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Date:   Wed, 1 Aug 2018 17:14:14 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Jeremy Linton <jeremy.linton@arm.com>
Cc:     linux-mm@kvack.org, cl@linux.com, penberg@kernel.org,
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        linux-kernel@vger.kernel.org
Subject: Re: [RFC 0/2] harden alloc_pages against bogus nid
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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?