linux-kernel - Re: [PATCH 1/2] memfd: Fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE

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Date:   Fri, 8 Nov 2019 10:34:10 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Nicolas Geoffray <ngeoffray@...gle.com>,
        kernel-team@...roid.com, Hugh Dickins <hughd@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 1/2] memfd: Fix COW issue on MAP_PRIVATE and
 F_SEAL_FUTURE_WRITE mappings

On Fri, Nov 08, 2019 at 07:37:15AM +0100, Greg KH wrote:
> On Thu, Nov 07, 2019 at 09:06:14PM -0500, Joel Fernandes wrote:
> > On Thu, Nov 07, 2019 at 05:00:23PM -0800, Andrew Morton wrote:
> > > On Thu,  7 Nov 2019 14:53:54 -0500 "Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:
> > > 
> > > > F_SEAL_FUTURE_WRITE has unexpected behavior when used with MAP_PRIVATE:
> > > > A private mapping created after the memfd file that gets sealed with
> > > > F_SEAL_FUTURE_WRITE loses the copy-on-write at fork behavior, meaning
> > > > children and parent share the same memory, even though the mapping is
> > > > private.
> > > 
> > > That sounds fairly serious.  Should this be backported into -stable kernels?
> > 
> > Yes, it should be. The F_SEAL_FUTURE_WRITE feature was introduced in v5.1 so
> > v5.3.x stable kernels would need a backport. I can submit a backport tomorrow
> > unless we are Ok with stable automatically picking it up (I believe the
> > stable folks "auto select" fixes which should detect this is a fix since I
> > have said it is a fix in the subject line).
> 
> Never rely on "auto select" to pick up a patch for stable if you already
> know it should go to stable.  Just mark it as such, or tell stable@...r
> after the fact.

Sure, agreed.

Thanks Andrew for adding the tags!

thanks,

 - Joel