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

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Date:   Tue, 17 Jan 2023 19:11:26 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Suren Baghdasaryan <surenb@...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 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.