linux-kernel - Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO

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Date:   Fri, 19 May 2023 12:20:25 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jiaqi Yan <jiaqiyan@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>,
        Huang Ying <ying.huang@...el.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Nadav Amit <namit@...are.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Shuah Khan <shuah@...nel.org>,
        ZhangPeng <zhangpeng362@...wei.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Anish Moorthy <amoorthy@...gle.com>
Subject: Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl

Hi, Jiaqi,

On Fri, May 19, 2023 at 08:04:09AM -0700, Jiaqi Yan wrote:
> I don't think CAP_ADMIN is something we can work around: a VMM must be
> a good citizen to avoid introducing any vulnerability to the host or
> guest.
> 
> On the other hand, "Userfaults allow the implementation of on-demand
> paging from userland and more generally they allow userland to take
> control of various memory page faults, something otherwise only the
> kernel code could do." [3]. I am not familiar with the UFFD internals,
> but our use case seems to match what UFFD wants to provide: without
> affecting the whole world, give a specific userspace (without
> CAP_ADMIN) the ability to handle page faults (indirectly emulate a
> HWPOISON page (in my mind I treat it as SetHWPOISON(page) +
> TestHWPOISON(page) operation in kernel's PF code)). So is it fair to
> say what Axel provided here is "provide !ADMIN somehow"?
> 
> [3]https://docs.kernel.org/admin-guide/mm/userfaultfd.html

Userfault keywords on "user", IMHO.  We don't strictly need userfault to
resolve anything regarding CAP_ADMIN problems.  MADV_DONTNEED also dosn't
need CAP_ADMIN, same to any new madvise() if we want to make it useful for
injecting poisoned ptes with !ADMIN and limit it within current->mm.

But I think you're right that userfaultfd always tried to avoid having
ADMIN and keep everything within its own scope of permissions.

So again, totally no objection on make it uffd specific for now if you guys
are all happy with it, but just to be clear that it's (to me) mostly for
avoiding another WAKE, and afaics that's not really for solving the ADMIN
issue here.

Thanks,

-- 
Peter Xu