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

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Date:   Mon, 27 Sep 2021 22:11:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <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 22:08, Nadav Amit wrote:
> 
> 
>> On Sep 27, 2021, at 10:06 AM, David Hildenbrand <david@...hat.com> wrote:
>>
>> 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!
> 
> 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