linux-kernel - Re: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Mon, 13 Jun 2022 18:29:27 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Charan Teja Reddy <charante@...eaurora.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>, Nadav Amit <namit@...are.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-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine
 grained access control

On Mon, Jun 13, 2022 at 02:55:40PM -0700, Andrew Morton wrote:
> On Wed,  1 Jun 2022 14:09:47 -0700 Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> 
> > To achieve this, add a /dev/userfaultfd misc device. This device
> > provides an alternative to the userfaultfd(2) syscall for the creation
> > of new userfaultfds. The idea is, any userfaultfds created this way will
> > be able to handle kernel faults, without the caller having any special
> > capabilities. Access to this mechanism is instead restricted using e.g.
> > standard filesystem permissions.
> 
> The use of a /dev node isn't pretty.  Why can't this be done by
> tweaking sys_userfaultfd() or by adding a sys_userfaultfd2()?
> 
> Peter, will you be completing review of this patchset?

Sorry to not have reviewed it proactively..

I think it's because I never had a good picture/understanding of what
should be the best security model for uffd, meanwhile I am (it seems) just
seeing more and more ways to "provide a safer uffd" by different people
using different ways.. and I never had time (and probably capability too..)
to figure out the correct approach if not to accept all options provided.

I think I'll just assume the whole thing is acked already from you
generally, then I'll read at least the implementation before the end of
tomorrow.

Thanks,

-- 
Peter Xu