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

[Daniel Vetter]

On Mon, Oct 5, 2020 at 7:58 PM Jason Gunthorpe <jgg@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > Fixes: 8025e5ddf9c1 ("[media] mm: Provide new get_vaddr_frames() helper")
> > > Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > >  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 :\

Also follow_pfn checks for this same conditionat already too, so this
isn't really stopping anything bad from happening.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch