[randstruct] Automatically randomize a structure of function pointers

Strutures of function pointers are a good surface area for attacks. We
should therefore randomize them unless explicitly told not to.

Reviewed By: aaron.ballman, MaskRay

Differential Revision: https://reviews.llvm.org/D123544

Author: isanbard

clang/lib/Sema/SemaDecl.cpp

@@ -18057,8 +18057,25 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
     // Handle attributes before checking the layout.
     ProcessDeclAttributeList(S, Record, Attrs);
 
-    // Maybe randomize the record's decls.
-    if (!getLangOpts().CPlusPlus && Record->hasAttr<RandomizeLayoutAttr>() &&
+    // Check to see if a FieldDecl is a pointer to a function.
+    auto IsFunctionPointer = [&](const Decl *D) {
+      const FieldDecl *FD = dyn_cast<FieldDecl>(D);
+      if (!FD)
+        return false;
+      QualType FieldType = FD->getType().getDesugaredType(Context);
+      if (isa<PointerType>(FieldType)) {
+        QualType PointeeType = cast<PointerType>(FieldType)->getPointeeType();
+        return PointeeType.getDesugaredType(Context)->isFunctionType();
+      }
+      return false;
+    };
+
+    // Maybe randomize the record's decls. We automatically randomize a record
+    // of function pointers, unless it has the "no_randomize_layout" attribute.
+    if (!getLangOpts().CPlusPlus &&
+        (Record->hasAttr<RandomizeLayoutAttr>() ||
+         (!Record->hasAttr<NoRandomizeLayoutAttr>() &&
+          llvm::all_of(Record->decls(), IsFunctionPointer))) &&
         !Record->isUnion() && !getLangOpts().RandstructSeed.empty() &&
         !Record->isRandomized()) {
       SmallVector<Decl *, 32> NewDeclOrdering;

clang/unittests/AST/RandstructTest.cpp

@@ -583,5 +583,49 @@ TEST(RANDSTRUCT_TEST, AnonymousStructsAndUnionsReferenced) {
   EXPECT_EQ(OriginalDeclCount, declCount(RD));
 }
 
+TEST(RANDSTRUCT_TEST, AutoRandomizeStructOfFunctionPointers) {
+  const std::unique_ptr<ASTUnit> AST = makeAST(R"c(
+    typedef void (*func_ptr)();
+
+    struct test {
+      func_ptr a;
+      func_ptr b;
+      func_ptr c;
+      func_ptr d;
+      func_ptr e;
+      func_ptr f;
+      func_ptr g;
+    };
+  )c");
+
+  EXPECT_FALSE(AST->getDiagnostics().hasErrorOccurred());
+
+  const RecordDecl *RD = getRecordDeclFromAST(AST->getASTContext(), "test");
+
+  EXPECT_TRUE(RD->isRandomized());
+}
+
+TEST(RANDSTRUCT_TEST, DisableAutoRandomizeStructOfFunctionPointers) {
+  const std::unique_ptr<ASTUnit> AST = makeAST(R"c(
+    typedef void (*func_ptr)();
+
+    struct test {
+      func_ptr a;
+      func_ptr b;
+      func_ptr c;
+      func_ptr d;
+      func_ptr e;
+      func_ptr f;
+      func_ptr g;
+    } __attribute__((no_randomize_layout));
+  )c");
+
+  EXPECT_FALSE(AST->getDiagnostics().hasErrorOccurred());
+
+  const RecordDecl *RD = getRecordDeclFromAST(AST->getASTContext(), "test");
+
+  EXPECT_FALSE(RD->isRandomized());
+}
+
 } // namespace ast_matchers
 } // namespace clang