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

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Message-ID: <50c2ea19-6ae8-1f42-97ef-ba5c95e40475@schaufler-ca.com>
Date:   Tue, 4 Jun 2019 13:31:37 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, 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>, casey@...aufler-ca.com
Subject: Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver
 #2]

n 6/4/2019 10:43 AM, Andy Lutomirski wrote:
> 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?

We've spent the last 18 months reeling from the implications
of what can happen when one process has the ability to snoop
on another. Introducing yet another mechanism that is trivial
to exploit is a very bad idea.

I will try to explain the problem once again. If process A
sends a signal (writes information) to process B the kernel
checks that either process A has the same UID as process B
or that process A has privilege to override that policy.
Process B is passive in this access control decision, while
process A is active. In the event delivery case, process A
does something (e.g. modifies a keyring) that generates an
event, which is then sent to process B's event buffer. Again,
A is active and B is passive. Process A must have write access
(defined by some policy) to process B's event buffer. To
implement such a policy requires A's credential, and some
information about the object (passive entity) to which the
event is being delivered. You can't just use the credential
from Process B because it is not the active entity, it is the
passive entity.

>
>> 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.