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

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Date:   Tue, 4 Jun 2019 21:19:22 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Stephen Smalley <stephen.smalley@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        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 6:18 PM Stephen Smalley
<stephen.smalley@...il.com> wrote:
>
> On Tue, Jun 4, 2019 at 4:58 PM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> On Tue, Jun 4, 2019 at 1:39 PM David Howells <dhowells@...hat.com> wrote:
>> >
>> > Andy Lutomirski <luto@...nel.org> wrote:
>> >
>> > > > 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?
>> >
>> > Casey responded to you.  It's one of his requirements.
>> >
>>
>> It being a "requirement" doesn't make it okay.
>>
>> > However, the LSMs (or at least SELinux) ignore f_cred and use current_cred()
>> > when checking permissions.  See selinux_revalidate_file_permission() for
>> > example - it uses current_cred() not file->f_cred to re-evaluate the perms,
>> > and the fd might be shared between a number of processes with different creds.
>>
>> That's a bug.  It's arguably a rather severe bug.  If I ever get
>> around to writing the patch I keep thinking of that will warn if we
>> use creds from invalid contexts, it will warn.
>
>
> No, not a bug.  Working as designed. Initial validation on open, but revalidation upon read/write if something has changed since open (process SID differs from opener, inode SID has changed, policy has changed). Current subject SID should be used for the revalidation. It's a MAC vs DAC difference.
>

Can you explain how the design is valid, then?  Consider nasty cases like this:

$ sudo -u lotsofgarbage 2>/dev/whatever

It is certainly the case that drivers, fs code, and other core code
MUST NOT look at current_cred() in the context of syscalls like
open().  Jann, I, and others have found quite a few rootable bugs of
this sort.  What makes MAC special here?

I would believe there are cases where auditing write() callers makes
some sense, but anyone reading those logs needs to understand that the
creds are dubious at best.