Date:   Thu, 23 Dec 2021 13:54:38 +0100
From:   Jan Kara <jack@suse.cz>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jan Kara <jack@suse.cz>, David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Nadav Amit <namit@vmware.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On Wed 22-12-21 10:40:18, Linus Torvalds wrote:
> On Wed, Dec 22, 2021 at 8:08 AM Jan Kara <jack@suse.cz> wrote:
> >
> > Well, but O_DIRECT reads must use FOLL_PIN in any case because they modify
> > page data (and so we need to detect them both for COW and filesystem needs).
> 
> Well, O_DIRECT reads do, but not necessarily writes.

I agree.

> And hey, even reads have been dodgy in the past when we didn't really
> have the pinning logic - there's been a lot of users that just wanted
> it to work for their particular use-case rather than in general and in
> all situations..

Yes, but currently a malicious user can take the system down (BUG_ON) or
cause DIF/DIX failures if he is nasty and tries hard enough with O_DIRECT
reads (practically, the window is small so I haven't really seen a report
that I could trace to O_DIRECT reads but in principle the problem is the
same as with pinning & dirtying done e.g. by video capture drivers and
there we've seen these problem happen). So forcing pinning for O_DIRECT
reads is IMO mandatory.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR