linux-kernel - Re: [RFC][PATCH 00/10] Mount, FS, Block and Keyrings notifications [ver #3]

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Date:   Thu, 6 Jun 2019 14:54:27 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     David Howells <dhowells@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Al Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>, 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>,
        Paul Moore <paul@...l-moore.com>
Subject: Re: [RFC][PATCH 00/10] Mount, FS, Block and Keyrings notifications [ver #3]



> On Jun 6, 2019, at 2:17 PM, David Howells <dhowells@...hat.com> wrote:
> 
> Andy Lutomirski <luto@...nel.org> wrote:
> 
>>>> You are allowing arbitrary information flow between T and W above.  Who
>>>> cares about notifications?
>>> 
>>> I do. If Watched object is /dev/null no data flow is possible.
>>> There are many objects on a modern Linux system for which this
>>> is true. Even if it's "just a file" the existence of one path
>>> for data to flow does not justify ignoring the rules for other
>>> data paths.
>> 
>> Aha!
>> 
>> Even ignoring security, writes to things like /dev/null should
>> probably not trigger notifications to people who are watching
>> /dev/null.  (There are probably lots of things like this: /dev/zero,
>> /dev/urandom, etc.)
> 
> Even writes to /dev/null might generate access notifications; leastways,
> vfs_read() will call fsnotify_access() afterwards on success.

Hmm. I can see this being an issue, but I guess not with your patch set.

> 
> Whether or not you can set marks on open device files is another matter.
> 
>> David, are there any notification types that have this issue in your
>> patchset?  If so, is there a straightforward way to fix it?
> 
> I'm not sure what issue you're referring to specifically.  Do you mean whether
> writes to device files generate notifications?

I mean: are there cases where some action generates a notification but does not otherwise have an effect visible to the users who can receive the notification. It looks like the answer is probably “no”, which is good.

Casey, is this good enough for you, or is there still an issue?