Re: [PATCH v6 09/17] media/videbuf1|2: Mark follow_pfn usage as unsafe

[Tomasz Figa <tfiga@chromium.org>]

On Fri, Nov 20, 2020 at 9:08 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> On 20/11/2020 11:51, Daniel Vetter wrote:
> > On Fri, Nov 20, 2020 at 11:39 AM Hans Verkuil <hverkuil@xs4all.nl> wrote:
> >>
> >> On 20/11/2020 10:18, Daniel Vetter wrote:
> >>> On Fri, Nov 20, 2020 at 9:28 AM Hans Verkuil <hverkuil@xs4all.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@chromium.org>
> >>>>>
> >>>>> Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.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.

> Regards,
>
>         Hans
>
> > -Daniel
> >
> >>
> >> Regards,
> >>
> >>         Hans
> >>
> >>>
> >>> Thanks, Daniel
> >>>
> >>>>
> >>>> Regards,
> >>>>
> >>>>         Hans
> >>>>
> >>>>>
> >>>>> Thanks!
> >>>>>
> >>>>>       Hans
> >>>>>
> >>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> >>>>>> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> >>>>>> Cc: Kees Cook <keescook@chromium.org>
> >>>>>> Cc: Dan Williams <dan.j.williams@intel.com>
> >>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
> >>>>>> Cc: John Hubbard <jhubbard@nvidia.com>
> >>>>>> Cc: Jérôme Glisse <jglisse@redhat.com>
> >>>>>> Cc: Jan Kara <jack@suse.cz>
> >>>>>> Cc: Dan Williams <dan.j.williams@intel.com>
> >>>>>> Cc: linux-mm@kvack.org
> >>>>>> Cc: linux-arm-kernel@lists.infradead.org
> >>>>>> Cc: linux-samsung-soc@vger.kernel.org
> >>>>>> Cc: linux-media@vger.kernel.org
> >>>>>> Cc: Pawel Osciak <pawel@osciak.com>
> >>>>>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> >>>>>> Cc: Kyungmin Park <kyungmin.park@samsung.com>
> >>>>>> Cc: Tomasz Figa <tfiga@chromium.org>
> >>>>>> Cc: Laurent Dufour <ldufour@linux.ibm.com>
> >>>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
> >>>>>> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> >>>>>> Cc: Michel Lespinasse <walken@google.com>
> >>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.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;
> >>>>>>
> >>>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >>
> >
> >
>