linux-kernel - Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL

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Message-ID: <CAHk-=wi_PfZ1_8pjMmRftiq1dshheTFnctEwRt8PZMGndCisdA@mail.gmail.com>
Date:   Tue, 9 Aug 2022 13:38:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

On Tue, Aug 9, 2022 at 1:30 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> And here we are, 30 years later, and it still does that, but it leaves
> the VM_MAYSHARE flag so that /proc/<pid>/maps can show that it's a
> shared mapping.

.. thinking about it, we end up still having some things that this helps.

For example, because we clear the VM_SHARED flags for read-only shared
mappings, they don't end up going through mapping_{un}map_writable(),
and don't update i_mmap_writable, and don't cause issues with
mapping_deny_writable() or mapping_writably_mapped().

So it ends up actually having random small semantic details due to
those almost three decades of history.

I'm sure there are other odd pieces like that.

                  Linus