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

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Message-Id: <ea9e43fff6b6531af0620f9df62e015af66d4535.camel@au1.ibm.com>
Date:   Tue, 27 Aug 2019 16:39:56 +1000
From:   "Alastair D'Silva" <alastair@....ibm.com>
To:     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>,
        David Hildenbrand <david@...hat.com>,
        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 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.

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