Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	stable@vger.kernel.org,
	 Andrew Morton <akpm@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	 Axel Rasmussen <axelrasmussen@google.com>,
	Peter Xu <peterx@redhat.com>,  Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	 Matthew Wilcox <willy@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	John Hubbard <jhubbard@nvidia.com>,
	 Jason Gunthorpe <jgg@nvidia.com>
Subject: Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW
Date: Tue, 9 Aug 2022 13:38:30 -0700	[thread overview]
Message-ID: <CAHk-=wi_PfZ1_8pjMmRftiq1dshheTFnctEwRt8PZMGndCisdA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wh1q7ZSWhDWOyqmVawqjq55sUVkn8ASjE_b2VOcE1vFaA@mail.gmail.com>

On Tue, Aug 9, 2022 at 1:30 PM Linus Torvalds
<torvalds@linux-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