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

[David Hildenbrand <david@redhat.com>]

On 27.09.21 12:19, Nadav Amit wrote:
> 
> 
>> On Sep 27, 2021, at 2:29 AM, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 26.09.21 19:06, Nadav Amit wrote:
>>> From: Nadav Amit <namit@vmware.com>
>>> 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