Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -3580,3 +3580,19 @@
   let SemaHandler = 0;
   let Documentation = [Undocumented];
 }
+
+def EnforceTCB : InheritableAttr {
+  let Spellings = [GCC<"enforce_tcb">];
+  let Subjects = SubjectList<[Function]>;
+  let Args = [StringArgument<"TCBName">];
+  let Documentation = [EnforceTCBDocs];
+  bit InheritEvenIfAlreadyPresent = 1;
+}
+
+def EnforceTCBLeaf : InheritableAttr {
+  let Spellings = [GCC<"enforce_tcb_leaf">];
+  let Subjects = SubjectList<[Function]>;
+  let Args = [StringArgument<"TCBName">];
+  let Documentation = [EnforceTCBLeafDocs];
+  bit InheritEvenIfAlreadyPresent = 1;
+}
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -5486,3 +5486,28 @@
 <https://developer.arm.com/docs/ecm0359818/latest/>`_ for more information.
   }];
 }
+
+def EnforceTCBDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+  The ``enforce_tcb`` attribute can be placed on functions to enforce that a
+  trusted compute base (TCB) does not call out of the TCB. This generates a
+  warning everytime a function not marked with an enforce_tcb attribute is
+  called from a function with the enforce_tcb attribute. A function may be
+  a part of multiple TCBs. Invocations of function pointers and C++ methods
+  are not checked. Builtins are considered to a part of every TCB.
+
+  - ``enforce_tcb(Name)`` indicates that this function is a part of the TCB named ``Name``
+  }];
+}
+
+def EnforceTCBLeafDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+  The ``enforce_tcb_leaf`` attribute satisfies the requirement enforced by
+  ``enforce_tcb`` for the marked function to be in the named TCB but does not
+  continue to check the functions called from within the leaf function.
+
+  - ``enforce_tcb_leaf(Name)`` indicates that this function is a part of the TCB named ``Name``
+  }];
+}
Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -1242,3 +1242,5 @@
 def WebAssemblyExceptionSpec : DiagGroup<"wasm-exception-spec">;
 
 def RTTI : DiagGroup<"rtti">;
+
+def TCBEnforcement : DiagGroup<"tcb-enforcement">;
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -11046,4 +11046,9 @@
 def err_probability_out_of_range : Error<
    "probability argument to __builtin_expect_with_probability is outside the "
    "range [0.0, 1.0]">;
+
+// TCB warnings
+def warn_tcb_enforcement_violation : Warning<
+  "TCB [%0] has been violated by calling a function [%1] that is not in the TCB.">,
+  InGroup<TCBEnforcement>;
 } // end of sema component.
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -12451,6 +12451,8 @@
   /// attempts to add itself into the container
   void CheckObjCCircularContainer(ObjCMessageExpr *Message);
 
+  void CheckTCBEnforcement(CallExpr *TheCall,  FunctionDecl *Callee);
+
   void AnalyzeDeleteExprMismatch(const CXXDeleteExpr *DE);
   void AnalyzeDeleteExprMismatch(FieldDecl *Field, SourceLocation DeleteLoc,
                                  bool DeleteWasArrayForm);
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -75,6 +75,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Support/AtomicOrdering.h"
@@ -4568,6 +4569,8 @@
   if (!FnInfo)
     return false;
 
+  CheckTCBEnforcement(TheCall, FDecl);
+
   CheckAbsoluteValueFunction(TheCall, FDecl);
   CheckMaxUnsignedZero(TheCall, FDecl);
 
@@ -16058,3 +16061,34 @@
 
   return CallResult;
 }
+
+/// \brief Enforce the bounds of a TCB
+/// CheckTCBEnforcement - Enforces that every function in a named TCB only
+/// directly calls other functions in the same TCB as marked by the enforce_tcb
+/// and enforce_tcb_leaf attributes.
+void Sema::CheckTCBEnforcement(CallExpr *TheCall,  FunctionDecl *Callee) {
+  FunctionDecl *Caller = getCurFunctionDecl();
+
+  // Calls to builtins are not enforced
+  if (!Caller || !Caller->hasAttr<EnforceTCBAttr>() || Callee->getBuiltinID() != 0) {
+    return;
+  }
+
+  // Search through the enforce_tcb and enforce_tcb_leaf attributes to find all TCBs the callee is a part of
+  llvm::StringSet<> CalleeTCBs;
+  for (const auto *Attr : Callee->specific_attrs<EnforceTCBAttr>()) {
+    CalleeTCBs.insert(Attr->getTCBName());
+  }
+  for (const auto *Attr : Callee->specific_attrs<EnforceTCBLeafAttr>()) {
+    CalleeTCBs.insert(Attr->getTCBName());
+  }
+
+  // Go through the TCBs the caller is a part of and emit warnings if Caller is in a TCB that the Callee is not
+  for (const auto *Attr : Caller->specific_attrs<EnforceTCBAttr>()) {
+    llvm::StringRef CallerTCB = Attr->getTCBName();
+
+    if (CalleeTCBs.count(CallerTCB) == 0) {
+      Diag(TheCall->getExprLoc(), diag::warn_tcb_enforcement_violation) << CallerTCB << Callee->getName();
+    }
+  }
+}
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -7336,6 +7336,14 @@
   D->addAttr(::new (S.Context) CFGuardAttr(S.Context, AL, Arg));
 }
 
+template<typename Attr>
+static void handleEnforceTCBAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  StringRef Argument;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, Argument))
+    return;
+  D->addAttr(Attr::Create(S.Context, Argument, AL));
+}
+
 //===----------------------------------------------------------------------===//
 // Top Level Sema Entry Points
 //===----------------------------------------------------------------------===//
@@ -8015,6 +8023,14 @@
   case ParsedAttr::AT_UseHandle:
     handleHandleAttr<UseHandleAttr>(S, D, AL);
     break;
+
+  case ParsedAttr::AT_EnforceTCB:
+    handleEnforceTCBAttr<EnforceTCBAttr>(S, D, AL);
+    break;
+
+  case ParsedAttr::AT_EnforceTCBLeaf:
+    handleEnforceTCBAttr<EnforceTCBLeafAttr>(S, D, AL);
+    break;
   }
 }
 
Index: clang/test/Sema/attr-enforce-tcb.c
===================================================================
--- /dev/null
+++ clang/test/Sema/attr-enforce-tcb.c
@@ -0,0 +1,54 @@
+// RUN: %clang_cc1  -fsyntax-only -verify %s
+
+#define PLACE_IN_TCB(NAME) __attribute__ ((enforce_tcb(NAME)))
+#define PLACE_IN_TCB_LEAF(NAME) __attribute__ ((enforce_tcb_leaf(NAME)))
+
+void foo1 (void) PLACE_IN_TCB("bar");
+void foo2 (void) PLACE_IN_TCB("bar");
+void foo3 (void); // not in any TCB
+void foo4 (void) PLACE_IN_TCB("bar2");
+void foo5 (void) PLACE_IN_TCB_LEAF("bar");
+void foo6 (void) PLACE_IN_TCB("bar2") PLACE_IN_TCB("bar");
+void foo7 (void) PLACE_IN_TCB("bar3");
+void foo8 (void) PLACE_IN_TCB("bar") PLACE_IN_TCB("bar2");
+void foo9 (void);
+
+void
+foo1()
+{
+    foo2(); // OK - function in same TCB
+    foo3(); // expected-warning {{TCB [bar] has been violated by calling a function [foo3]}}
+    foo4(); // expected-warning {{TCB [bar] has been violated by calling a function [foo4]}}
+    foo5(); // OK - in leaf node
+    foo6(); // OK - in multiple TCBs, one of which is the same
+    foo7(); // expected-warning {{TCB [bar] has been violated by calling a function [foo7]}}
+    (void) __builtin_clz(5); // OK - builtins are excluded
+}
+
+// Normal use without any attributes works
+void
+foo3()
+{
+    foo9(); // OK
+}
+
+void
+foo5()
+{
+    // all calls should be okay, function in TCB leaf
+    foo2();
+    foo3();
+    foo4();
+}
+
+
+void
+foo6()
+{
+    foo1(); // expected-warning {{TCB [bar2] has been violated by calling a function [foo1]}}
+    foo4(); // expected-warning {{TCB [bar] has been violated by calling a function [foo4]}}
+    foo8(); // OK
+    foo7(); // #1
+    // expected-warning@#1 {{TCB [bar] has been violated by calling a function [foo7] that is not in the TCB.}}
+    // expected-warning@#1 {{TCB [bar2] has been violated by calling a function [foo7] that is not in the TCB.}}
+}