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

[Daniel Vetter]

On Tue, Oct 6, 2020 at 1:41 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Tue, Oct 06, 2020 at 12:43:31AM +0200, Daniel Vetter wrote:
>
> > > iow I think I can outright delete the frame vector stuff.
> >
> > Ok this doesn't work, because dma_mmap always uses a remap_pfn_range,
> > which is a VM_IO | VM_PFNMAP vma and so even if it's cma backed and
> > not a carveout, we can't get the pages.
>
> If CMA memory has struct pages it probably should be mmap'd with
> different flags, and the lifecycle of the CMA memory needs to respect
> the struct page refcount?

I guess yes and no. The problem is if there's pagecache in the cma
region, pup(FOLL_LONGTERM) needs to first migrate those pages out of
the cma range. Because all normal page allocation in cma regions must
be migratable at all times. But when you use cma as the contig
allocator (mostly with dma_alloc_coherent) and then remap that for
userspace (e.g. dma_mmap_wc), then anyone doing pup or gup should not
try to migrate anything. Also in the past these contig ranges where
generally carveouts without any struct page, changing that would break
too much I guess.

> > Plus trying to move the cma pages out of cma for FOLL_LONGTERM would
> > be kinda bad when they've been allocated as a contig block by
> > dma_alloc_coherent :-)
>
> Isn't holding a long term reference to a CMA page one of those really
> scary use-after-free security issues I've been talking about?
>
> I know nothing about CMA, so can't say too much, sorry

Uh ... yes :-/

This is actually worse than the gpu case I had in mind, where at most
you can sneak access other gpu buffers. With cma you should be able to
get at arbitrary pagecache (well anything that's GFP_MOVEABLE really).
Nice :-(
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch