Re: [PATCH v3 1/3] mmap: Fix vma_iterator in error path of vma_merge()

[Vlastimil Babka]

On 9/29/23 20:30, 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@xxxxxxxxxxxxxx/
> Closes: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@xxxxxxxxxxxxxx/
> Fixes: 18b098af2890 ("vma_merge: set vma iterator to correct position.")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  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;
>  }
>  
>  /*