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Date:   Tue, 17 Jan 2023 18:48:00 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Jann Horn <jannh@google.com>, akpm@linux-foundation.org,
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock
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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@infradead.org> 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@google.com> wrote:
> > > >
> > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@google.com> 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@google.com/
> 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.