Re: [RFC PATCH 1/9] ntsync: Introduce the ntsync driver and character device.

[Elizabeth Figura]

On Thursday, 25 January 2024 12:55:04 CST Andy Lutomirski wrote:
> On Wed, Jan 24, 2024 at 7:42 PM Elizabeth Figura
> <zfigura@xxxxxxxxxxxxxxx> wrote:
> >
> > On Wednesday, 24 January 2024 16:56:23 CST Elizabeth Figura wrote:
> > > On Wednesday, 24 January 2024 15:26:15 CST Andy Lutomirski wrote:
> > >
> > > > On Tue, Jan 23, 2024 at 4:59 PM Elizabeth Figura
> > > > <zfigura@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > >
> > > > >
> > > > > ntsync uses a misc device as the simplest and least intrusive uAPI
> > > > > interface.
> > > >
> > > > >
> > > > >
> > > > > Each file description on the device represents an isolated NT instance,
> > > > > intended to correspond to a single NT virtual machine.
> > > >
> > > >
> > > > If I understand this text right, and if I understood the code right,
> > > > you're saying that each open instance of the device represents an
> > > > entire universe of NT synchronization objects, and no security or
> > > > isolation is possible between those objects.  For single-process use,
> > > > this seems fine.  But fork() will be a bit odd (although NT doesn't
> > > > really believe in fork, so maybe this is fine).
> > > >
> > > > Except that NT has *named* semaphores and such.  And I'm pretty sure
> > > > I've written GUI programs that use named synchronization objects (IIRC
> > > > they were events, and this was a *very* common pattern, regularly
> > > > discussed in MSDN, usenet, etc) to detect whether another instance of
> > > > the program is running.  And this all works on real Windows because
> > > > sessions have sufficiently separated namespaces, and the security all
> > > > works out about as any other security on Windows, etc.  But
> > > > implementing *that* on top of this
> > > > file-description-plus-integer-equals-object will be fundamentally
> > > > quite subject to one buggy program completely clobbering someone
> > > > else's state.
> > > >
> > > > Would it make sense and scale appropriately for an NT synchronization
> > > > *object* to be a Linux open file description?  Then SCM_RIGHTS could
> > > > pass them around, an RPC server could manage *named* objects, and
> > > > they'd generally work just like other "Object Manager" objects like,
> > > > say, files.
> > >
> > >
> > > It's a sensible concern. I think when I discussed this with Alexandre
> > > Julliard (the Wine maintainer, CC'd) the conclusion was this wasn't
> > > something we were concerned about.
> > >
> > > While the current model *does* allow for processes to arbitrarily mess
> > > with each other, accidentally or not, I think we're not concerned with
> > > the scope of that than we are about implementing a whole scheduler in
> > > user space.
> > >
> > > For one, you can't corrupt the wineserver state this way—wineserver
> > > being sort of like a dedicated process that handles many of the things
> > > that a kernel would, and so sometimes needs to set or reset events, or
> > > perform NTSYNC_IOC_KILL_MUTEX, but never relies on ntsync object state.
> > > Whereas trying to implement a scheduler in user space would involve the
> > > wineserver taking locks, and hence other processes could deadlock.
> > >
> > > For two, it's probably a lot harder to mess with that internal state
> > > accidentally.
> > >
> > > [There is also a potential problem where some broken applications
> > > create a million (literally) sync objects. Making these into files runs
> > > into NOFILE. We did specifically push distributions and systemd to
> > > increase those limits because an older solution *did* use eventfds and
> > > *did* run into those limits. Since that push was successful I don't
> > > know if this is *actually* a concern anymore, but avoiding files is
> > > probably not a bad thing either.]
> >
> > Of course, looking at it from a kernel maintainer's perspective, it wouldn't
> > be insane to do this anyway. If we at some point do start to care about cross-
> > process isolation in this way, or if another NT emulator wants to use this
> > interface and does care about cross-process isolation, it'll be necessary. At
> > least it'd make sense to make them separate files even if we don't implement
> > granular permission handling just yet.
> 
> I'm not convinced that any complexity at all beyond using individual
> files is needed for granular permission handling.  Unless something
> actually needs permission bits on different files pointing at the same
> sync object (which I believe NT supports, but it's sort of an odd
> concept and I'm not immediately convinced that anything uses it),
> merely having individual files ought to do the trick.  Handling of who
> has permission to open a given named object can live in a daemon, and
> I'd guess that Wine even already implements this.

This is mostly correct. NT has file descriptors and descriptions (the
former is called a "handle"), though unlike Unix access bits are
specific to the *descriptor* (handle). I don't know if anything uses 
it, but we do currently implement that basic functionality, so I can't
say that nothing does either.

So inasmuch as access to someone else's object is a concern, access to
your object with bits you don't have permission for could be a concern
along the same lines. However, from conversation with Alexandre I
believe it'd be fine to just implement those checks in user space.

> And keeping everything together gives me flashbacks of Windows 95 and
> Mac OS pre-X.  Sure, in principle the software wasn't malicious, but
> there was no shortage whatsoever of buggy crap out there, and systems
> were quite unstable.  Even just:
> 
> CreateSemaphore();
> fork();
> sleep a few seconds;
> exit();
> 
> seems like it could corrupt the shared namespace world.  (Obviously no
> one would ever do that, right?)
> 
> Also, handle leaks:
> 
> while(true) {
>   make a subprocess, which creates a semaphore and crashes;
> }

For whatever it's worth, this particular thing wouldn't be a concern;
Wine's "kernel" daemon already has to detect when a process dies and
close all its outstanding handles.