linux-kernel - Re: [RFC v3 7/7] shm: isolate pinned pages when sealing files

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Message-ID: <CALCETrWaUsq_D2Z1PwbbwQQWKrnsWTLOdUR6bqPuedi8ZHgvEQ@mail.gmail.com>
Date:	Fri, 13 Jun 2014 08:06:31 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>
Subject: Re: [RFC v3 7/7] shm: isolate pinned pages when sealing files

On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrmann@...il.com> wrote:
> When setting SEAL_WRITE, we must make sure nobody has a writable reference
> to the pages (via GUP or similar). We currently check references and wait
> some time for them to be dropped. This, however, might fail for several
> reasons, including:
>  - the page is pinned for longer than we wait
>  - while we wait, someone takes an already pinned page for read-access
>
> Therefore, this patch introduces page-isolation. When sealing a file with
> SEAL_WRITE, we copy all pages that have an elevated ref-count. The newpage
> is put in place atomically, the old page is detached and left alone. It
> will get reclaimed once the last external user dropped it.
>
> Signed-off-by: David Herrmann <dh.herrmann@...il.com>

Won't this have unexpected effects?

Thread 1:  start read into mapping backed by fd

Thread 2:  SEAL_WRITE

Thread 1: read finishes.  now the page doesn't match the sealed page

Is this okay?  Or am I missing something?

Are there really things that keep unnecessary writable pins around?

--Andy
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