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Subject: Re: [PATCH v4] memory_hotplug: Add a bounds check to __add_pages
From:   David Hildenbrand <david@redhat.com>
To:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190926013406.16133-1-alastair@au1.ibm.com>
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Organization: Red Hat GmbH
Message-ID: <c448db33-75cc-e74d-8b41-9e6cc5071703@redhat.com>
Date:   Thu, 26 Sep 2019 09:37:31 +0200
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On 26.09.19 09:12, 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")
>> http://lkml.kernel.org/r/20190827052047.31547-1-alastair@au1.ibm.com
>>
>> 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. But as I don't have any
> power here, the code looks fine, although I consider the computations in
> check_hotplug_memory_addressable() fairly ugly.
> 

Forgot to add

Acked-by: David Hildenbrand <david@redhat.com>

:)

-- 

Thanks,

David / dhildenb