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Subject: Re: [PATCH v4] memory_hotplug: Add a bounds check to __add_pages
To:     Alastair D'Silva <alastair@au1.ibm.com>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   David Hildenbrand <david@redhat.com>
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Date:   Fri, 27 Sep 2019 09:24:20 +0200
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On 27.09.19 08:33, Alastair D'Silva wrote:
> On Thu, 2019-09-26 at 09:46 +0200, David Hildenbrand wrote:
>> On 26.09.19 09:43, Michal Hocko wrote:
>>> On Thu 26-09-19 09:12:50, David Hildenbrand wrote:
>>>> On 26.09.19 03:34, Alastair D'Silva wrote:
>>>>> From: Alastair D'Silva <alastair@d-silva.org>
>>>>>
>>>>> On PowerPC, the address ranges allocated to OpenCAPI LPC memory
>>>>> are allocated from firmware. These address ranges may be higher
>>>>> than what older kernels permit, as we increased the maximum
>>>>> permissable address in commit 4ffe713b7587
>>>>> ("powerpc/mm: Increase the max addressable memory to 2PB"). It
>>>>> is
>>>>> possible that the addressable range may change again in the
>>>>> future.
>>>>>
>>>>> In this scenario, we end up with a bogus section returned from
>>>>> __section_nr (see the discussion on the thread "mm: Trigger bug
>>>>> on
>>>>> if a section is not found in __section_nr").
>>>>>
>>>>> Adding a check here means that we fail early and have an
>>>>> opportunity to handle the error gracefully, rather than
>>>>> rumbling
>>>>> on and potentially accessing an incorrect section.
>>>>>
>>>>> Further discussion is also on the thread ("powerpc: Perform a
>>>>> bounds
>>>>> check in arch_add_memory")
>>>>> https://urldefense.proofpoint.com/v2/url?u=http-3A__lkml.kernel.org_r_20190827052047.31547-2D1-2Dalastair-40au1.ibm.com&d=DwICaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=cT4tgeEQ0Ll3SIlZDHE5AEXyKy6uKADMtf9_Eb7-vec&m=p9ZS4kSnvF0zq81jcCFd2nYj1zfTMvfbApCtmKI2KNA&s=yif-duzz_RESW3LUyU_0kkmefRAnKWjjn_p5Et-9B2g&e= 
>>>>>
>>>>> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
>>>>> ---
>>>>>  mm/memory_hotplug.c | 20 ++++++++++++++++++++
>>>>>  1 file changed, 20 insertions(+)
>>>>>
>>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>>> index c73f09913165..212804c0f7f5 100644
>>>>> --- a/mm/memory_hotplug.c
>>>>> +++ b/mm/memory_hotplug.c
>>>>> @@ -278,6 +278,22 @@ static int check_pfn_span(unsigned long
>>>>> pfn, unsigned long nr_pages,
>>>>>  	return 0;
>>>>>  }
>>>>>  
>>>>> +static int check_hotplug_memory_addressable(unsigned long pfn,
>>>>> +					    unsigned long
>>>>> nr_pages)
>>>>> +{
>>>>> +	unsigned long max_addr = ((pfn + nr_pages) <<
>>>>> PAGE_SHIFT) - 1;
>>>>> +
>>>>> +	if (max_addr >> MAX_PHYSMEM_BITS) {
>>>>> +		WARN(1,
>>>>> +		     "Hotplugged memory exceeds maximum
>>>>> addressable address, range=%#lx-%#lx, maximum=%#lx\n",
>>>>> +		     pfn << PAGE_SHIFT, max_addr,
>>>>> +		     (1ul << (MAX_PHYSMEM_BITS + 1)) - 1);
>>>>> +		return -E2BIG;
>>>>> +	}
>>>>> +
>>>>> +	return 0;
>>>>> +}
>>>>> +
>>>>>  /*
>>>>>   * Reasonably generic function for adding memory.  It is
>>>>>   * expected that archs that support memory hotplug will
>>>>> @@ -291,6 +307,10 @@ int __ref __add_pages(int nid, unsigned
>>>>> long pfn, unsigned long nr_pages,
>>>>>  	unsigned long nr, start_sec, end_sec;
>>>>>  	struct vmem_altmap *altmap = restrictions->altmap;
>>>>>  
>>>>> +	err = check_hotplug_memory_addressable(pfn, nr_pages);
>>>>> +	if (err)
>>>>> +		return err;
>>>>> +
>>>>>  	if (altmap) {
>>>>>  		/*
>>>>>  		 * Validate altmap is within bounds of the
>>>>> total request
>>>>>
>>>>
>>>> I know Michal suggested this, but I still prefer checking early
>>>> instead
>>>> of when we're knees-deep into adding of memory.
>>>
>>> What is your concern here? Unwinding the state should be pretty
>>> straightfoward from this failure path.
>>
>> Just the general "check what you can check early without locks"
>> approach. But yeah, this series is probably not worth a v5, so I can
>> live with this change just fine :)
>>
>>
> 
> I'm going to spin a V5 anyway - where were you suggesting?

I preferred the previous places where we checked, but I think we settled
on __add_pages(). So I am fine with the changes Oscar proposed. You
might want to turn "max_addr" into a const if you feel fancy. :)

-- 

Thanks,

David / dhildenb