linux-kernel - Re: [PATCH v3 1/3] mmap: Fix vma_iterator in error path of vma

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Message-ID: <f3193ba5-32de-4f61-9b36-07373a21d751@lucifer.local>
Date:   Fri, 29 Sep 2023 23:21:36 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH v3 1/3] mmap: Fix vma_iterator in error path of
 vma_merge()

On Fri, Sep 29, 2023 at 02:30:39PM -0400, Liam R. Howlett wrote:
> During the error path, the vma iterator may not be correctly positioned
> or set to the correct range.  Undo the vma_prev() call by resetting to
> the passed in address.  Re-walking to the same range will fix the range
> to the area previously passed in.
>
> Users would notice increased cycles as vma_merge() would be called an
> extra time with vma == prev, and thus would fail to merge and return.
>
> Link: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
> Closes: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
> Fixes: 18b098af2890 ("vma_merge: set vma iterator to correct position.")
> Cc: stable@...r.kernel.org
> Cc: Jann Horn <jannh@...gle.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b56a7f0c9f85..acb7dea49e23 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -975,7 +975,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>
>  	/* Error in anon_vma clone. */
>  	if (err)
> -		return NULL;
> +		goto anon_vma_fail;
>
>  	if (vma_start < vma->vm_start || vma_end > vma->vm_end)
>  		vma_expanded = true;
> @@ -988,7 +988,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	}
>
>  	if (vma_iter_prealloc(vmi, vma))
> -		return NULL;
> +		goto prealloc_fail;
>
>  	init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
>  	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
> @@ -1016,6 +1016,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	vma_complete(&vp, vmi, mm);
>  	khugepaged_enter_vma(res, vm_flags);
>  	return res;
> +
> +prealloc_fail:
> +anon_vma_fail:
> +	vma_iter_set(vmi, addr);
> +	vma_iter_load(vmi);
> +	return NULL;
>  }
>
>  /*
> --
> 2.40.1
>
Looks good to me,

Reviewed-by: Lorenzo Stoakes <lstoakes@...il.com>