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

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Date:   Thu, 7 Nov 2019 21:06:14 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 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).

thanks,

 - Joel