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SubjectRe: [RFC 0/2] harden alloc_pages against bogus nid
On Wed, 1 Aug 2018 17:56:46 -0500 Jeremy Linton <jeremy.linton@arm.com> wrote:

> Hi,
>
> On 08/01/2018 04:50 PM, Andrew Morton wrote:
> > On Wed, 1 Aug 2018 15:04:16 -0500 Jeremy Linton <jeremy.linton@arm.com> wrote:
> >
> >> The thread "avoid alloc memory on offline node"
> >>
> >> https://lkml.org/lkml/2018/6/7/251
> >>
> >> Asked at one point why the kzalloc_node was crashing rather than
> >> returning memory from a valid node. The thread ended up fixing
> >> the immediate causes of the crash but left open the case of bad
> >> proximity values being in DSDT tables without corrisponding
> >> SRAT/SLIT entries as is happening on another machine.
> >>
> >> Its also easy to fix that, but we should also harden the allocator
> >> sufficiently that it doesn't crash when passed an invalid node id.
> >> There are a couple possible ways to do this, and i've attached two
> >> separate patches which individually fix that problem.
> >>
> >> The first detects the offline node before calling
> >> the new_slab code path when it becomes apparent that the allocation isn't
> >> going to succeed. The second actually hardens node_zonelist() and
> >> prepare_alloc_pages() in the face of NODE_DATA(nid) returning a NULL
> >> zonelist. This latter case happens if the node has never been initialized
> >> or is possibly out of range. There are other places (NODE_DATA &
> >> online_node) which should be checking if the node id's are > MAX_NUMNODES.
> >>
> >
> > What is it that leads to a caller requesting memory from an invalid
> > node? A race against offlining? If so then that's a lack of
> > appropriate locking, isn't it?
>
> There were a couple unrelated cases, both having to do with the PXN
> associated with a PCI port. The first case AFAIK, the domain wasn't
> really invalid if the entire SRAT was parsed and nodes created even when
> there weren't associated CPUs. The second case (a different machine) is
> simply a PXN value that is completely invalid (no associated
> SLIT/SRAT/etc entries) due to firmware making a mistake when a socket
> isn't populated.
>
> There have been a few other suggested or merged patches for the
> individual problems above, this set is just an attempt at avoiding a
> full crash if/when another similar problem happens.

Please add the above info to the changelog.

>
> >
> > I don't see a problem with emitting a warning and then selecting a
> > different node so we can keep running. But we do want that warning, so
> > we can understand the root cause and fix it?
>
> Yes, we do want to know when an invalid id is passed, i will add the
> VM_WARN in the first one.
>
> The second one I wasn't sure about as failing prepare_alloc_pages()
> generates a couple of error messages, but the system then continues
> operation.
>
> I guess my question though is which method (or both/something else?) is
> the preferred way to harden this up?

The first patch looked neater. Can we get a WARN_ON in there as well?

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