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Date:   Fri, 29 Sep 2023 12:07:53 +0200
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Subject: Re: [PATCH 2/3] mmap: Fix error paths with dup_anon_vma()
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To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     maple-tree@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Jann Horn <jannh@google.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Matthew Wilcox <willy@infradead.org>, stable@vger.kernel.org
References: <20230927160746.1928098-1-Liam.Howlett@oracle.com>
 <20230927160746.1928098-3-Liam.Howlett@oracle.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20230927160746.1928098-3-Liam.Howlett@oracle.com>
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On 9/27/23 18:07, Liam R. Howlett wrote:
> When the calling function fails after the dup_anon_vma(), the
> duplication of the anon_vma is not being undone.  Add the necessary
> unlink_anon_vma() call to the error paths that are missing them.
> 
> This issue showed up during inspection of the error path in vma_merge()
> for an unrelated vma iterator issue.
> 
> Users may experience increased memory usage, which may be problematic as
> the failure would likely be caused by a low memory situation.
> 
> Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
> Cc: stable@vger.kernel.org
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  mm/mmap.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b5bc4ca9bdc4..2f0ee489db8a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -587,7 +587,7 @@ static inline void vma_complete(struct vma_prepare *vp,
>   * Returns: 0 on success.
>   */
>  static inline int dup_anon_vma(struct vm_area_struct *dst,
> -			       struct vm_area_struct *src)
> +		struct vm_area_struct *src, struct vm_area_struct **dup)
>  {
>  	/*
>  	 * Easily overlooked: when mprotect shifts the boundary, make sure the
> @@ -597,6 +597,7 @@ static inline int dup_anon_vma(struct vm_area_struct *dst,
>  	if (src->anon_vma && !dst->anon_vma) {
>  		vma_assert_write_locked(dst);
>  		dst->anon_vma = src->anon_vma;
> +		*dup = dst;
>  		return anon_vma_clone(dst, src);

So the code is simpler that way and seems current callers are fine, but
shouldn't we rather only assign *dup if the clone succeeds?

>  	}
>  
> @@ -624,6 +625,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	       unsigned long start, unsigned long end, pgoff_t pgoff,
>  	       struct vm_area_struct *next)
>  {
> +	struct vm_area_struct *anon_dup = NULL;
>  	bool remove_next = false;
>  	struct vma_prepare vp;
>  
> @@ -633,7 +635,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  
>  		remove_next = true;
>  		vma_start_write(next);
> -		ret = dup_anon_vma(vma, next);
> +		ret = dup_anon_vma(vma, next, &anon_dup);
>  		if (ret)
>  			return ret;
>  	}
> @@ -661,6 +663,8 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	return 0;
>  
>  nomem:
> +	if (anon_dup)
> +		unlink_anon_vmas(anon_dup);
>  	return -ENOMEM;
>  }
>  
> @@ -860,6 +864,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  {
>  	struct vm_area_struct *curr, *next, *res;
>  	struct vm_area_struct *vma, *adjust, *remove, *remove2;
> +	struct vm_area_struct *anon_dup = NULL;
>  	struct vma_prepare vp;
>  	pgoff_t vma_pgoff;
>  	int err = 0;
> @@ -927,18 +932,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		vma_start_write(next);
>  		remove = next;				/* case 1 */
>  		vma_end = next->vm_end;
> -		err = dup_anon_vma(prev, next);
> +		err = dup_anon_vma(prev, next, &anon_dup);
>  		if (curr) {				/* case 6 */
>  			vma_start_write(curr);
>  			remove = curr;
>  			remove2 = next;
>  			if (!next->anon_vma)
> -				err = dup_anon_vma(prev, curr);
> +				err = dup_anon_vma(prev, curr, &anon_dup);
>  		}
>  	} else if (merge_prev) {			/* case 2 */
>  		if (curr) {
>  			vma_start_write(curr);
> -			err = dup_anon_vma(prev, curr);
> +			err = dup_anon_vma(prev, curr, &anon_dup);
>  			if (end == curr->vm_end) {	/* case 7 */
>  				remove = curr;
>  			} else {			/* case 5 */
> @@ -954,7 +959,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			vma_end = addr;
>  			adjust = next;
>  			adj_start = -(prev->vm_end - addr);
> -			err = dup_anon_vma(next, prev);
> +			err = dup_anon_vma(next, prev, &anon_dup);
>  		} else {
>  			/*
>  			 * Note that cases 3 and 8 are the ONLY ones where prev
> @@ -1018,6 +1023,9 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	return res;
>  
>  prealloc_fail:
> +	if (anon_dup)
> +		unlink_anon_vmas(anon_dup);
> +
>  anon_vma_fail:
>  	if (merge_prev)
>  		vma_next(vmi);