Re: [PATCH RFT v8 4/9] fork: Add shadow stack support to clone3()

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Fri, 2024-08-16 at 09:44 +0100, Catalin Marinas wrote:
> > After a token is consumed normally, it doesn't set it to zero. Instead it
> > sets
> > it to a "previous-ssp token". I don't think we actually want to do that here
> > though because it involves the old SSP, which doesn't really apply in this
> > case.
> > I don't see any problem with zero, but was there any special thinking behind
> > it?
> 
> BTW, since it's the parent setting up the shadow stack in its own
> address space before forking, I think at least the read can avoid
> access_remote_vm() and we could do it earlier, even before the new
> process is created.

Hmm. Makes sense. It's a bit racy since the parent could consume that token from
another thread, but it would be a race in any case.

> 
> > > +       if (access_remote_vm(mm, addr, &val, sizeof(val),
> > > +                            FOLL_FORCE | FOLL_WRITE) != sizeof(val))
> > > +               goto out;
> > 
> > The GUPs still seem a bit unfortunate for a couple reasons:
> >   - We could do a CMPXCHG version and are just not (I see ARM has identical
> > code
> > in gcs_consume_token()). It's not the only race like this though FWIW.
> >   - I *think* this is the only unprivileged FOLL_FORCE that can write to the
> > current process in the kernel. As is, it could be used on normal RO
> > mappings, at
> > least in a limited way. Maybe another point for the VMA check. We'd want to
> > check that it is normal shadow stack?
> >   - Lingering doubts about the wisdom of doing GUPs during task creation.
> 
> I don't like the access_remote_vm() either. In the common (practically
> only) case with CLONE_VM, the mm is actually current->mm, so no need for
> a GUP.

On the x86 side, we don't have a shadow stack access CMPXCHG. We will have to
GUP and do a normal CMPXCHG off of the direct map to handle it fully properly in
any case (CLONE_VM or not).

> 
> We could, in theory, consume this token in the parent before the child
> mm is created. The downside is that if a parent forks multiple
> processes using the same shadow stack, it will have to set the token
> each time. I'd be fine with this, that's really only for the mostly
> theoretical case where one doesn't use CLONE_VM and still want a
> separate stack and shadow stack.
> 
> > I don't think they are show stoppers, but the VMA check would be nice to
> > have in
> > the first upstream support.
> 
> Good point.