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From: Andy Lutomirski <luto@kernel.org>
To: Stephen Smalley <stephen.smalley@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	David Howells <dhowells@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Casey Schaufler <casey@schaufler-ca.com>,
	raven@themaw.net, Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	linux-block@vger.kernel.org, keyrings@vger.kernel.org,
	LSM List <linux-security-module@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]
Date: Tue, 4 Jun 2019 21:19:22 -0700
Message-ID: <CALCETrU_5djawkwW-GRyHZXHwOUjaei1Cp7NEJaVFDm_bK6G3w@mail.gmail.com> (raw)
In-Reply-To: <CAB9W1A0AgMYOwGx9c-TmAt=1O6Bjsr2P3Nhd=2+QV39dgw0CrA@mail.gmail.com>

On Tue, Jun 4, 2019 at 6:18 PM Stephen Smalley
<stephen.smalley@gmail.com> wrote:
>
> On Tue, Jun 4, 2019 at 4:58 PM Andy Lutomirski <luto@kernel.org> wrote:
>>
>> On Tue, Jun 4, 2019 at 1:39 PM David Howells <dhowells@redhat.com> wrote:
>> >
>> > Andy Lutomirski <luto@kernel.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.

  parent reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-04 16:34 David Howells
2019-06-04 16:35 ` [PATCH 1/8] security: Override creds in __fput() with last fputter's creds " David Howells
2019-06-04 18:15   ` Andy Lutomirski
2019-06-04 16:35 ` [PATCH 2/8] General notification queue with user mmap()'able ring buffer " David Howells
2019-06-04 16:35 ` [PATCH 3/8] keys: Add a notification facility " David Howells
2019-06-04 16:35 ` [PATCH 4/8] vfs: Add a mount-notification " David Howells
2019-06-04 16:35 ` [PATCH 5/8] vfs: Add superblock notifications " David Howells
2019-06-04 16:36 ` [PATCH 6/8] fsinfo: Export superblock notification counter " David Howells
2019-06-04 16:36 ` [PATCH 7/8] block: Add block layer notifications " David Howells
2019-06-04 16:36 ` [PATCH 8/8] Add sample notification program " David Howells
2019-06-04 17:43 ` [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications " Andy Lutomirski
2019-06-04 20:31   ` Casey Schaufler
2019-06-04 21:05     ` Andy Lutomirski
2019-06-04 22:03       ` Casey Schaufler
2019-06-05  8:41   ` David Howells
2019-06-05 14:50     ` Casey Schaufler
2019-06-05 16:04       ` Andy Lutomirski
2019-06-05 17:01         ` Casey Schaufler
2019-06-05 17:47           ` Andy Lutomirski
2019-06-05 18:12             ` Casey Schaufler
2019-06-05 18:25             ` Stephen Smalley
2019-06-05 19:28               ` Greg KH
2019-06-05 21:01                 ` Stephen Smalley
2019-06-05 16:56     ` Rational model for UID based controls David Howells
2019-06-05 17:40       ` Casey Schaufler
2019-06-05 21:06       ` David Howells
2019-06-05 17:21     ` [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2] David Howells
2019-06-04 20:39 ` David Howells
2019-06-04 20:57   ` Andy Lutomirski
     [not found]     ` <CAB9W1A0AgMYOwGx9c-TmAt=1O6Bjsr2P3Nhd=2+QV39dgw0CrA@mail.gmail.com>
2019-06-05  4:19       ` Andy Lutomirski [this message]
2019-06-05 13:47         ` Stephen Smalley
2019-06-04 21:11   ` Casey Schaufler

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