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

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Message-ID: <20210107202525.GD504133@ziepe.ca>
Date:   Thu, 7 Jan 2021 16:25:25 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     linux-mm@...ck.org, 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 Thu, Jan 07, 2021 at 03:04:00PM -0500, Andrea Arcangeli wrote:

> vmsplice syscall API is insecure allowing long term GUP PINs without
> privilege.

Lots of places are relying on pin_user_pages long term pins of memory,
and cannot be converted to notifiers.

I don't think it is reasonable to just declare that insecure and
requires privileges, it is a huge ABI break.

FWIW, vhost tries to use notifiers as a replacement for GUP, and I
think it ended up quite strange and complicated. It is hard to
maintain performance when every access to the pages needs to hold some
protection against parallel invalidation.

Jason