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

From: Andy Lutomirski
Date: Wed Jun 05 2019 - 00:23:36 EST

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