(resolved)
 	return absDomainName(cname), nil
 }
@@ -415,7 +425,10 @@ const dnsSectionMask = 0x0003
 
 // returns only results applicable to name and resolves CNAME entries.
 func validRecs(r *syscall.DNSRecord, dnstype uint16, name string) []*syscall.DNSRecord {
-	cname := syscall.StringToUTF16Ptr(name)
+	cname, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return nil
+	}
 	if dnstype != syscall.DNS_TYPE_CNAME {
 		cname = resolveCNAME(cname, r)
 	}

src/syscall/syscall_windows.go

@@ -1375,7 +1375,11 @@ func LoadCreateSymbolicLink() error {
 
 // Readlink returns the destination of the named symbolic link.
 func Readlink(path string, buf []byte) (n int, err error) {
-	fd, err := CreateFile(StringToUTF16Ptr(path), GENERIC_READ, 0, nil, OPEN_EXISTING,
+	pathp, err := UTF16PtrFromString(path)
+	if err != nil {
+		return -1, err
+	}
+	fd, err := CreateFile(pathp, GENERIC_READ, 0, nil, OPEN_EXISTING,
 		FILE_FLAG_OPEN_REPARSE_POINT|FILE_FLAG_BACKUP_SEMANTICS, 0)
 	if err != nil {
 		return -1, err