linux-kernel - Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpFruHecz9NssGGFrG-RqbJFHCfRuZ7c6GbTBD4x4AU8aA@mail.gmail.com>
Date:   Tue, 17 Jan 2023 10:26:32 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

On Tue, Jan 17, 2023 at 10:12 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@...gle.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.