Date:   Tue, 24 Nov 2020 15:16:52 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: Re: [PATCH v6 09/17] media/videbuf1|2: Mark follow_pfn usage as
 unsafe

On Fri, Nov 20, 2020 at 09:23:12PM +0900, Tomasz Figa wrote:
> On Fri, Nov 20, 2020 at 9:08 PM Hans Verkuil <hverkuil@...all.nl> wrote:
> >
> > On 20/11/2020 11:51, Daniel Vetter wrote:
> > > On Fri, Nov 20, 2020 at 11:39 AM Hans Verkuil <hverkuil@...all.nl> wrote:
> > >>
> > >> On 20/11/2020 10:18, Daniel Vetter wrote:
> > >>> On Fri, Nov 20, 2020 at 9:28 AM Hans Verkuil <hverkuil@...all.nl> wrote:
> > >>>>
> > >>>> On 20/11/2020 09:06, Hans Verkuil wrote:
> > >>>>> On 19/11/2020 15:41, Daniel Vetter wrote:
> > >>>>>> The media model assumes that buffers are all preallocated, so that
> > >>>>>> when a media pipeline is running we never miss a deadline because the
> > >>>>>> buffers aren't allocated or available.
> > >>>>>>
> > >>>>>> This means we cannot fix the v4l follow_pfn usage through
> > >>>>>> mmu_notifier, without breaking how this all works. The only real fix
> > >>>>>> is to deprecate userptr support for VM_IO | VM_PFNMAP mappings and
> > >>>>>> tell everyone to cut over to dma-buf memory sharing for zerocopy.
> > >>>>>>
> > >>>>>> userptr for normal memory will keep working as-is, this only affects
> > >>>>>> the zerocopy userptr usage enabled in 50ac952d2263 ("[media]
> > >>>>>> videobuf2-dma-sg: Support io userptr operations on io memory").
> > >>>>>>
> > >>>>>> Acked-by: Tomasz Figa <tfiga@...omium.org>
> > >>>>>
> > >>>>> Acked-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> > >>>>
> > >>>> Actually, cancel this Acked-by.
> > >>>>
> > >>>> So let me see if I understand this right: VM_IO | VM_PFNMAP mappings can
> > >>>> move around. There is a mmu_notifier that can be used to be notified when
> > >>>> that happens, but that can't be used with media buffers since those buffers
> > >>>> must always be available and in the same place.
> > >>>>
> > >>>> So follow_pfn is replaced by unsafe_follow_pfn to signal that what is attempted
> > >>>> is unsafe and unreliable.
> > >>>>
> > >>>> If CONFIG_STRICT_FOLLOW_PFN is set, then unsafe_follow_pfn will fail, if it
> > >>>> is unset, then it writes a warning to the kernel log but just continues while
> > >>>> still unsafe.
> > >>>>
> > >>>> I am very much inclined to just drop VM_IO | VM_PFNMAP support in the media
> > >>>> subsystem. For vb2 there is a working alternative in the form of dmabuf, and
> > >>>> frankly for vb1 I don't care. If someone really needs this for a vb1 driver,
> > >>>> then they can do the work to convert that driver to vb2.
> > >>>>
> > >>>> I've added Mauro to the CC list and I'll ping a few more people to see what
> > >>>> they think, but in my opinion support for USERPTR + VM_IO | VM_PFNMAP
> > >>>> should just be killed off.
> > >>>>
> > >>>> If others would like to keep it, then frame_vector.c needs a comment before
> > >>>> the 'while' explaining why the unsafe_follow_pfn is there and that using
> > >>>> dmabuf is the proper alternative to use. That will make it easier for
> > >>>> developers to figure out why they see a kernel warning and what to do to
> > >>>> fix it, rather than having to dig through the git history for the reason.
> > >>>
> > >>> I'm happy to add a comment, but otherwise if you all want to ditch
> > >>> this, can we do this as a follow up on top? There's quite a bit of
> > >>> code that can be deleted and I'd like to not hold up this patch set
> > >>> here on that - it's already a fairly sprawling pain touching about 7
> > >>> different subsystems (ok only 6-ish now since the s390 patch landed).
> > >>> For the comment, is the explanation next to unsafe_follow_pfn not good
> > >>> enough?
> > >>
> > >> No, because that doesn't mention that you should use dma-buf as a replacement.
> > >> That's really the critical piece of information I'd like to see. That doesn't
> > >> belong in unsafe_follow_pfn, it needs to be in frame_vector.c since it's
> > >> vb2 specific.
> > >
> > > Ah makes sense, I'll add that.
> > >
> > >>>
> > >>> So ... can I get you to un-cancel your ack?
> > >>
> > >> Hmm, I really would like to see support for this to be dropped completely.
> > >>
> > >> How about this: just replace follow_pfn() by -EINVAL instead of unsafe_follow_pfn().
> > >>
> > >> Add a TODO comment that this code now can be cleaned up a lot. Such a clean up patch
> > >> can be added on top later, and actually that is something that I can do once this
> > >> series has landed.
> > >>
> > >> Regardless, frame_vector.c should mention dma-buf as a replacement in a comment
> > >> since I don't want users who hit this issue to have to dig through git logs
> > >> to find that dma-buf is the right approach.
> > >>
> > >> BTW, nitpick: the subject line of this patch says 'videbuf' instead of 'videobuf'.
> > >
> > > Will fix to, and next round will have the additional -EINVAL on top.
> > > Iirc Mauro was pretty clear that we can't just delete this, so I kinda
> > > don't want to get stuck in this discussion with my patches :-)
> >
> > Ah, I found that discussion for the v2 of this series.
> >
> > Yes, add that on top and we can discuss whether to Ack that -EINVAL patch or
> > not.
> >
> > I don't see why we would want to continue supporting a broken model that is
> > also a security risk, as I understand it.
> >
> > Tomasz, can you look at the discussion for this old RFC patch of mine:
> >
> > https://patchwork.linuxtv.org/project/linux-media/patch/20200221084531.576156-9-hverkuil-cisco@xs4all.nl/
> >
> > Specifically, if we just drop support for follow_pfn(), would that cause
> > problems for Chromium since that is apparently still using USERPTR for encoders?
> >
> 
> Nope, we use regular page-backed user pointers and not IO/PFNMAP ones.
> 
> By the way, for any inter-device sharing we're using DMABUF. USERPTR
> is left only in case of the data coming from the CPU, e.g. network.

Yeah Mauro wasn't too enthusiastic even about this patch here, so I think
I'll just leave it as-is. I fixed the typo in the commit message subject.
-Daniel

> 
> > Regards,
> >
> >         Hans
> >
> > > -Daniel
> > >
> > >>
> > >> Regards,
> > >>
> > >>         Hans
> > >>
> > >>>
> > >>> Thanks, Daniel
> > >>>
> > >>>>
> > >>>> Regards,
> > >>>>
> > >>>>         Hans
> > >>>>
> > >>>>>
> > >>>>> Thanks!
> > >>>>>
> > >>>>>       Hans
> > >>>>>
> > >>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> > >>>>>> Cc: Jason Gunthorpe <jgg@...pe.ca>
> > >>>>>> Cc: Kees Cook <keescook@...omium.org>
> > >>>>>> Cc: Dan Williams <dan.j.williams@...el.com>
> > >>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
> > >>>>>> Cc: John Hubbard <jhubbard@...dia.com>
> > >>>>>> Cc: Jérôme Glisse <jglisse@...hat.com>
> > >>>>>> Cc: Jan Kara <jack@...e.cz>
> > >>>>>> Cc: Dan Williams <dan.j.williams@...el.com>
> > >>>>>> Cc: linux-mm@...ck.org
> > >>>>>> Cc: linux-arm-kernel@...ts.infradead.org
> > >>>>>> Cc: linux-samsung-soc@...r.kernel.org
> > >>>>>> Cc: linux-media@...r.kernel.org
> > >>>>>> Cc: Pawel Osciak <pawel@...iak.com>
> > >>>>>> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> > >>>>>> Cc: Kyungmin Park <kyungmin.park@...sung.com>
> > >>>>>> Cc: Tomasz Figa <tfiga@...omium.org>
> > >>>>>> Cc: Laurent Dufour <ldufour@...ux.ibm.com>
> > >>>>>> Cc: Vlastimil Babka <vbabka@...e.cz>
> > >>>>>> Cc: Daniel Jordan <daniel.m.jordan@...cle.com>
> > >>>>>> Cc: Michel Lespinasse <walken@...gle.com>
> > >>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
> > >>>>>> --
> > >>>>>> v3:
> > >>>>>> - Reference the commit that enabled the zerocopy userptr use case to
> > >>>>>>   make it abundandtly clear that this patch only affects that, and not
> > >>>>>>   normal memory userptr. The old commit message already explained that
> > >>>>>>   normal memory userptr is unaffected, but I guess that was not clear
> > >>>>>>   enough.
> > >>>>>> ---
> > >>>>>>  drivers/media/common/videobuf2/frame_vector.c | 2 +-
> > >>>>>>  drivers/media/v4l2-core/videobuf-dma-contig.c | 2 +-
> > >>>>>>  2 files changed, 2 insertions(+), 2 deletions(-)
> > >>>>>>
> > >>>>>> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
> > >>>>>> index a0e65481a201..1a82ec13ea00 100644
> > >>>>>> --- a/drivers/media/common/videobuf2/frame_vector.c
> > >>>>>> +++ b/drivers/media/common/videobuf2/frame_vector.c
> > >>>>>> @@ -70,7 +70,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > >>>>>>                      break;
> > >>>>>>
> > >>>>>>              while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
> > >>>>>> -                    err = follow_pfn(vma, start, &nums[ret]);
> > >>>>>> +                    err = unsafe_follow_pfn(vma, start, &nums[ret]);
> > >>>>>>                      if (err) {
> > >>>>>>                              if (ret == 0)
> > >>>>>>                                      ret = err;
> > >>>>>> diff --git a/drivers/media/v4l2-core/videobuf-dma-contig.c b/drivers/media/v4l2-core/videobuf-dma-contig.c
> > >>>>>> index 52312ce2ba05..821c4a76ab96 100644
> > >>>>>> --- a/drivers/media/v4l2-core/videobuf-dma-contig.c
> > >>>>>> +++ b/drivers/media/v4l2-core/videobuf-dma-contig.c
> > >>>>>> @@ -183,7 +183,7 @@ static int videobuf_dma_contig_user_get(struct videobuf_dma_contig_memory *mem,
> > >>>>>>      user_address = untagged_baddr;
> > >>>>>>
> > >>>>>>      while (pages_done < (mem->size >> PAGE_SHIFT)) {
> > >>>>>> -            ret = follow_pfn(vma, user_address, &this_pfn);
> > >>>>>> +            ret = unsafe_follow_pfn(vma, user_address, &this_pfn);
> > >>>>>>              if (ret)
> > >>>>>>                      break;
> > >>>>>>
> > >>>>>>
> > >>>>>
> > >>>>
> > >>>
> > >>>
> > >>
> > >
> > >
> >

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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