Re: [RFC PATCH 0/1] lsm: Add hook unix_path_connect

[Justin Suess <utilityemal77@gmail.com>]

On 1/7/26 02:33, Kuniyuki Iwashima wrote:
> +VFS maintainers
>
> On Mon, Jan 5, 2026 at 3:04 AM Günther Noack <gnoack@google.com> wrote:
>> Hello!
>>
>> On Sun, Jan 04, 2026 at 11:46:46PM -0800, Kuniyuki Iwashima wrote:
>>> On Wed, Dec 31, 2025 at 1:33 PM Justin Suess <utilityemal77@gmail.com> wrote:
>>>> Motivation
>>>> ---
>>>>
>>>> For AF_UNIX sockets bound to a filesystem path (aka named sockets), one
>>>> identifying object from a policy perspective is the path passed to
>>>> connect(2). However, this operation currently restricts LSMs that rely
>>>> on VFS-based mediation, because the pathname resolved during connect()
>>>> is not preserved in a form visible to existing hooks before connection
>>>> establishment.
>>> Why can't LSM use unix_sk(other)->path in security_unix_stream_connect()
>>> and security_unix_may_send() ?
>> Thanks for bringing it up!
>>
>> That path is set by the process that acts as the listening side for
>> the socket.  The listening and the connecting process might not live
>> in the same mount namespace, and in that case, it would not match the
>> path which is passed by the client in the struct sockaddr_un.
> Thanks for the explanation !
>
> So basically what you need is resolving unix_sk(sk)->addr.name
> by kern_path() and comparing its d_backing_inode(path.dentry)
> with d_backing_inode (unix_sk(sk)->path.dendtry).
>
> If the new hook is only used by Landlock, I'd prefer doing that on
> the existing connect() hooks.
I see. Did you have a particular hook in mind to extend?

One complication I see is whatever hook this gets added to
would also need CONFIG_SECURITY_PATH, since logically this restriction
would fall under it:

From security/Kconfig:

config SECURITY_PATH
    bool "Security hooks for pathname based access control"
    depends on SECURITY
    help
      This enables the security hooks for pathname based access control.
      If enabled, a security module can use these hooks to
      implement pathname based access controls.
      If you are unsure how to answer this question, answer N.

config SECURITY_NETWORK
    bool "Socket and Networking Security Hooks"
    depends on SECURITY
    help
      This enables the socket and networking security hooks.
      If enabled, a security module can use these hooks to
      implement socket and networking access controls.
      If you are unsure how to answer this question, answer N.

Logically, this type of access control falls under both categories, so must be
gated by both features. No existing LSM hooks are gated by both afaik, so
there is not really an existing logical place to extend an existing hook without
changing what features are required to be enabled for existing users.

I do see more uses for this hook that just landlock, bpf lsm hooks
or other non-labeling LSMs like apparmor or TOMOYO could take advantage
of this as well.

Günther did you have anything to add?

>> For more details, see
>> https://lore.kernel.org/all/20260101134102.25938-1-gnoack3000@gmail.com/
>> and
>> https://github.com/landlock-lsm/linux/issues/36#issuecomment-2950632277
>>
>> Justin: Maybe we could add that reasoning to the cover letter in the
>> next version of the patch?
>>
>> –Günther