linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Date:   Fri, 17 Dec 2021 20:50:29 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

On Fri, Dec 17, 2021 at 10:04:11PM +0100, David Hildenbrand wrote:

> > To remind all, the GUP users, like RDMA, VFIO use
> > FOLL_FORCE|FOLL_WRITE to get a 'r/o pin' specifically because of the
> 
> I heard that statement often. Can you point me at the code?
> 
> VFIO: drivers/vfio/vfio_iommu_type1.c
> 
> vaddr_get_pfns() will end up doing a 
> pin_user_pages_remote(FOLL_LONGTERM) without 
> FOLL_FORCE|FOLL_WRITE.

> Is that added automatically internally?

No, it is just that VFIO is broken in this regard. AFAIK VFIO users
rarely use the read-only mode and haven't noticed this bug yet.

You can search for FOLL_FORCE and see the drivers that do it this way:

drivers/misc/habanalabs/common/memory.c:                                 FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
drivers/infiniband/core/umem.c:         gup_flags |= FOLL_FORCE;
drivers/media/v4l2-core/videobuf-dma-sg.c:      unsigned int flags = FOLL_FORCE;
drivers/media/common/videobuf2/frame_vector.c:                            FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,

[etc]

No doubt there are others that do it right and wrong, this is badly
documented and misunderstood.

> But this series really just wants to fix the security issue as "part 1".
> Without any more breakages.

Sure, I'm just trying to understand why this example was brought up.

Jason