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

[Suren Baghdasaryan]

On Tue, Jan 17, 2023 at 10:36 AM Jann Horn <jannh at google.com> wrote:
>
> On Tue, Jan 17, 2023 at 7:31 PM Matthew Wilcox <willy at 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 at google.com> wrote:
> > > >
> > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb at 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?
>
> I don't see that. When do_user_addr_fault() is calling
> vma_read_unlock(), there's no RCU read lock held, right?

We free VMAs using call_rcu() after removing them from VMA tree. OTOH
page fault handlers are searching for VMAs from inside RCU read
section and calling vma_read_unlock() from there, see
https://lore.kernel.org/all/20230109205336.3665937-29-surenb@google.com/.
Once we take the VMA read-lock, it ensures that it can't be
write-locked and if someone is destroying or isolating the VMA, it
needs to write-lock it first.