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

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.

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>
---

(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@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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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.

> 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.

Regards,
Paul.

(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.
>