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

[Michael Ellerman <mpe@ellerman.id.au>]

On Mon, 2014-12-01 at 16:24 +1100, Paul Mackerras wrote:
> On Mon, Dec 01, 2014 at 04:02:14PM +1100, Michael Ellerman wrote:
> > 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?
> 
> Commit 14e50c6a9bc2, which went into 3.10-rc1.

OK. So a Fixes tag is nice:

Fixes: 14e50c6a9bc2 ("mm: slab: Verify the nodeid passed to ____cache_alloc_node")

> You'll only notice if you have CONFIG_SLAB=y and CONFIG_DEBUG_VM=y
> and you're running on a machine with discontiguous node IDs.

Right. And we have SLUB=y for all the defconfigs that are likely to hit that.

> > Also needs:
> > 
> > Cc: stable@vger.kernel.org
> 
> It does.  I remembered that a minute after I sent the patch.

OK. Hopefully one of the slab maintainers will be happy to add it for us when
they merge this?

cheers