Kernel: Switch the first-8MB-of-upper-3GB pseudo mappings to 4KB pages

This memory range was set up using 2MB pages by the code in boot.S.
Because of that, the kernel image protection code didn't work, since it
assumed 4KB pages.

We now switch to 4KB pages during MemoryManager initialization. This
makes the kernel image protection code work correctly again. :^)

Kernel/VM/MemoryManager.cpp

@@ -63,6 +63,7 @@ MemoryManager::MemoryManager()
     parse_memory_map();
     write_cr3(kernel_page_directory().cr3());
     setup_low_identity_mapping();
+    setup_low_pseudo_identity_mapping();
     protect_kernel_image();
 
     m_shared_zero_page = allocate_user_physical_page();
@@ -72,6 +73,36 @@ MemoryManager::~MemoryManager()
 {
 }
 
+void MemoryManager::setup_low_pseudo_identity_mapping()
+{
+    // This code switches the pseudo-identity mapping (8 first MB above 3G mark) from 2MB pages to 4KB pages.
+    // The boot code sets it up as 2MB huge pages for convenience. But we need 4KB pages to be able to protect
+    // the kernel soon!
+
+    for (size_t i = 0; i < 4; ++i) {
+        m_low_pseudo_identity_mapping_pages[i] = allocate_supervisor_physical_page();
+        FlatPtr base = i * (2 * MB);
+        auto* page_table = (PageTableEntry*)quickmap_page(*m_low_pseudo_identity_mapping_pages[i]);
+        for (size_t j = 0; j < 512; ++j) {
+            auto& pte = page_table[j];
+            pte.set_physical_page_base(base + j * PAGE_SIZE);
+            pte.set_writable(true);
+            pte.set_present(true);
+            pte.set_execute_disabled(false);
+            pte.set_user_allowed(false);
+        }
+        unquickmap_page();
+    }
+
+    auto* pd = quickmap_pd(*m_kernel_page_directory, 3);
+    for (size_t i = 0; i < 4; ++i) {
+        pd[i].set_huge(false);
+        pd[i].set_page_table_base(m_low_pseudo_identity_mapping_pages[i]->paddr().get());
+    }
+
+    flush_entire_tlb();
+}
+
 void MemoryManager::protect_kernel_image()
 {
     // Disable writing to the kernel text and rodata segments.

Kernel/VM/MemoryManager.h

@@ -150,6 +150,7 @@ private:
 
     void detect_cpu_features();
     void setup_low_identity_mapping();
+    void setup_low_pseudo_identity_mapping();
     void protect_kernel_image();
     void parse_memory_map();
     void flush_entire_tlb();
@@ -191,6 +192,8 @@ private:
     InlineLinkedList<VMObject> m_vmobjects;
 
     bool m_quickmap_in_use { false };
+
+    RefPtr<PhysicalPage> m_low_pseudo_identity_mapping_pages[4];
 };
 
 template<typename Callback>