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

[Jason Gunthorpe]

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.

Jason