Re: [PATCH 07/13] mm: close race in generic_access_phys

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Thu, Oct 8, 2020 at 2:44 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 10/7/20 9:44 AM, Daniel Vetter wrote:
> > Way back it was a reasonable assumptions that iomem mappings never
> > change the pfn range they point at. But this has changed:
> >
> > - gpu drivers dynamically manage their memory nowadays, invalidating
> >    ptes with unmap_mapping_range when buffers get moved
> >
> > - contiguous dma allocations have moved from dedicated carvetouts to
>
> s/carvetouts/carveouts/
>
> >    cma regions. This means if we miss the unmap the pfn might contain
> >    pagecache or anon memory (well anything allocated with GFP_MOVEABLE)
> >
> > - even /dev/mem now invalidates mappings when the kernel requests that
> >    iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
> >    ("/dev/mem: Revoke mappings when a driver claims the region")
>
> Thanks for putting these references into the log, it's very helpful.
> ...
> > diff --git a/mm/memory.c b/mm/memory.c
> > index fcfc4ca36eba..8d467e23b44e 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4873,28 +4873,68 @@ int follow_phys(struct vm_area_struct *vma,
> >       return ret;
> >   }
> >
> > +/**
> > + * generic_access_phys - generic implementation for iomem mmap access
> > + * @vma: the vma to access
> > + * @addr: userspace addres, not relative offset within @vma
> > + * @buf: buffer to read/write
> > + * @len: length of transfer
> > + * @write: set to FOLL_WRITE when writing, otherwise reading
> > + *
> > + * This is a generic implementation for &vm_operations_struct.access for an
> > + * iomem mapping. This callback is used by access_process_vm() when the @vma is
> > + * not page based.
> > + */
> >   int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
> >                       void *buf, int len, int write)
> >   {
> >       resource_size_t phys_addr;
> >       unsigned long prot = 0;
> >       void __iomem *maddr;
> > +     pte_t *ptep, pte;
> > +     spinlock_t *ptl;
> >       int offset = addr & (PAGE_SIZE-1);
> > +     int ret = -EINVAL;
> > +
> > +     if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > +             return -EINVAL;
> > +
> > +retry:
> > +     if (follow_pte(vma->vm_mm, addr, &ptep, &ptl))
> > +             return -EINVAL;
> > +     pte = *ptep;
> > +     pte_unmap_unlock(ptep, ptl);
> >
> > -     if (follow_phys(vma, addr, write, &prot, &phys_addr))
> > +     prot = pgprot_val(pte_pgprot(pte));
> > +     phys_addr = (resource_size_t)pte_pfn(pte) << PAGE_SHIFT;
> > +
> > +     if ((write & FOLL_WRITE) && !pte_write(pte))
> >               return -EINVAL;
> >
> >       maddr = ioremap_prot(phys_addr, PAGE_ALIGN(len + offset), prot);
> >       if (!maddr)
> >               return -ENOMEM;
> >
> > +     if (follow_pte(vma->vm_mm, addr, &ptep, &ptl))
> > +             goto out_unmap;
> > +
> > +     if (pte_same(pte, *ptep)) {
>
>
> The ioremap area is something I'm sorta new to, so a newbie question:
> is it possible for the same pte to already be there, ever? If so, we
> be stuck in an infinite loop here.  I'm sure that's not the case, but
> it's not yet obvious to me why it's impossible. Resource reservations
> maybe?

It's just buggy, it should be !pte_same. And I need to figure out how
to test this I guess.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch