Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

Kirill A. Shutemov
2018-07-16T14:04:40Z

On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> >
> > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > >
> > > False-positive vma_is_anonymous() may lead to crashes:
> > >
> > > ...
> > >
> > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > on it being NULL.
> > >
> > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> >
> > Is there a smaller, simpler fix which we can use for backporting
> > purposes and save the larger rework for development kernels?
>
> Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> for all users who do not initialize it in their mmap callbacks?
> Basically have a sanity check&fixup in call_mmap?

As I said, there's a corner case of MAP_PRIVATE of /dev/zero. It has to
produce anonymous VMA, but in map_region() we cannot distinguish it from
broken ->mmap handler.

See my attempt

6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")

and it's revert

28c553d0aa0a ("revert "mm: make sure all file VMAs have ->vm_ops set"")

--
Kirill A. Shutemov