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

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Date:   Tue, 9 Aug 2022 11:48:25 -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 Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@...hat.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.

This, btw, just makes me go "uffd-wp is broken garbage" once more.

It also makes me go "if uffd-wp can disallow ptrace writes, then why
doesn't regular write protect do it"?

IOW, I don't think the patch is wrong (apart from the VM_BUG_ON's that
absolutely must go away), but I get the strong feelign that we instead
should try to get rid of FOLL_FORCE entirely.

If some other user action can stop FOLL_FORCE anyway, then why do we
support it at all?

             Linus