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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>, 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>
 <20190926013406.16133-2-alastair@au1.ibm.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <8e00cf16-76b9-6655-86b6-288b454d6fe5@redhat.com>
Date:   Thu, 26 Sep 2019 09:12:50 +0200
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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.

-- 

Thanks,

David / dhildenb