Re: [PATCH v2 8/8] binder: use per-vma lock in page installation

[Carlos Llamas <cmllamas@google.com>]

On Thu, Nov 07, 2024 at 10:04:23AM -0800, Suren Baghdasaryan wrote:
> On Thu, Nov 7, 2024 at 9:55 AM Carlos Llamas <cmllamas@google.com> wrote:
> > On Thu, Nov 07, 2024 at 08:16:39AM -0800, Suren Baghdasaryan wrote:
> > > On Wed, Nov 6, 2024 at 8:03 PM Carlos Llamas <cmllamas@google.com> wrote:
> > > > +static int binder_page_insert(struct binder_alloc *alloc,
> > > > +                             unsigned long addr,
> > > > +                             struct page *page)
> > > > +{
> > > > +       struct mm_struct *mm = alloc->mm;
> > > > +       struct vm_area_struct *vma;
> > > > +       int ret = -ESRCH;
> > > > +
> > > > +       if (!mmget_not_zero(mm))
> > > > +               return -ESRCH;
> > > > +
> > > > +       /* attempt per-vma lock first */
> > > > +       vma = lock_vma_under_rcu(mm, addr);
> > > > +       if (!vma)
> > > > +               goto lock_mmap;
> > > > +
> > > > +       if (binder_alloc_is_mapped(alloc))
> > >
> > > I don't think you need this check here. lock_vma_under_rcu() ensures
> > > that the VMA was not detached from the tree after locking the VMA, so
> > > if you got a VMA it's in the tree and it can't be removed (because
> > > it's locked). remove_vma()->vma_close()->vma->vm_ops->close() is
> > > called after VMA gets detached from the tree and that won't happen
> > > while VMA is locked. So, if lock_vma_under_rcu() returns a VMA,
> > > binder_alloc_is_mapped() has to always return true. A WARN_ON() check
> > > here to ensure that might be a better option.
> >
> > Yes we are guaranteed to have _a_ non-isolated vma. However, the check
> > validates that it's the _expected_ vma. IIUC, our vma could have been
> > unmapped (clearing alloc->mapped) and a _new_ unrelated vma could have
> > gotten the same address space assigned?
> 
> No, this should never happen. lock_vma_under_rcu() specifically checks
> the address range *after* it locks the VMA:
> https://elixir.bootlin.com/linux/v6.11.6/source/mm/memory.c#L6026

The scenario I'm describing is the following:

Proc A				Proc B
				mmap(addr, binder_fd)
binder_page_insert()
mmget_not_zero()
				munmap(addr)
				alloc->mapped = false;
				[...]
				// mmap other vma but same addr
				mmap(addr, other_fd)

vma = lock_vma_under_rcu()

Isn't there a chance for the vma that Proc A receives is an unrelated
vma that was placed in the same address range?