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From:   Daniel Colascione <dancol@google.com>
Date:   Fri, 9 Nov 2018 14:42:18 -0800
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Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Jann Horn <jannh@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        kernel list <linux-kernel@vger.kernel.org>,
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        Christoph Hellwig <hch@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Bruce Fields <bfields@fieldses.org>,
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        Khalid Aziz <khalid.aziz@oracle.com>, Lei.Yang@windriver.com,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>, marcandre.lureau@redhat.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Minchan Kim <minchan@kernel.org>,
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        Linux API <linux-api@vger.kernel.org>
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On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <luto@amacapital.net> wrote=
:
>> Another, more general fix might be to prevent /proc/pid/fd/N opens
>> from "upgrading" access modes. But that'd be a bigger ABI break.
>
> I think we should fix that, too.  I consider it a bug fix, not an ABI bre=
ak, personally.

Someone, somewhere is probably relying on it though, and that means
that we probably can't change it unless it's actually causing
problems.

<mumble>spacebar heating</mumble>

>>> That aside: I wonder whether a better API would be something that
>>> allows you to create a new readonly file descriptor, instead of
>>> fiddling with the writability of an existing fd.
>>
>> That doesn't work, unfortunately. The ashmem API we're replacing with
>> memfd requires file descriptor continuity. I also looked into opening
>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
>> work in the case that the old FD has already leaked to some other
>> context (e.g., another dup, SCM_RIGHTS). See
>> https://developer.android.com/ndk/reference/group/memory. We can't
>> break ASharedMemory_setProt.
>
>
> Hmm.  If we fix the general reopen bug, a way to drop write access from a=
n existing struct file would do what Android needs, right?  I don=E2=80=99t=
 know if there are general VFS issues with that.

I also proposed that. :-) Maybe it'd work best as a special case of
the perennial revoke(2) that people keep proposing. You'd be able to
selectively revoke all access or just write access.