Kernel: User pointer validation should reject kernel-only addresses

We were happily allowing syscalls with pointers into kernel-only
regions (virtual address >= 0xc0000000).

This patch fixes that by only considering user regions in the current
process, and also double-checking the Region::is_user_accessible() flag
before approving an access.

Thanks to Fire30 for finding the bug! :^)

Kernel/VM/MemoryManager.cpp

@@ -592,14 +592,14 @@ bool MemoryManager::validate_user_stack(const Process& process, VirtualAddress v
 
 bool MemoryManager::validate_user_read(const Process& process, VirtualAddress vaddr) const
 {
-    auto* region = region_from_vaddr(process, vaddr);
-    return region && region->is_readable();
+    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
 {
-    auto* region = region_from_vaddr(process, vaddr);
-    return region && region->is_writable();
+    auto* region = user_region_from_vaddr(const_cast<Process&>(process), vaddr);
+    return region && region->is_user_accessible() && region->is_writable();
 }
 
 void MemoryManager::register_vmobject(VMObject& vmobject)