windows: fix dangling pointers in (*SECURITY_DESCRIPTOR).ToAbsolute

Prior to this CL, a byte slice was allocated via make to use as the
absoluteSD argument passed to the Windows API MakeAbsoluteSD.

MakeAbsoluteSD then sets pointers outside the view of the GC, including
pointers within absoluteSD that point to other chunks of memory
we pass into MakeAbsoluteSD.

CL 653856 recently allowed more make results to be stack allocated,
which worsened the problems here and made it easier for those
pointers in absoluteSD to become dangling pointers, though the
core problems here existed before.

This CL instead allocates absoluteSD as a proper SECURITY_DESCRIPTOR
struct so that the GC can be aware of its pointers. We also verify the
pointers are as we expect, and then set them explicitly
in view of the GC.

Updates golang/go#73199

Change-Id: Id8038d38a887bb8ff3ffc6eae603589b97e92cdc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/663355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

Author: thepudds

windows/security_windows.go

@@ -1303,7 +1303,10 @@ func (selfRelativeSD *SECURITY_DESCRIPTOR) ToAbsolute() (absoluteSD *SECURITY_DE
 		return nil, err
 	}
 	if absoluteSDSize > 0 {
-		absoluteSD = (*SECURITY_DESCRIPTOR)(unsafe.Pointer(&make([]byte, absoluteSDSize)[0]))
+		absoluteSD = new(SECURITY_DESCRIPTOR)
+		if unsafe.Sizeof(*absoluteSD) < uintptr(absoluteSDSize) {
+			panic("sizeof(SECURITY_DESCRIPTOR) too small")
+		}
 	}
 	var (
 		dacl  *ACL
@@ -1312,19 +1315,55 @@ func (selfRelativeSD *SECURITY_DESCRIPTOR) ToAbsolute() (absoluteSD *SECURITY_DE
 		group *SID
 	)
 	if daclSize > 0 {
-		dacl = (*ACL)(unsafe.Pointer(&make([]byte, daclSize)[0]))
+		dacl = (*ACL)(unsafe.Pointer(unsafe.SliceData(make([]byte, daclSize))))
 	}
 	if saclSize > 0 {
-		sacl = (*ACL)(unsafe.Pointer(&make([]byte, saclSize)[0]))
+		sacl = (*ACL)(unsafe.Pointer(unsafe.SliceData(make([]byte, saclSize))))
 	}
 	if ownerSize > 0 {
-		owner = (*SID)(unsafe.Pointer(&make([]byte, ownerSize)[0]))
+		owner = (*SID)(unsafe.Pointer(unsafe.SliceData(make([]byte, ownerSize))))
 	}
 	if groupSize > 0 {
-		group = (*SID)(unsafe.Pointer(&make([]byte, groupSize)[0]))
+		group = (*SID)(unsafe.Pointer(unsafe.SliceData(make([]byte, groupSize))))
 	}
+	// We call into Windows via makeAbsoluteSD, which sets up
+	// pointers within absoluteSD that point to other chunks of memory
+	// we pass into makeAbsoluteSD, and that happens outside the view of the GC.
+	// We therefore take some care here to then verify the pointers are as we expect
+	// and set them explicitly in view of the GC. See https://go.dev/issue/73199.
+	// TODO: consider weak pointers once Go 1.24 is appropriate. See suggestion in https://go.dev/cl/663575.
 	err = makeAbsoluteSD(selfRelativeSD, absoluteSD, &absoluteSDSize,
 		dacl, &daclSize, sacl, &saclSize, owner, &ownerSize, group, &groupSize)
+	if err != nil {
+		// Don't return absoluteSD, which might be partially initialized.
+		return nil, err
+	}
+	// Before using any fields, verify absoluteSD is in the format we expect according to Windows.
+	// See https://learn.microsoft.com/en-us/windows/win32/secauthz/absolute-and-self-relative-security-descriptors
+	absControl, _, err := absoluteSD.Control()
+	if err != nil {
+		panic("absoluteSD: " + err.Error())
+	}
+	if absControl&SE_SELF_RELATIVE != 0 {
+		panic("absoluteSD not in absolute format")
+	}
+	if absoluteSD.dacl != dacl {
+		panic("dacl pointer mismatch")
+	}
+	if absoluteSD.sacl != sacl {
+		panic("sacl pointer mismatch")
+	}
+	if absoluteSD.owner != owner {
+		panic("owner pointer mismatch")
+	}
+	if absoluteSD.group != group {
+		panic("group pointer mismatch")
+	}
+	absoluteSD.dacl = dacl
+	absoluteSD.sacl = sacl
+	absoluteSD.owner = owner
+	absoluteSD.group = group
+
 	return
 }