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

[Tomasz Figa]

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.

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.

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?

Best regards,
Tomasz