linux-kernel - Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE

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Message-Id: <F8A6A5DC-3BA0-43BD-B7EC-EDE199B33A02@amacapital.net>
Date:   Fri, 9 Nov 2018 15:14:02 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        John Reck <jreck@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bruce Fields <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>, Lei.Yang@...driver.com,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>, marcandre.lureau@...hat.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Minchan Kim <minchan@...nel.org>,
        Shuah Khan <shuah@...nel.org>, valdis.kletnieks@...edu,
        Hugh Dickins <hughd@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd



> On Nov 9, 2018, at 2:42 PM, Daniel Colascione <dancol@...gle.com> wrote:
> 
> On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <luto@...capital.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 break, 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>

I think it has caused problems in the past. It’s certainly extremely surprising behavior.  I’d say it should be fixed and, if needed, a sysctl to unfix it might be okay.

> 
>>>> 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 an existing struct file would do what Android needs, right?  I don’t 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.

Sounds good to me, modulo possible races, but that shouldn’t be too hard to deal with.