Re: [RFC PATCH] userfaultfd: support control over mm of remote PIDs

[David Hildenbrand]

On 27.09.21 22:08, Nadav Amit wrote:
> > On Sep 27, 2021, at 10:06 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 27.09.21 12:19, Nadav Amit wrote:
> > > > On Sep 27, 2021, at 2:29 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > >
> > > > On 26.09.21 19:06, Nadav Amit wrote:
> > > > > From: Nadav Amit <namit@xxxxxxxxxx>
> > > > > Non-cooperative mode is useful but only for forked processes.
> > > > > Userfaultfd can be useful to monitor, debug and manage memory of remote
> > > > > processes.
> > > > > To support this mode, add a new flag, UFFD_REMOTE_PID, and an optional
> > > > > second argument to the userfaultfd syscall. When the flag is set, the
> > > > > second argument is assumed to be the PID of the process that is to be
> > > > > monitored. Otherwise the flag is ignored.
> > > > > The syscall enforces that the caller has CAP_SYS_PTRACE to prevent
> > > > > misuse of this feature.
> > > >
> > > > What supposed to happen if the target process intents to use uffd itself?
> > > Thanks for the quick response.
> > > First, sorry that I mistakenly dropped the changes to userfaultfd.h
> > > that define UFFD_REMOTE_PID.
> >
> > Didn't even notice it :)
> >
> > > As for your question: there are standard ways to deal with such cases,
> > > similarly to when a debugged program wants to use PTRACE. One way is
> > > to block the userfaultfd syscall, using seccomp. Another way is to do
> > > chaining using ptrace (although using ptrace for anything is
> > > challenging).
> > > It is also possible to add tailor something specific to userfaultfd,
> > > but I think seccomp is a good enough solution. I am open to suggestions.
> >
> > If we have something already in place to handle PTRACE, we'd better reuse what's already there. Thanks!
>
> Just to ensure we are on the same page: I meant that this is usually
> left for the user application to handle. The 2 basic solutions are to
> not expose userfaultfd to the monitored process (easy using seccomp)
> or to chain the two monitors (hard using ptrace).

Yes, and I agree that the first approach then makes sense. Chaining 
might be way to complicated to support.

As long as the kernel will continue working when a second one tries to 
register (which I think is the case), that should be good enough.

> Since ptrace is hard, in theory we can have facilities to “hijack”
> a context and “inject” uffd event to another monitor. I just think
> it is a total overkill at this stage.

Agreed


--
Thanks,

David / dhildenb