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Subject: Re: [PATCH v4] memory_hotplug: Add a bounds check to __add_pages
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org,
        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
References: <20190926013406.16133-1-alastair@au1.ibm.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <10237d54-f182-be5d-1b83-3d0780d71671@redhat.com>
Date:   Thu, 26 Sep 2019 09:46:48 +0200
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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")
>>> 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.
> 
> 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 :)


-- 

Thanks,

David / dhildenb