UserVM: Validate the entire range when freeing multiple VM pages

kernel/src/memory/UserVM.cpp

@@ -100,8 +100,7 @@ Result<void> UserVM::free_several_pages(u64 address, usize count)
     const u64 index = (address - VM_BASE) / ARCH_PAGE_SIZE;
     if ((index + count) > (MAX_VM_SIZE * 8)) return err(EINVAL);
 
-    // FIXME: Is it necessary to check all pages?
-    if (!m_bitmap.get(index)) return err(EFAULT);
+    if (!m_bitmap.match_region(index, count, true)) return err(EFAULT);
 
     m_bitmap.clear_region(index, count, false);
 

luna/include/luna/Bitmap.h

@@ -43,6 +43,8 @@ class Bitmap
 
     Option<usize> find_and_toggle_region(bool value, usize count, usize begin = 0);
 
+    bool match_region(usize start, usize bits, bool value);
+
     void clear(bool value);
     void clear_region(usize start, usize bits, bool value);
 

luna/src/Bitmap.cpp

@@ -167,3 +167,40 @@ Option<usize> Bitmap::find_and_toggle_region(bool value, usize count, usize begi
     clear_region(index, count, !value);
     return index;
 }
+
+bool Bitmap::match_region(usize start, usize bits, bool value)
+{
+    expect(initialized(), "Bitmap was never initialized");
+    expect((start + bits) <= size(), "Bitmap match out of range");
+
+    if (!bits) return true;
+
+    // Match individual bits while not on a byte boundary.
+    while ((start % 8) && bits)
+    {
+        if (get(start) != value) return false;
+        start++;
+        bits--;
+    }
+
+    // Match the rest in bytes.
+    usize bytes = bits / 8;
+    const u8 byte_that_contains_only_value = value_byte(value);
+
+    for (usize i = start; i < start + bytes; i += 8)
+    {
+        if (m_location[i / 8] != byte_that_contains_only_value) return false;
+    }
+
+    start += bytes * 8;
+    bits -= bytes * 8;
+
+    // Match the remaining individual bits.
+    while (bits--)
+    {
+        if (get(start) != value) return false;
+        start++;
+    }
+
+    return true;
+}