[PATCH] slab: Fix nodeid bounds check for non-contiguous node IDs

[Yasuaki Ishimatsu]

(2014/12/01 7:16), Paul Mackerras wrote:
> The bounds check for nodeid in ____cache_alloc_node gives false
> positives on machines where the node IDs are not contiguous, leading
> to a panic at boot time.  For example, on a POWER8 machine the node
> IDs are typically 0, 1, 16 and 17.  This means that num_online_nodes()
> returns 4, so when ____cache_alloc_node is called with nodeid = 16 the
> VM_BUG_ON triggers.

Do you have the call trace? If you have it, please add it in the description.

> To fix this, we instead compare the nodeid with MAX_NUMNODES, and
> additionally make sure it isn't negative (since nodeid is an int).
> The check is there mainly to protect the array dereference in the
> get_node() call in the next line, and the array being dereferenced is
> of size MAX_NUMNODES.  If the nodeid is in range but invalid, the
> BUG_ON in the next line will catch that.
>
> Signed-off-by: Paul Mackerras <paulus at samba.org>

Do you need to backport it into -stable kernels?

> ---
> diff --git a/mm/slab.c b/mm/slab.c
> index eb2b2ea..f34e053 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3076,7 +3076,7 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
>   	void *obj;
>   	int x;
>

> -	VM_BUG_ON(nodeid > num_online_nodes());
> +	VM_BUG_ON(nodeid < 0 || nodeid >= MAX_NUMNODES);

How about use:
	VM_BUG_ON(!node_online(nodeid));

When allocating the memory, the node of the memory being allocated must be
online. But your code cannot check the condition.

Thanks,
Yasuaki Ishimatsu

>   	n = get_node(cachep, nodeid);
>   	BUG_ON(!n);
>
>
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