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Date:   Sun, 11 Oct 2020 00:11:11 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn
Message-ID: <20201010211111.GA3939@pendragon.ideasonboard.com>
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Hi Daniel,

On Fri, Oct 09, 2020 at 07:52:05PM +0200, Daniel Vetter wrote:
> On Fri, Oct 9, 2020 at 2:48 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > On Fri, Oct 09, 2020 at 02:37:23PM +0200, Mauro Carvalho Chehab wrote:
> >
> > > I'm not a mm/ expert, but, from what I understood from Daniel's patch
> > > description is that this is unsafe *only if*  __GFP_MOVABLE is used.
> >
> > No, it is unconditionally unsafe. The CMA movable mappings are
> > specific VMAs that will have bad issues here, but there are other
> > types too.
> >
> > The only way to do something at a VMA level is to have a list of OK
> > VMAs, eg because they were creatd via a special mmap helper from the
> > media subsystem.
> >
> > > Well, no drivers inside the media subsystem uses such flag, although
> > > they may rely on some infrastructure that could be using it behind
> > > the bars.
> >
> > It doesn't matter, nothing prevents the user from calling media APIs
> > on mmaps it gets from other subsystems.
> 
> I think a good first step would be to disable userptr of non struct
> page backed storage going forward for any new hw support. Even on
> existing drivers. dma-buf sharing has been around for long enough now
> that this shouldn't be a problem. Unfortunately right now this doesn't
> seem to exist, so the entire problem keeps getting perpetuated.

On the V4L2 side, I think we should disable USERPTR for any new driver,
period. That's what I've been recommended when reviewing patches for
several years already. It's a deprecated API, it should be phased out,
which starts by not allowing any new use case.

> > > If this is the case, the proper fix seems to have a GFP_NOT_MOVABLE
> > > flag that it would be denying the core mm code to set __GFP_MOVABLE.
> >
> > We can't tell from the VMA these kinds of details..
> >
> > It has to go the other direction, evey mmap that might be used as a
> > userptr here has to be found and the VMA specially created to allow
> > its use. At least that is a kernel only change, but will need people
> > with the HW to do this work.
> 
> I think the only reasonable way to keep this working is:
> - add a struct dma_buf *vma_tryget_dma_buf(struct vm_area_struct *vma);
> - add dma-buf export support to fbdev and v4l
> - roll this out everywhere we still need it.
> 
> Realistically this just isn't going to happen. And anything else just
> reimplements half of dma-buf, which is kinda pointless (you need
> minimally refcounting and some way to get at a promise of a permanent
> sg list for dma. Plus probably the vmap for kernel cpu access.
> 
> > > Please let address the issue on this way, instead of broken an
> > > userspace API that it is there since 1991.
> >
> > It has happened before :( It took 4 years for RDMA to undo the uAPI
> > breakage caused by a security fix for something that was a 15 years
> > old bug.
> 
> Yeah we have a bunch of these on the drm side too. Some of them are
> really just "you have to upgrade userspace", and there's no real fix
> for the security nightmare without that.

-- 
Regards,

Laurent Pinchart