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

[Daniel Vetter]

On Mon, Oct 5, 2020 at 7:58 PM Jason Gunthorpe <jgg at nvidia.com> wrote:
>
> On Mon, Oct 05, 2020 at 07:53:08PM +0200, Jan Kara wrote:
> > On Mon 05-10-20 14:38:54, Jason Gunthorpe wrote:
> > > When get_vaddr_frames() does its hacky follow_pfn() loop it should never
> > > be allowed to extract a struct page from a normal VMA. This could allow a
> > > serious use-after-free problem on any kernel memory.
> > >
> > > Restrict this to only work on VMA's with one of VM_IO | VM_PFNMAP
> > > set. This limits the use-after-free problem to only IO memory, which while
> > > still serious, is an improvement.
> > >
> > > Cc: stable at vger.kernel.org
> > > Fixes: 8025e5ddf9c1 ("[media] mm: Provide new get_vaddr_frames() helper")
> > > Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>
> > >  mm/frame_vector.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> > > index 10f82d5643b6de..26cb20544b6c37 100644
> > > +++ b/mm/frame_vector.c
> > > @@ -99,6 +99,10 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > >             if (ret >= nr_frames || start < vma->vm_end)
> > >                     break;
> > >             vma = find_vma_intersection(mm, start, start + 1);
> > > +           if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> > > +                   ret = -EINVAL;
> > > +                   goto out;
> > > +           }
> > >     } while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
> >
> > Hum, I fail to see how this helps. If vma has no VM_IO or VM_PFNMAP flag,
> > we'd exit the loop (to out: label) anyway due to the loop termination
> > condition and why not return the frames we already have? Furthermore
> > find_vma_intersection() can return NULL which would oops in your check
> > then. What am I missing?
>
> Oh, nothing, you are right. It just didn't read naturally because
> hitting the wrong kind of VMA should be an error condition :\

Afaik these mmio maps should all be VM_DONTEXPAND (or at least the
ones in drivers/gpu are all), so not sure why we need the loop here.
But maybe there's some drivers that don't set that, or have other
funny things going on with userspace piecing the mmap together, and
I'm not going to audit them all :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch