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 11:40:50 -0700	[thread overview]
Message-ID: <CAHk-=wi81ujYGP0gmyy2kDke_ExL742Lo_hLepGjCa8mS81A7w@mail.gmail.com> (raw)
In-Reply-To: <20220808073232.8808-1-david@redhat.com>

On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@redhat.com> wrote:
>
> For example, a write() via /proc/self/mem to a uffd-wp-protected range has
> to fail instead of silently granting write access and bypassing the
> userspace fault handler. Note that FOLL_FORCE is not only used for debug
> access, but also triggered by applications without debug intentions, for
> example, when pinning pages via RDMA.

So this made me go "Whaa?"

I didn't even realize that the media drivers and rdma used FOLL_FORCE.

That's just completely bogus.

Why do they do that?

It seems to be completely bogus, and seems to have no actual valid
reason for it. Looking through the history, it goes back to the
original code submission in 2006, and doesn't have a mention of why.

I think the original reason was that the code didn't have pinning, so
it used "do a write" as a pin mechanism - even for reads.

IOW, I think the non-ptrace use of FOLL_FORCE should just be removed.

                 Linus