rewriter: Omit register scrubbing when caller/target pkey are zero

If a call gate's caller has pkey zero then its memory is accessible to all
compartments and we shouldn't care about scrubbing registers before calling the
target since its memory is not protected anyway. If the call gate's target has
pkey zero then the same argument holds for the register scrubbing after calling
the target function.

rewriter/GenCallAsm.cpp

@@ -286,6 +286,8 @@ std::string emit_asm_wrapper(const CAbiSignature &sig,
                              WrapperKind kind, int caller_pkey, int target_pkey,
                              bool as_macro) {
 
+  // Small sanity check
+  assert(caller_pkey != target_pkey);
   std::string terminator = {};
   // Code generated as a macro needs to terminate each line with '\'
   if (as_macro) {
@@ -462,30 +464,35 @@ std::string emit_asm_wrapper(const CAbiSignature &sig,
     }
   }
 
-  // Zero out all unused registers. First we save all registers containing args.
-  // These pushes have matching pops before calling the wrapped function so this
-  // stack space is not shown in the diagram above.
-  if (reg_arg_count > 0) {
-    add_comment_line(aw, "Save registers containing arguments");
-    for (const auto &loc : param_locs) {
-      if (!loc.is_stack()) {
-        emit_reg_push(aw, loc);
+  // Before calling the target function, we only need to zero out registers not
+  // used for arguments if the caller is a protected compartment (i.e. pkey is
+  // not zero).
+  if (caller_pkey != 0) {
+    // Zero out all unused registers. First we save all registers containing args.
+    // These pushes have matching pops before calling the wrapped function so this
+    // stack space is not shown in the diagram above.
+    if (reg_arg_count > 0) {
+      add_comment_line(aw, "Save registers containing arguments");
+      for (const auto &loc : param_locs) {
+        if (!loc.is_stack()) {
+          emit_reg_push(aw, loc);
+        }
       }
     }
-  }
-
-  // Zero all registers except rsp
-  add_comment_line(aw, "Zero all registers except rsp");
-  // FIXME: If this will use the System V ABI make sure that the %rsp is aligned
-  // before this call
-  add_asm_line(aw, "call __libia2_scrub_registers");
 
-  // Restore used arg regs after zeroing registers
-  if (reg_arg_count > 0) {
-    add_comment_line(aw, "Restore registers containing arguments");
-    for (auto loc = param_locs.rbegin(); loc != param_locs.rend(); loc++) {
-      if (!loc->is_stack()) {
-        emit_reg_pop(aw, *loc);
+    // Zero all registers except rsp
+    add_comment_line(aw, "Zero all registers except rsp");
+    // FIXME: If this will use the System V ABI make sure that the %rsp is aligned
+    // before this call
+    add_asm_line(aw, "call __libia2_scrub_registers");
+
+    // Restore used arg regs after zeroing registers
+    if (reg_arg_count > 0) {
+      add_comment_line(aw, "Restore registers containing arguments");
+      for (auto loc = param_locs.rbegin(); loc != param_locs.rend(); loc++) {
+        if (!loc->is_stack()) {
+          emit_reg_pop(aw, *loc);
+        }
       }
     }
   }
@@ -563,27 +570,31 @@ std::string emit_asm_wrapper(const CAbiSignature &sig,
   // Switch back to the caller's stack
   emit_switch_stacks(aw, target_pkey, caller_pkey, "r11"s);
 
-  add_comment_line(
-      aw, "Push return regs to caller's stack before scrubbing registers");
-  // Push return regs to the caller's stack. These pushes have matching pops
-  // before the return so it has no net effect on the caller's stack pointer.
-  for (auto loc : return_locs) {
-    if (!loc.is_stack()) {
-      emit_reg_push(aw, loc);
+  // After calling the target function, we only need to zero out registers not
+  // used for return values if the target is a protected compartment
+  if (target_pkey != 0) {
+    add_comment_line(
+        aw, "Push return regs to caller's stack before scrubbing registers");
+    // Push return regs to the caller's stack. These pushes have matching pops
+    // before the return so it has no net effect on the caller's stack pointer.
+    for (auto loc : return_locs) {
+      if (!loc.is_stack()) {
+        emit_reg_push(aw, loc);
+      }
     }
-  }
 
-  // Zero all registers except rsp
-  add_comment_line(aw, "Scrub registers after call");
-  // FIXME: If this will use the System V ABI make sure that the %rsp is aligned
-  // before this call
-  add_asm_line(aw, "call __libia2_scrub_registers");
-
-  // pop return regs
-  add_comment_line(aw, "Pop return regs");
-  for (auto loc = return_locs.rbegin(); loc != return_locs.rend(); loc++) {
-    if (!loc->is_stack()) {
-      emit_reg_pop(aw, *loc);
+    // Zero all registers except rsp
+    add_comment_line(aw, "Scrub registers after call");
+    // FIXME: If this will use the System V ABI make sure that the %rsp is aligned
+    // before this call
+    add_asm_line(aw, "call __libia2_scrub_registers");
+
+    // pop return regs
+    add_comment_line(aw, "Pop return regs");
+    for (auto loc = return_locs.rbegin(); loc != return_locs.rend(); loc++) {
+      if (!loc->is_stack()) {
+        emit_reg_pop(aw, *loc);
+      }
     }
   }
   // Once again use r10 and r11 as scratch registers