Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

[Jann Horn]

On Tue, Jan 17, 2023 at 7:55 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> On Tue, Jan 17, 2023 at 10:47 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jan 17, 2023 at 10:36:42AM -0800, Suren Baghdasaryan wrote:
> > > On Tue, Jan 17, 2023 at 10:31 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote:
> > > > > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > > > > > rw_semaphore is a sizable structure of 40 bytes and consumes
> > > > > > > considerable space for each vm_area_struct. However vma_lock has
> > > > > > > two important specifics which can be used to replace rw_semaphore
> > > > > > > with a simpler structure:
> > > > > > [...]
> > > > > > >  static inline void vma_read_unlock(struct vm_area_struct *vma)
> > > > > > >  {
> > > > > > > -       up_read(&vma->vm_lock->lock);
> > > > > > > +       if (atomic_dec_and_test(&vma->vm_lock->count))
> > > > > > > +               wake_up(&vma->vm_mm->vma_writer_wait);
> > > > > > >  }
> > > > > >
> > > > > > I haven't properly reviewed this, but this bit looks like a
> > > > > > use-after-free because you're accessing the vma after dropping your
> > > > > > reference on it. You'd have to first look up the vma->vm_mm, then do
> > > > > > the atomic_dec_and_test(), and afterwards do the wake_up() without
> > > > > > touching the vma. Or alternatively wrap the whole thing in an RCU
> > > > > > read-side critical section if the VMA is freed with RCU delay.
> > > > >
> > > > > vm_lock->count does not control the lifetime of the VMA, it's a
> > > > > counter of how many readers took the lock or it's negative if the lock
> > > > > is write-locked.
> > > >
> > > > Yes, but ...
> > > >
> > > >         Task A:
> > > >         atomic_dec_and_test(&vma->vm_lock->count)
> > > >                         Task B:
> > > >                         munmap()
> > > >                         write lock
> > > >                         free VMA
> > > >                         synchronize_rcu()
> > > >                         VMA is really freed
> > > >         wake_up(&vma->vm_mm->vma_writer_wait);
> > > >
> > > > ... vma is freed.
> > > >
> > > > Now, I think this doesn't occur.  I'm pretty sure that every caller of
> > > > vma_read_unlock() is holding the RCU read lock.  But maybe we should
> > > > have that assertion?
> > >
> > > Yep, that's what this patch is doing
> > > https://lore.kernel.org/all/20230109205336.3665937-27-surenb@xxxxxxxxxx/
> > > by calling vma_assert_no_reader() from __vm_area_free().
> >
> > That's not enough though.  Task A still has a pointer to vma after it
> > has called atomic_dec_and_test(), even after vma has been freed by
> > Task B, and before Task A dereferences vma->vm_mm.
>
> Ah, I see your point now. I guess I'll have to store vma->vm_mm in a
> local variable and call mmgrab() before atomic_dec_and_test(), then
> use it in wake_up() and call mmdrop(). Is that what you are thinking?

You shouldn't need mmgrab()/mmdrop(), because whoever is calling you
for page fault handling must be keeping the mm_struct alive.