linux-kernel - Re: Why add the general notification queue and its sources

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Date:   Fri, 06 Sep 2019 15:53:38 -0400
From:   Robbie Harwood <rharwood@...hat.com>
To:     Ray Strode <rstrode@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>, raven@...maw.net,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        Christian Brauner <christian@...uner.io>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Ray\, Debarshi" <debarshi.ray@...il.com>
Subject: Re: Why add the general notification queue and its sources

Ray Strode <rstrode@...hat.com> writes:

> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>> That is *way* too specific to make for any kind of generic
>> notification mechanism.
>
> Well from my standpoint, I just don't want to have to poll... I don't
> have a strong opinion about how it looks architecturally to reach that
> goal.
>
> Ideally, at a higher level, I want the userspace api that gnome uses
> to be something like:
>
> err = krb5_cc_watch (ctx, ccache, (krb5_cc_change_fct) on_cc_change ,
> &watch_fd);
>
> then a watch_fd would get handed back and caller could poll on it. if
> it woke up poll(), caller would do
>
> krb5_cc_watch_update (ctx, ccache, watch_fd)
>
> or so and it would trigger on_cc_change to get called (or something like that).
>
> If under the hood, fd comes from opening /dev/watch_queue, and
> krb5_cc_watch_update reads from some mmap'ed buffer to decide whether
> or not to call on_cc_change, that's fine with me.
>
> If under the hood, fd comes from a pipe fd returned from some ioctl or
> syscall, and krb5_cc_watch_update reads messages directly from that fd
> to decide whether or not to call on_cc_change, that's fine with
> me. too.
>
> it could be an eventfd too, or whatever, too, just as long as its
> something I can add to poll() and don't have to intermittently poll
> ... :-)
>
>
>> And why would you do a broken big-key thing in the kernel in the
>> first place? Why don't you just have a kernel key to indirectly
>> encrypt using a key and "additional user space data". The kernel
>> should simply not take care of insane 1MB keys.
>
> 🤷 dunno.  I assume you're referencing the discussions from comment 0
> on that 2013 bug.  I wasn't involved in those discussions, I just
> chimed in after they happened trying to avoid having to add polling
> :-)
>
> I have no idea why a ticket would get that large. I assume it only is
> in weird edge cases.

Sadly they're not weird, but yes.  Kerberos tickets can get decently
large due to Microsoft's PACs (see MS-PAC and MS-KILE), which we need to
process, understand, and store for Active Directory interop.  I'm not
sure if I've personally made one over 1MB, but it could easily occur
given enough claims (MS-ADTS).

> Anyway, gnome treats the tickets as opaque blobs.  it doesn't do anything
> with them other than tell the user when they need to get refreshed...
>
> all the actual key manipulation happens from krb5 libraries.
>
> of course, one advantage of having the tickets kernel side is nfs
> could in theory access them directly, rather than up calling back to
> userspace...

Easy availability to filesystems is in fact the main theoretical
advantage of the keyring for us in krb5 (and, for whatever it's worth,
is why we're interested in namespaced keyrings for containers).  Our
privilege separation mechanism (gssproxy) is cache-type agnostic, and we
do have other credential cache types (KCM and DIR/FILE) in the
implementation.

Thanks,
--Robbie

--
Robbie Harwood
Kerberos Development Lead
Security Engineering Team
Red Hat, Inc.
Boston, MA, US

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