Kernel: Reject create links on paths that were not unveiled as writable

This solves one of the security issues being mentioned in issue SerenityOS#15996.
We simply don't allow creating hardlinks on paths that were not unveiled
as writable to prevent possible bypass on a certain path that was
unveiled as non-writable.

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -723,7 +723,9 @@ static bool hard_link_allowed(Credentials const& credentials, Inode const& inode
 
 ErrorOr<void> VirtualFileSystem::link(Credentials const& credentials, StringView old_path, StringView new_path, Custody& base)
 {
- auto old_custody = TRY(resolve_path(credentials, old_path, base));
+ // NOTE: To prevent unveil bypass by creating an hardlink after unveiling a path as read-only,
+ // check that if write permission is allowed by the veil info on the old_path.
+ auto old_custody = TRY(resolve_path(credentials, old_path, base, nullptr, O_RDWR));
  auto& old_inode = old_custody->inode();
 
  RefPtr<Custody> parent_custody;