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Subject: Re: [PATCH] powerpc: Perform a bounds check in arch_add_memory
To:     Alastair D'Silva <alastair@au1.ibm.com>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <20190827052047.31547-1-alastair@au1.ibm.com>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Tue, 27 Aug 2019 09:13:59 +0200
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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@d-silva.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.

-- 

Thanks,

David / dhildenb