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

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:

quoted

On Mon, 2014-12-01 at 15:28 +1100, Paul Mackerras wrote:

quoted

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:

...

quoted

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.

quoted

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