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

[Yasuaki Ishimatsu]

(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@xxxxxxxxx>
> >
> > 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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