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

From: Paul Mackerras <[email-masked]>
Date: Mon, 1 Dec 2014 11:42:10 +1100

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 <[email-masked]>
> 
> 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.