Re: [PATCH] maple_tree: Fix potential out-of-bounds access in mas_wr_end_piv()

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* Peng Zhang <zhangpeng.00@bytedance.com> [230503 23:14]:
> Access to the pivots array may be out of bounds. Fix it by changing the
> code to ensure that the index of the pivots does not go out of bounds.
> It is difficult to assess user-visible impact.

This is indeed an issue.  There isn't any user-visible impact for
current node types, since the overflow will access the slots and be
corrected in the next if clause, but it's certainly better to fix this.

The commit message is also not as descriptive as necessary, perhaps
something like:

Check the write offset end bounds before using it as the offset into the
pivot array.  This avoids a possible out-of-bounds access on the pivot
array if the write extends to the last slot in the node, in which case
the node maximum should be used as the end pivot.

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> 
> Fixes: 54a611b60590 ("Maple Tree: add new data structure")

Cc stable ?

> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> ---
>  lib/maple_tree.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 110a36479dced..5a49327444d76 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -4263,11 +4263,13 @@ static inline bool mas_wr_slot_store(struct ma_wr_state *wr_mas)
>  
>  static inline void mas_wr_end_piv(struct ma_wr_state *wr_mas)
>  {
> -	while ((wr_mas->mas->last > wr_mas->end_piv) &&
> -	       (wr_mas->offset_end < wr_mas->node_end))
> -		wr_mas->end_piv = wr_mas->pivots[++wr_mas->offset_end];
> +	while ((wr_mas->offset_end < wr_mas->node_end) &&
> +	       (wr_mas->mas->last > wr_mas->pivots[wr_mas->offset_end]))
> +		wr_mas->offset_end++;
>  
> -	if (wr_mas->mas->last > wr_mas->end_piv)
> +	if (wr_mas->offset_end < wr_mas->node_end)
> +		wr_mas->end_piv = wr_mas->pivots[wr_mas->offset_end];
> +	else
>  		wr_mas->end_piv = wr_mas->mas->max;
>  }
>  
> @@ -4424,7 +4426,6 @@ static inline void *mas_wr_store_entry(struct ma_wr_state *wr_mas)
>  	}
>  
>  	/* At this point, we are at the leaf node that needs to be altered. */
> -	wr_mas->end_piv = wr_mas->r_max;
>  	mas_wr_end_piv(wr_mas);
>  
>  	if (!wr_mas->entry)
> -- 
> 2.20.1
>