diff --git a/src/libos/src/lib.rs b/src/libos/src/lib.rs
index 61daa25b..bc51de48 100644
--- a/src/libos/src/lib.rs
+++ b/src/libos/src/lib.rs
@@ -11,6 +11,7 @@
 #![feature(alloc_layout_extra)]
 #![feature(concat_idents)]
 #![feature(trace_macros)]
+#![feature(slice_fill)]
 
 #[macro_use]
 extern crate alloc;
diff --git a/src/libos/src/vm/vm_manager.rs b/src/libos/src/vm/vm_manager.rs
index a0b7f642..1fee8865 100644
--- a/src/libos/src/vm/vm_manager.rs
+++ b/src/libos/src/vm/vm_manager.rs
@@ -24,26 +24,18 @@ impl VMInitializer {
                 // Do nothing
             }
             VMInitializer::FillZeros() => {
-                for b in buf {
-                    *b = 0;
-                }
+                // Filling zero is done in munmap
             }
             VMInitializer::CopyFrom { range } => {
                 let src_slice = unsafe { range.as_slice() };
                 let copy_len = min(buf.len(), src_slice.len());
                 buf[..copy_len].copy_from_slice(&src_slice[..copy_len]);
-                for b in &mut buf[copy_len..] {
-                    *b = 0;
-                }
             }
             VMInitializer::LoadFromFile { file, offset } => {
                 // TODO: make sure that read_at does not move file cursor
                 let len = file
                     .read_at(*offset, buf)
                     .cause_err(|_| errno!(EIO, "failed to init memory from file"))?;
-                for b in &mut buf[len..] {
-                    *b = 0;
-                }
             }
         }
         Ok(())
@@ -341,6 +333,9 @@ impl VMManager {
 
                 // Reset memory permissions
                 Self::apply_perms(&intersection_range, VMPerms::default());
+                unsafe {
+                    intersection_range.as_slice_mut().fill(0);
+                }
 
                 vma.subtract(&intersection_range)
             })
@@ -733,12 +728,18 @@ impl VMManager {
 
 impl Drop for VMManager {
     fn drop(&mut self) {
-        // Ensure that memory permissions are recovered
+        // Ensure that all allocated memories are restored to the default permissions and zeroed
         for vma in &self.vmas {
-            if vma.size() == 0 || vma.perms() == VMPerms::default() {
-                continue;
+            if vma.size() != 0 {
+                warn!("There are unmapped memories");
+
+                if vma.perms() != VMPerms::default() {
+                    Self::apply_perms(vma, VMPerms::default());
+                }
+                unsafe {
+                    vma.as_slice_mut().fill(0);
+                }
             }
-            Self::apply_perms(vma, VMPerms::default());
         }
     }
 }