linux-kernel - Re: [PATCH v3] landlock: Add abstract unix socket connect restriction

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Message-ID: <CAG48ez3MBGi6i9Xcj29+pD9Lo1_hKkEtWYn-jNqvOZ_WWQZjYA@mail.gmail.com>
Date: Mon, 10 Jun 2024 23:49:21 +0200
From: Jann Horn <jannh@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Tahera Fahimi <fahimitahera@...il.com>, Günther Noack <gnoack@...gle.com>, 
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, linux-security-module@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	outreachy@...ts.linux.dev, netdev@...r.kernel.org
Subject: Re: [PATCH v3] landlock: Add abstract unix socket connect restriction

On Mon, Jun 10, 2024 at 6:36 PM Mickaël Salaün <mic@...ikod.net> wrote:
> On Fri, Jun 07, 2024 at 01:41:39PM -0600, Tahera Fahimi wrote:
> > On Fri, Jun 07, 2024 at 10:28:35AM +0200, Günther Noack wrote:
> > > Is it intentional that you are both restricting the connection and the sending
> > > with the same flag (security_unix_may_send)?  If an existing Unix Domain Socket
> > > gets passed in to a program from the outside (e.g. as stdout), shouldn't it
> > > still be possible that the program enables a Landlock policy and then still
> > > writes to it?  (Does that work?  Am I mis-reading the patch?)
>
> If a passed socket is already connected, then a write/send should work.

If I'm reading unix_dgram_sendmsg() correctly, we'll always hit
security_unix_may_send() for any UNIX socket type other than
SOCK_SEQPACKET (meaning SOCK_STREAM and SOCK_DGRAM), even if the
socket is already connected, and then we'll do the landlock check.
That's probably not the intended behavior for Landlock, unless I'm
misreading the code?

Maybe to get nice semantics it's necessary to add a parameter to
security_unix_may_send() that says whether the destination address
came from the caller or from the socket?