From 9ed502c59c6456bf8984bb6de49472da69ef4c90 Mon Sep 17 00:00:00 2001
From: Ted Campbell <tcampbell@mozilla.com>
Date: Sat, 07 Nov 2020 05:36:31 +0000 (2020-11-07)
Subject: [PATCH] CVE-2020-26950

Simplify IonBuilder::createThisScripted. r=jandem,iain a=RyanVM

---
 js/src/jit/IonBuilder.cpp | 31 ++++++++-----------------------
 js/src/jit/IonIC.cpp      |  8 ++++++++
 2 files changed, 16 insertions(+), 23 deletions(-)

diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 1b2a62a268..865e7bb322 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -5206,31 +5206,16 @@ MDefinition* IonBuilder::createThisScripted(MDefinition* callee,
   // explicit operation in the bytecode, we cannot use resumeAfter().
   // Getters may not override |prototype| fetching, so this operation is
   // indeed idempotent.
-  // - First try an idempotent property cache.
-  // - Upon failing idempotent property cache, we can't use a non-idempotent
-  //   cache, therefore we fallback to CallGetProperty
-  //
-  // Note: both CallGetProperty and GetPropertyCache can trigger a GC,
-  //       and thus invalidation.
-  MInstruction* getProto;
-  if (!invalidatedIdempotentCache()) {
-    MConstant* id = constant(StringValue(names().prototype));
-    MGetPropertyCache* getPropCache =
-        MGetPropertyCache::New(alloc(), newTarget, id,
-                               /* monitored = */ false);
-    getPropCache->setIdempotent();
-    getProto = getPropCache;
-  } else {
-    MCallGetProperty* callGetProp =
-        MCallGetProperty::New(alloc(), newTarget, names().prototype);
-    callGetProp->setIdempotent();
-    getProto = callGetProp;
-  }
-  current->add(getProto);
-
+   // Note: GetPropertyCache can trigger a GC, and thus invalidation.
+  MConstant* id = constant(StringValue(names().prototype));
+  MGetPropertyCache* getPropCache =
+      MGetPropertyCache::New(alloc(), newTarget, id,
+                              /* monitored = */ false);
+  getPropCache->setIdempotent();
+  current->add(getPropCache); 
   // Create this from prototype
   MCreateThisWithProto* createThis =
-      MCreateThisWithProto::New(alloc(), callee, newTarget, getProto);
+                MCreateThisWithProto::New(alloc(), callee, newTarget, getPropCache);
   current->add(createThis);
 
   return createThis;
diff --git a/js/src/jit/IonIC.cpp b/js/src/jit/IonIC.cpp
index 2c3ba44782..3cd06a13e7 100644
--- a/js/src/jit/IonIC.cpp
+++ b/js/src/jit/IonIC.cpp
@@ -216,6 +216,14 @@ bool IonGetPropertyIC::update(JSContext* cx, HandleScript outerScript,
     if (outerScript->hasIonScript()) {
       Invalidate(cx, outerScript);
     }
+    // IonBuilder::createScriptedThis does not use InvalidedIdempotentCache
+    // flag so prevent bailout-loop by disabling Ion for the script.
+    MOZ_ASSERT(ic->kind() == CacheKind::GetProp);
+    if (idVal.toString()->asAtom().asPropertyName() == cx->names().prototype) {
+      if (val.isObject() && val.toObject().is<JSFunction>()) {
+        outerScript->disableIon();
+      }
+    }
 
     // We will redo the potentially effectful lookup in Baseline.
     return true;
-- 
2.27.0