Re: [PATCH 1/3] lib/maple_tree: fix potential NULL dereference in mas_pop_node()

[Josh Law]

12 Mar 2026 20:45:32 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:

> On Thu, 12 Mar 2026 18:40:53 +0000 Josh Law <hlcj1234567@xxxxxxxxx> wrote:
>
>> If kmem_cache_alloc_from_sheaf() returns NULL (possible under
>> GFP_NOWAIT pressure), mas_pop_node() falls through to the out label
>> and dereferences the NULL pointer in memset(ret, 0, sizeof(*ret)).
>
> This is such a glaring bug that I wonder if we're missing something.
>
>> Add a WARN_ON_ONCE NULL check after the sheaf allocation to bail out
>> early, matching the existing pattern for the !mas->sheaf case above.
>>
>> Signed-off-by: Josh Law <objecting@xxxxxxxxxxxxx>
>> ---
>> lib/maple_tree.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>> index 739918e859e5..87a2ba6468ca 100644
>> --- a/lib/maple_tree.c
>> +++ b/lib/maple_tree.c
>> @@ -1063,6 +1063,8 @@ static __always_inline struct maple_node *mas_pop_node(struct ma_state *mas)
>>         return NULL;
>>
>>     ret = kmem_cache_alloc_from_sheaf(maple_node_cache, GFP_NOWAIT, mas->sheaf);
>> +   if (WARN_ON_ONCE(!ret))
>> +       return NULL;
>
> If we're going to do this then we may as well restore !__GFP_NOWARN,
> get more relevant information.
>
> But a GFP_NOWAIT allocation attempt can fail relatively easily so
> callers must be equipped to handle it - perhaps no need for any
> warning.

Well, fair enough, but WARN_ON is equivalent to a "oops! Something went wrong! We will continue anyway", NOWARN is quite bad for logging that that went wrong, usually it's BUG_ON that causes said kernel panics and that, which is a bit overkill, that's why I didn't add it, and it warns once, then bails, that's why I'm a bit on the iffy side about adding NOWARN, what's your opinion on this, do you think a NOWARN is better then warn on once?


V/R



Josh law