linux-kernel - Re: [PATCH v2] slab: Fix nodeid bounds check for non-contiguous node IDs

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Message-ID: <1417410134.16178.2.camel@concordia>
Date:	Mon, 01 Dec 2014 16:02:14 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	linuxppc-dev@...abs.org, David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2] slab: Fix nodeid bounds check for non-contiguous
 node IDs

On Mon, 2014-12-01 at 15:28 +1100, 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, like this:
...
> 
> 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 (for
> example if the node is off-line), the BUG_ON in the next line will
> catch that.

When did this break? How come we only just noticed?

Also needs:

Cc: stable@...r.kernel.org

cheers



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