syscall: allow setting security attributes on processes

This allows creating processes that can only be debugged/accessed by
certain tokens, according to a particular security descriptor. We
already had everything ready for this but just neglected to pass through
the value from the user-accessible SysProcAttr.

Change-Id: I4a3fcc9f5078aa0058b26c103355c984093ae03f
Reviewed-on: https://go-review.googlesource.com/c/go/+/174197
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/syscall/exec_windows.go

@@ -219,10 +219,12 @@ type ProcAttr struct {
 }
 
 type SysProcAttr struct {
-	HideWindow    bool
-	CmdLine       string // used if non-empty, else the windows command line is built by escaping the arguments passed to StartProcess
-	CreationFlags uint32
-	Token         Token // if set, runs new process in the security context represented by the token
+	HideWindow        bool
+	CmdLine           string // used if non-empty, else the windows command line is built by escaping the arguments passed to StartProcess
+	CreationFlags     uint32
+	Token             Token               // if set, runs new process in the security context represented by the token
+	ProcessAttributes *SecurityAttributes // if set, applies these security attributes as the descriptor for the new process
+	ThreadAttributes  *SecurityAttributes // if set, applies these security attributes as the descriptor for the main thread of the new process
 }
 
 var zeroProcAttr ProcAttr
@@ -323,9 +325,9 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 
 	flags := sys.CreationFlags | CREATE_UNICODE_ENVIRONMENT
 	if sys.Token != 0 {
-		err = CreateProcessAsUser(sys.Token, argv0p, argvp, nil, nil, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
+		err = CreateProcessAsUser(sys.Token, argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
 	} else {
-		err = CreateProcess(argv0p, argvp, nil, nil, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
+		err = CreateProcess(argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
 	}
 	if err != nil {
 		return 0, 0, err