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

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

(2014/12/01 9:42), Paul Mackerras wrote:
> On Mon, Dec 01, 2014 at 09:14:40AM +0900, Yasuaki Ishimatsu wrote:
>> (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.
>
> I can get it easily enough.
>
>>> 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@samba.org>
>>
>> Do you need to backport it into -stable kernels?
>
> It does need to go to stable, yes, for 3.10 and later.
>
>>> ---
>>> 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));
>
> That would not be better, since node_online() doesn't bounds-check its
> argument.
>

Ah. You are right.

>> When allocating the memory, the node of the memory being allocated must be
>> online. But your code cannot check the condition.
>
> The following two lines:
>
>>>   	n = get_node(cachep, nodeid);
>>>   	BUG_ON(!n);
>
> effectively check that condition already, as I tried to explain in the
> commit message.

O.K. I understood.

Thansk,
Yasuaki Ishimatsu

>
> Regards,
> Paul.
>


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