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

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Message-ID: <21c6a41d-3f65-6a49-f604-b75ef53d2910@redhat.com>
Date:   Mon, 27 Sep 2021 19:06:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [RFC PATCH] userfaultfd: support control over mm of remote PIDs

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