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Date:   Sat, 9 Jan 2021 21:54:05 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, Yu Zhao <yuzhao@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse
Message-ID: <X/psTXZp9xNgX3gE@redhat.com>
References: <20210110004435.26382-1-aarcange@redhat.com>
 <20210110004435.26382-2-aarcange@redhat.com>
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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