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

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 05, 2019 at 02:25:33PM -0400, Stephen Smalley wrote:
> On 6/5/19 1:47 PM, Andy Lutomirski wrote:
> > 
> > > On Jun 5, 2019, at 10:01 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> > > 
> > > > On 6/5/2019 9:04 AM, Andy Lutomirski wrote:
> > > > > On Wed, Jun 5, 2019 at 7:51 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > > > > > On 6/5/2019 1:41 AM, David Howells wrote:
> > > > > > Casey Schaufler <casey@schaufler-ca.com> wrote:
> > > > > > 
> > > > > > > I will try to explain the problem once again. If process A
> > > > > > > sends a signal (writes information) to process B the kernel
> > > > > > > checks that either process A has the same UID as process B
> > > > > > > or that process A has privilege to override that policy.
> > > > > > > Process B is passive in this access control decision, while
> > > > > > > process A is active. In the event delivery case, process A
> > > > > > > does something (e.g. modifies a keyring) that generates an
> > > > > > > event, which is then sent to process B's event buffer.
> > > > > > I think this might be the core sticking point here.  It looks like two
> > > > > > different situations:
> > > > > > 
> > > > > > (1) A explicitly sends event to B (eg. signalling, sendmsg, etc.)
> > > > > > 
> > > > > > (2) A implicitly and unknowingly sends event to B as a side effect of some
> > > > > >      other action (eg. B has a watch for the event A did).
> > > > > > 
> > > > > > The LSM treats them as the same: that is B must have MAC authorisation to send
> > > > > > a message to A.
> > > > > YES!
> > > > > 
> > > > > Threat is about what you can do, not what you intend to do.
> > > > > 
> > > > > And it would be really great if you put some thought into what
> > > > > a rational model would be for UID based controls, too.
> > > > > 
> > > > > > But there are problems with not sending the event:
> > > > > > 
> > > > > > (1) B's internal state is then corrupt (or, at least, unknowingly invalid).
> > > > > Then B is a badly written program.
> > > > Either I'm misunderstanding you or I strongly disagree.
> > > 
> > > A program needs to be aware of the conditions under
> > > which it gets event, *including the possibility that
> > > it may not get an event that it's not allowed*. Do you
> > > regularly write programs that go into corrupt states
> > > if an open() fails? Or where read() returns less than
> > > the amount of data you ask for?
> > 
> > I do not regularly write programs that handle read() omitting data in the middle of a TCP stream.  I also don’t write programs that wait for processes to die and need to handle the case where a child is dead, waitid() can see it, but SIGCHLD wasn’t sent because “security”.
> > 
> > > 
> > > >   If B has
> > > > authority to detect a certain action, and A has authority to perform
> > > > that action, then refusing to notify B because B is somehow missing
> > > > some special authorization to be notified by A is nuts.
> > > 
> > > You are hand-waving the notion of authority. You are assuming
> > > that if A can read X and B can read X that A can write B.
> > 
> > No, read it again please. I’m assuming that if A can *write* X and B can read X then A can send information to B.
> 
> I guess the questions here are:
> 
> 1) How do we handle recursive notification support, since we can't check
> that B can read everything below a given directory easily?  Perhaps we can
> argue that if I have watch permission to / then that implies visibility to
> everything below it but that is rather broad.

How do you handle fanotify today which I think can do this?

thanks,

greg k-h