linux-kernel - RE: [PATCH] powerpc: Perform a bounds check in arch_add

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Date:   Mon, 02 Sep 2019 09:54:34 +1000
From:   "Alastair D'Silva" <alastair@....ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: RE: [PATCH] powerpc: Perform a bounds check in arch_add_memory

On Tue, 2019-08-27 at 09:13 +0200, David Hildenbrand wrote:
> On 27.08.19 08:39, Alastair D'Silva wrote:
> > On Tue, 2019-08-27 at 08:28 +0200, Michal Hocko wrote:
> > > On Tue 27-08-19 15:20:46, Alastair D'Silva wrote:
> > > > From: Alastair D'Silva <alastair@...ilva.org>
> > > > 
> > > > It is possible for firmware to allocate memory ranges outside
> > > > the range of physical memory that we support
> > > > (MAX_PHYSMEM_BITS).
> > > 
> > > Doesn't that count as a FW bug? Do you have any evidence of that
> > > in
> > > the
> > > field? Just wondering...
> > > 
> > 
> > Not outside our lab, but OpenCAPI attached LPC memory is assigned
> > addresses based on the slot/NPU it is connected to. These addresses
> > prior to:
> > 4ffe713b7587 ("powerpc/mm: Increase the max addressable memory to
> > 2PB")
> > were inaccessible and resulted in bogus sections - see our
> > discussion
> > on 'mm: Trigger bug on if a section is not found in __section_nr'.
> > Doing this check here was your suggestion :)
> > 
> > It's entirely possible that a similar problem will occur in the
> > future,
> > and it's cheap to guard against, which is why I've added this.
> > 
> 
> If you keep it here, I guess this should be wrapped by a
> WARN_ON_ONCE().
> 
> If we move it to common code (e.g., __add_pages() or add_memory()),
> then
> probably not. I can see that s390x allows to configure
> MAX_PHYSMEM_BITS,
> so the check could actually make sense.
> 

I couldn't see a nice platform indepedent way to determine the
allowable address range, but if there is, then I'll move this to the
generic code instead.

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819