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

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Message-ID: <1f2d967a-57a1-d3a3-4eb7-306b43709fee@redhat.com>
Date:   Tue, 27 Aug 2019 09:13:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alastair D'Silva <alastair@....ibm.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 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.

-- 

Thanks,

David / dhildenb