Re: [PATCH] binder: fix race between munmap() and direct reclaim

From: Todd Kjos
Date: Sat Mar 02 2019 - 11:28:00 EST

On Fri, Mar 1, 2019 at 11:57 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Mar 01, 2019 at 03:06:06PM -0800, Todd Kjos wrote:
> > An munmap() on a binder device causes binder_vma_close() to be called
> > which clears the alloc->vma pointer.
> >
> > If direct reclaim causes binder_alloc_free_page() to be called, there
> > is a race where alloc->vma is read into a local vma pointer and then
> > used later after the mm->mmap_sem is acquired. This can result in
> > calling zap_page_range() with an invalid vma which manifests as a
> > use-after-free in zap_page_range().
> >
> > The fix is to check alloc->vma after acquiring the mmap_sem (which we
> > were acquiring anyway) and skip zap_page_range() if it has changed
> > to NULL.
> >
> > Signed-off-by: Todd Kjos <tkjos@xxxxxxxxxx>
> > ---
> Any specific commit that this fixes?

No, it's been there a long time.

> And should it be marked for stable releases?

It is needed in stable (back to 4.4), but will need to be backported.
Should I post backported versions targeting the specific releases now?
I was thinking we'd wait for this one to land. I think we'll need 1
patch for 4.4/4.9 and a second one for 4.14/4.19 (and some of those
backported patches will have conflicts when merged down to android-4.X
-- I think the 4.14/4.19 version will apply to all the android
branches). Let me know how you want to handle this.

> thanks,
> greg k-h