linux-kernel - Re: [PATCH 0/2] page_count can't be used to decide when wp_page

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Date:   Fri, 8 Jan 2021 10:31:24 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, Yu Zhao <yuzhao@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

On Fri, Jan 8, 2021 at 10:19 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Fri, Jan 08, 2021 at 12:00:36PM -0500, Andrea Arcangeli wrote:
> > > The majority cannot be converted to notifiers because they are DMA
> > > based. Every one of those is an ABI for something, and does not expect
> > > extra privilege to function. It would be a major breaking change to
> > > have pin_user_pages require some cap.
> >
> > ... what makes them safe is to be transient GUP pin and not long
> > term.
> >
> > Please note the "long term" in the underlined line.
>
> Many of them are long term, though only 50 or so have been marked
> specifically with FOLL_LONGTERM. I don't see how we can make such a
> major ABI break.
>
> Looking at it, vmsplice() is simply wrong. A long term page pin must
> use pin_user_pages(), and either FOLL_LONGTERM|FOLL_WRITE (write mode)
> FOLL_LONGTERM|FOLL_FORCE|FOLL_WRITE (read mode)

Can we just remove vmsplice() support?  We could make it do a normal
copy, thereby getting rid of a fair amount of nastiness and potential
attacks.  Even ignoring issues relating to the length of time that the
vmsplice reference is alive, we also have whatever problems could be
caused by a malicious or misguided user vmsplice()ing some memory and
then modifying it.

--Andy