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

[Josh Law]

On 26 March 2026 18:02:35 GMT, "Vlastimil Babka (SUSE)" <vbabka@xxxxxxxxxx> wrote:
>On 3/6/26 23:58, Josh Law wrote:
>> From: Josh Law <objecting@xxxxxxxxxxxxx>
>> 
>> 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@xxxxxxxxxxxxx
>> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
>> Signed-off-by: Josh Law <objecting@xxxxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
>> Cc: Andrew Ballance <andrewjballance@xxxxxxxxx>
>> Cc: Liam Howlett <liam.howlett@xxxxxxxxxx>
>> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
>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@xxxxxxxxxx>
>
>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handle the fact that this patch has been withdrawn [1] however.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@xxxxxxxxxxxxx/
>
>> ---
>> Changes in v3:
>> - Included a changelog detailing modifications since v1.
>> 
>> Changes in v2:
>> - Added Link, Fixes, and Cc tags (including stable@xxxxxxxxxxxxxxx) 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++;
>>  	}
>>  }
>


Hi, this patch isn't pending!

I may come back to lib/ earlier, I'm just refining my workflow to be better, and to be honest!


Also, thanks for the ack


V/R


Josh Law