linux-kernel - Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]

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Date:   Tue, 4 Jun 2019 10:43:32 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>, raven@...maw.net,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-block@...r.kernel.org, keyrings@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]

On Tue, Jun 4, 2019 at 9:35 AM David Howells <dhowells@...hat.com> wrote:
>
>
> Hi Al,
>
> Here's a set of patches to add a general variable-length notification queue
> concept and to add sources of events for:

I asked before and didn't see a response, so I'll ask again.  Why are
you paying any attention at all to the creds that generate an event?
It seems like the resulting security model will be vary hard to
understand and probably buggy.  Can't you define a sensible model in
which only the listener creds matter?

> LSM support is included:
>
>  (1) The creds of the process that did the fput() that reduced the refcount
>      to zero are cached in the file struct.
>
>  (2) __fput() overrides the current creds with the creds from (1) whilst
>      doing the cleanup, thereby making sure that the creds seen by the
>      destruction notification generated by mntput() appears to come from
>      the last fputter.

That looks like duct tape that is, at best, likely to be very buggy.

>
>  (3) security_post_notification() is called for each queue that we might
>      want to post a notification into, thereby allowing the LSM to prevent
>      covert communications.

This seems like the wrong approach.  If an LSM wants to prevent covert
communication from, say, mount actions, then it shouldn't allow the
watch to be set up in the first place.