Kernel: Assert that copy_to/from_user() are called with user addresses

This will panic the kernel immediately if these functions are misused
so we can catch it and fix the misuse.

This patch fixes a couple of misuses:

    - create_signal_trampolines() writes to a user-accessible page
      above the 3GB address mark. We should really get rid of this
      page but that's a whole other thing.

    - CoW faults need to use copy_from_user rather than copy_to_user
      since it's the *source* pointer that points to user memory.

    - Inode faults need to use memcpy rather than copy_to_user since
      we're copying a kernel stack buffer into a quickmapped page.

This should make the copy_to/from_user() functions slightly less useful
for exploitation. Before this, they were essentially just glorified
memcpy() with SMAP disabled. :^)

Kernel/VM/MemoryManager.h

@@ -214,3 +214,10 @@ inline bool is_user_address(VirtualAddress vaddr)
 {
     return vaddr.get() < 0xc0000000;
 }
+
+inline bool is_user_range(VirtualAddress vaddr, size_t size)
+{
+    if (vaddr.offset(size) < vaddr)
+        return false;
+    return is_user_address(vaddr) && is_user_address(vaddr.offset(size));
+}

Kernel/VM/Region.cpp

@@ -424,7 +424,7 @@ PageFaultResponse Region::handle_cow_fault(size_t page_index_in_region)
 #ifdef PAGE_FAULT_DEBUG
     dbgprintf("      >> COW P%p <- P%p\n", physical_page->paddr().get(), physical_page_to_copy->paddr().get());
 #endif
-    copy_to_user(dest_ptr, src_ptr, PAGE_SIZE);
+    copy_from_user(dest_ptr, src_ptr, PAGE_SIZE);
     vmobject_physical_page_entry = move(physical_page);
     MM.unquickmap_page();
     set_should_cow(page_index_in_region, false);
@@ -481,7 +481,7 @@ PageFaultResponse Region::handle_inode_fault(size_t page_index_in_region)
     }
 
     u8* dest_ptr = MM.quickmap_page(*vmobject_physical_page_entry);
-    copy_to_user(dest_ptr, page_buffer, PAGE_SIZE);
+    memcpy(dest_ptr, page_buffer, PAGE_SIZE);
     MM.unquickmap_page();
 
     remap_page(page_index_in_region);

Kernel/VM/VirtualAddress.h

@@ -37,6 +37,11 @@ public:
     {
     }
 
+    explicit VirtualAddress(const void* address)
+        : m_address((u32)address)
+    {
+    }
+
     bool is_null() const { return m_address == 0; }
     bool is_page_aligned() const { return (m_address & 0xfff) == 0; }