linux-kernel - Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Wed, 20 Jul 2022 23:21:20 +0000
From:   Nadav Amit <namit@...are.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
CC:     "Schaufler, Casey" <casey.schaufler@...el.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Peter Xu <peterx@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        zhangyi <yi.zhang@...wei.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained
 access control

On Jul 20, 2022, at 4:04 PM, Axel Rasmussen <axelrasmussen@...gle.com> wrote:

> ⚠ External Email
> 
> On Wed, Jul 20, 2022 at 3:16 PM Schaufler, Casey
> <casey.schaufler@...el.com> wrote:
>>> -----Original Message-----
>>> From: Axel Rasmussen <axelrasmussen@...gle.com>
>>> Sent: Tuesday, July 19, 2022 12:56 PM
>>> To: Alexander Viro <viro@...iv.linux.org.uk>; Andrew Morton
>>> <akpm@...ux-foundation.org>; Dave Hansen
>>> <dave.hansen@...ux.intel.com>; Dmitry V . Levin <ldv@...linux.org>; Gleb
>>> Fotengauer-Malinovskiy <glebfm@...linux.org>; Hugh Dickins
>>> <hughd@...gle.com>; Jan Kara <jack@...e.cz>; Jonathan Corbet
>>> <corbet@....net>; Mel Gorman <mgorman@...hsingularity.net>; Mike
>>> Kravetz <mike.kravetz@...cle.com>; Mike Rapoport <rppt@...nel.org>;
>>> Amit, Nadav <namit@...are.com>; Peter Xu <peterx@...hat.com>;
>>> Shuah Khan <shuah@...nel.org>; Suren Baghdasaryan
>>> <surenb@...gle.com>; Vlastimil Babka <vbabka@...e.cz>; zhangyi
>>> <yi.zhang@...wei.com>
>>> Cc: Axel Rasmussen <axelrasmussen@...gle.com>; linux-
>>> doc@...r.kernel.org; linux-fsdevel@...r.kernel.org; linux-
>>> kernel@...r.kernel.org; linux-mm@...ck.org; linux-
>>> kselftest@...r.kernel.org
>>> Subject: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained
>>> access control
>> 
>> I assume that leaving the LSM mailing list off of the CC is purely
>> accidental. Please, please include us in the next round.
> 
> Honestly it just hadn't occurred to me, but I'm more than happy to CC
> it on future revisions.
> 
>>> This series is based on torvalds/master.
>>> 
>>> The series is split up like so:
>>> - Patch 1 is a simple fixup which we should take in any case (even by itself).
>>> - Patches 2-6 add the feature, configurable selftest support, and docs.
>>> 
>>> Why not ...?
>>> ============
>>> 
>>> - Why not /proc/[pid]/userfaultfd? The proposed use case for this is for one
>>> process to open a userfaultfd which can intercept another process' page
>>> faults. This seems to me like exactly what CAP_SYS_PTRACE is for, though,
>>> so I
>>> think this use case can simply use a syscall without the powers
>>> CAP_SYS_PTRACE
>>> grants being "too much".
>>> 
>>> - Why not use a syscall? Access to syscalls is generally controlled by
>>> capabilities. We don't have a capability which is used for userfaultfd access
>>> without also granting more / other permissions as well, and adding a new
>>> capability was rejected [1].
>>> 
>>> - It's possible a LSM could be used to control access instead. I suspect
>>> adding a brand new one just for this would be rejected,
>> 
>> You won't know if you don't ask.
> 
> Fair enough - I wonder if MM folks (Andrew, Peter, Nadav especially)
> would find that approach more palatable than /proc/[pid]/userfaultfd?
> Would it make sense from our perspective to propose a userfaultfd- or
> MM-specific LSM for controlling access to certain features?
> 
> I remember +Andrea saying Red Hat was also interested in some kind of
> access control mechanism like this. Would one or the other approach be
> more convenient for you?

To reiterate my position - I think that /proc/[pid]/userfaultfd is very
natural and can be easily extended to support cross-process access of
userfaultfd. The necessary access controls are simple in any case. For
cross-process access, they are similar to those that are used for other
/proc/[pid]/X such as pagemap.

I have little experience with LSM and I do not know how real deployments use
them. If they are easier to deploy and people prefer them over some
pseudo-file, I cannot argue against them.