Kernel: Always reject never-userspace addresses before checking regions

At the moment, addresses below 8MB and above 3GB are never accessible
to userspace, so just reject them without even looking at the current
process's memory regions.

Kernel/VM/MemoryManager.cpp

@@ -584,20 +584,31 @@ void MemoryManager::unquickmap_page()
     m_quickmap_in_use = false;
 }
 
+static inline bool is_user_address(VirtualAddress vaddr)
+{
+    return vaddr.get() >= (8 * MB) && vaddr.get() < 0xc0000000;
+}
+
 bool MemoryManager::validate_user_stack(const Process& process, VirtualAddress vaddr) const
 {
+    if (!is_user_address(vaddr))
+        return false;
     auto* region = user_region_from_vaddr(const_cast<Process&>(process), vaddr);
     return region && region->is_user_accessible() && region->is_stack();
 }
 
 bool MemoryManager::validate_user_read(const Process& process, VirtualAddress vaddr) const
 {
+    if (!is_user_address(vaddr))
+        return false;
     auto* region = user_region_from_vaddr(const_cast<Process&>(process), vaddr);
     return region && region->is_user_accessible() && region->is_readable();
 }
 
 bool MemoryManager::validate_user_write(const Process& process, VirtualAddress vaddr) const
 {
+    if (!is_user_address(vaddr))
+        return false;
     auto* region = user_region_from_vaddr(const_cast<Process&>(process), vaddr);
     return region && region->is_user_accessible() && region->is_writable();
 }