Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Tomasz Figa]

On Wed, Oct 7, 2020 at 4:23 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Wed, Oct 7, 2020 at 4:12 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Oct 7, 2020 at 4:09 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> > >
> > > On Wed, Oct 7, 2020 at 3:34 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Oct 7, 2020 at 3:06 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > > >
> > > > > On Wed, Oct 07, 2020 at 02:58:33PM +0200, Daniel Vetter wrote:
> > > > > > On Wed, Oct 7, 2020 at 2:48 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Wed, Oct 7, 2020 at 2:44 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On Wed, Oct 07, 2020 at 02:33:56PM +0200, Marek Szyprowski wrote:
> > > > > > > > > Well, it was in vb2_get_vma() function, but now I see that it has been
> > > > > > > > > lost in fb639eb39154 and 6690c8c78c74 some time ago...
> > > > > > > >
> > > > > > > > There is no guarentee that holding a get on the file says anthing
> > > > > > > > about the VMA. This needed to check that the file was some special
> > > > > > > > kind of file that promised the VMA layout and file lifetime are
> > > > > > > > connected.
> > > > > > > >
> > > > > > > > Also, cloning a VMA outside the mm world is just really bad. That
> > > > > > > > would screw up many assumptions the drivers make.
> > > > > > > >
> > > > > > > > If it is all obsolete I say we hide it behind a default n config
> > > > > > > > symbol and taint the kernel if anything uses it.
> > > > > > > >
> > > > > > > > Add a big comment above the follow_pfn to warn others away from this
> > > > > > > > code.
> > > > > > >
> > > > > > > Sadly it's just verbally declared as deprecated and not formally noted
> > > > > > > anyway. There are a lot of userspace applications relying on user
> > > > > > > pointer support.
> > > > > >
> > > > > > userptr can stay, it's the userptr abuse for zerocpy buffer sharing
> > > > > > which doesn't work anymore. At least without major surgery (you'd need
> > > > > > an mmu notifier to zap mappings and recreate them, and that pretty
> > > > > > much breaks the v4l model of preallocating all buffers to make sure we
> > > > > > never underflow the buffer queue). And static mappings are not coming
> > > > > > back I think, we'll go ever more into the direction of dynamic
> > > > > > mappings and moving stuff around as needed.
> > > > >
> > > > > Right, and to be clear, the last time I saw a security flaw of this
> > > > > magnitude from a subsystem badly mis-designing itself, Linus's
> > > > > knee-jerk reaction was to propose to remove the whole subsystem.
> > > > >
> > > > > Please don't take status-quo as acceptable, V4L community has to work
> > > > > to resolve this, uABI breakage or not. The follow_pfn related code
> > > > > must be compiled out of normal distro kernel builds.
> > > >
> > > > I think the userptr zero-copy hack should be able to go away indeed,
> > > > given that we now have CMA that allows having carveouts backed by
> > > > struct pages and having the memory represented as DMA-buf normally.
> > >
> > > Not sure whether there's a confusion here: dma-buf supports memory not
> > > backed by struct page.
> > >
> >
> > That's new to me. The whole API relies on sg_tables a lot, which in
> > turn rely on struct page pointers to describe the physical memory.
>
> You're not allowed to look at struct page pointers from the importer
> side, those might not be there. Which isn't the prettiest thing, but
> it works. And even if there's a struct page, you're still not allowed
> to look at it, since it's fully managed by the exporter under whatever
> rules that might need. So no touching it, ever.
>
> This is also not news, supporting this was in the design brief from
> the kickoff session 10+ years ago at some linaro connect thing (in
> Budapest iirc). And we have implementations doing that for almost as
> long merged in upstream.
>
> > > > How about the regular userptr use case, though?
> > > >
> > > > The existing code resolves the user pointer into pages by following
> > > > the get_vaddr_frames() -> frame_vector_to_pages() ->
> > > > sg_alloc_table_from_pages() / vm_map_ram() approach.
> > > > get_vaddr_frames() seems to use pin_user_pages() behind the scenes if
> > > > the vma is not an IO or a PFNMAP, falling back to follow_pfn()
> > > > otherwise.
> > >
> > > Yeah pin_user_pages is fine, it's just the VM_IO | VM_PFNMAP vma that
> > > don't work.
> >
> > Ack.
> >
> > > >
> > > > Is your intention to drop get_vaddr_frames() or we could still keep
> > > > using it and if vec->is_pfns is true:
> > > > a) if CONFIG_VIDEO_LEGACY_PFN_USERPTR is set, taint the kernel
> > > > b) otherwise just undo and fail?
> > >
> > > I'm typing that patch series (plus a pile more) right now.
> >
> > Cool, thanks!
> >
> > We also need to bring back the vma_open() that somehow disappeared
> > around 4.2, as Marek found.
>
> The vm_open isn't enough to stop the problems (it doesn't and cannot
> protect against unmap_mapping_range), I don't think keeping an
> incomplete solution around has much benefit. People who need this can
> disable CONFIG_STRICT_FOLLOW_PFN to keep things working, everyone else
> probably doesn't want these mm internals leaking into the media
> subsystem.

Okay, I think I mixed up the two different threads in the earlier
discussion. Since pin_user_pages() would atomically look up and pin
the underlying pages, we should be fine, so that vm_open() was only
needed for the bad follow_pfn path.

Best regards,
Tomasz