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

[Andrew Morton]

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.