--- a/xpcom/threads/ThreadStackHelper.cpp
+++ b/xpcom/threads/ThreadStackHelper.cpp
@@ -607,17 +607,17 @@ ThreadStackHelper::FillStackBuffer()
   // availableBufferSize < 0 if we needed a larger buffer than we reserved.
   // Calculate a new reserve size for next time.
   if (availableBufferSize < 0) {
     mMaxBufferSize = reservedBufferSize - availableBufferSize;
   }
 #endif
 }
 
-void
+MOZ_ASAN_BLACKLIST void
 ThreadStackHelper::FillThreadContext(void* aContext)
 {
 #ifdef MOZ_THREADSTACKHELPER_NATIVE
   if (!mContextToFill) {
     return;
   }
 
 #if defined(XP_LINUX)
@@ -752,16 +752,27 @@ ThreadStackHelper::FillThreadContext(voi
 
 #ifndef MOZ_THREADSTACKHELPER_STACK_GROWS_DOWN
   // If if the stack grows upwards, and we need to recalculate our
   // stack copy's base address. Subtract sizeof(void*) so that the
   // location pointed to by sp is included.
   sp -= stackSize - sizeof(void*);
 #endif
 
+#ifndef MOZ_ASAN
   memcpy(mContextToFill->mStack, reinterpret_cast<void*>(sp), stackSize);
+#else
+  // ASan will flag memcpy for access outside of stack frames,
+  // so roll our own memcpy here.
+  intptr_t* dst = reinterpret_cast<intptr_t*>(&mContextToFill->mStack[0]);
+  const intptr_t* src = reinterpret_cast<intptr_t*>(sp);
+  for (intptr_t len = stackSize; len > 0; len -= sizeof(*src)) {
+    *(dst++) = *(src++);
+  }
+#endif
+
   mContextToFill->mStackBase = uintptr_t(sp);
   mContextToFill->mStackSize = stackSize;
   mContextToFill->mValid = true;
 #endif // MOZ_THREADSTACKHELPER_NATIVE
 }
 
 } // namespace mozilla