linux-kernel - Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse

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Date:   Sat, 9 Jan 2021 21:54:05 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:     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@...temov.name>,
        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>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Nadav Amit <nadav.amit@...il.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse

Hello,

On Sat, Jan 09, 2021 at 07:44:35PM -0500, Andrea Arcangeli wrote:
> allowing a child to corrupt memory in the parent. That's a problem
> that could happen not-maliciously too. So the scenario described

I updated the above partly quoted sentence since in the previous
version it didn't have full accuracy:

https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=fc5a76b1c14e5e6cdc64ece306fc03773662d98a

"However since a single transient GUP pin on a tail page, would elevate
the page_count for all other tail pages (unlike the mapcount which is
subpage granular), the COW page reuse inaccuracy would then cross
different vmas and the effect would happen at a distance in vma of
different processes. A single GUP pin taken on a subpage mapped in a
different process could trigger 511 false positive COWs copies in the
local process, after a fork()."

This a best effort to try to document all side effects, but it'd be
great to hear from Kirill too on the above detail to have
confirmation.

Thanks and have a great weekend everyone,
Andrea