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

[David Hildenbrand]

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!

--
Thanks,

David / dhildenb