Re: [PATCH v3] lib/maple_tree: fix swapped arguments in mas_safe_pivot() call

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/6/26 23:58, Josh Law wrote:
> From: Josh Law <objecting@objecting.org>
> 
> The call to mas_safe_pivot() in mas_wr_extend_null() has the pivot index

The function is actually mas_extend_spanning_null() ?

> and maple type arguments swapped. The function signature expects
> (mas, pivots, piv, type) but the call passes (mas, pivots, type, piv).
> 
> This causes the pivot index to be interpreted as a maple node type and
> vice versa, leading to incorrect pivot lookups. In practice, this means
> a null-extending store into a maple tree node can read the wrong pivot
> value, potentially corrupting the range tracked by the maple state. For
> a VMA maple tree, this could cause an incorrect vm_area_struct range to
> be returned during operations like mmap or munmap, leading to silent
> memory mapping corruption.
> 
> Every other mas_safe_pivot() call site in the file passes the arguments
> in the correct (piv, type) order; this is the only one with them
> reversed.
> 
> Link: https://lkml.kernel.org/r/20260306200820.2819999-1-objecting@objecting.org
> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> Signed-off-by: Josh Law <objecting@objecting.org>
> Cc: stable@vger.kernel.org
> Cc: Alice Ryhl <aliceryhl@google.com>
> Cc: Andrew Ballance <andrewjballance@gmail.com>
> Cc: Liam Howlett <liam.howlett@oracle.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

I'm not a maple tree expert but this looks obviously correct enough. So I
won't speculate on the impact of this bug, but:

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

I guess since it's old and not in mm-hotfixes, we can afford to wait for
Liam who should be back before the merge window. I'm not sure how to
handle the fact that this patch has been withdrawn [1] however.

[1] https://lore.kernel.org/all/E1A667AB-DCE4-4034-A36B-DAA458780A81@objecting.org/

> ---
> Changes in v3:
> - Included a changelog detailing modifications since v1.
> 
> Changes in v2:
> - Added Link, Fixes, and Cc tags (including stable@vger.kernel.org) to the commit message.
> - Appended Andrew Morton's Signed-off-by to expedite merging.

No!

> 
>  lib/maple_tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 5aa4c9500018..f82000821293 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3279,7 +3279,7 @@ static inline void mas_extend_spanning_null(struct ma_wr_state *l_wr_mas,
>  	    (r_mas->last < r_mas->max) &&
>  	    !mas_slot_locked(r_mas, r_wr_mas->slots, r_mas->offset + 1)) {
>  		r_mas->last = mas_safe_pivot(r_mas, r_wr_mas->pivots,
> -					     r_wr_mas->type, r_mas->offset + 1);
> +					     r_mas->offset + 1, r_wr_mas->type);
>  		r_mas->offset++;
>  	}
>  }