oopsable race in xen-gntdev (unsafe vma access)

[Al Viro]

	1) find_vma() is *not* safe without ->mmap_sem and its result may
very well be freed just as it's returned to caller.  IOW,
gntdev_ioctl_get_offset_for_vaddr() is racy and may end up with
dereferencing freed memory.

	2) gntdev_vma_close() is putting NULL into map->vma with only
->mmap_sem held by caller.  Things like
                if (!map->vma)
                        continue;
                if (map->vma->vm_start >= end)
                        continue;
                if (map->vma->vm_end <= start)
done with just priv->lock held are racy.

	I'm not familiar with the code, but it looks like we need to
protect gntdev_vma_close() guts with the same spinlock and probably
hold ->mmap_sem shared around the "find_vma()+get to map->{index,count}"
in the ioctl.  Or replace the logics in ioctl with search through the
list of grant_map under the same spinlock...

	Comments?
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