Re: [PATCH 1/1] mm: fix a lockdep issue in vma_assert_write_locked

[Suren Baghdasaryan]

On Wed, Jul 12, 2023 at 12:56 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> __is_vma_write_locked() can be used only when mmap_lock is write-locked
> to guarantee vm_lock_seq and mm_lock_seq stability during the check.
> Therefore it asserts this condition before further checks. Because of
> that it can't be used unless the user expects the mmap_lock to be
> write-locked. vma_assert_locked() can't assume this before ensuring
> that VMA is not read-locked.
> Change the order of the checks in vma_assert_locked() to check if the
> VMA is read-locked first and only then assert if it's not write-locked.
>
> Fixes: 50b88b63e3e4 ("mm: handle userfaults under VMA lock")
> Reported-by: Liam R. Howlett <liam.howlett@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20230712022620.3yytbdh24b7i4zrn@revolver/
> Reported-by: syzbot+339b02f826caafd5f7a8@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/0000000000002db68f05ffb791bc@xxxxxxxxxx/
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Should have mentioned that this patch is for mm-unstable.

> ---
>  include/linux/mm.h | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9687b48dfb1b..e3b022a66343 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -668,6 +668,7 @@ static inline void vma_end_read(struct vm_area_struct *vma)
>         rcu_read_unlock();
>  }
>
> +/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
>  static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
>  {
>         mmap_assert_write_locked(vma->vm_mm);
> @@ -707,22 +708,17 @@ static inline bool vma_try_start_write(struct vm_area_struct *vma)
>         return true;
>  }
>
> -static inline void vma_assert_locked(struct vm_area_struct *vma)
> +static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>  {
>         int mm_lock_seq;
>
> -       if (__is_vma_write_locked(vma, &mm_lock_seq))
> -               return;
> -
> -       lockdep_assert_held(&vma->vm_lock->lock);
> -       VM_BUG_ON_VMA(!rwsem_is_locked(&vma->vm_lock->lock), vma);
> +       VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
>  }
>
> -static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> +static inline void vma_assert_locked(struct vm_area_struct *vma)
>  {
> -       int mm_lock_seq;
> -
> -       VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
> +       if (!rwsem_is_locked(&vma->vm_lock->lock))
> +               vma_assert_write_locked(vma);
>  }
>
>  static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
> --
> 2.41.0.455.g037347b96a-goog
>