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

[Suren Baghdasaryan]

On Thu, Nov 7, 2024 at 10:19 AM Carlos Llamas <cmllamas@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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?

Ah, I see now. The VMA is a valid one and at the address we specified
but it does not belong to the binder. Yes, then you do need this
check.