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Subject: Re: [PATCH v3 1/2] memory_hotplug: Add a bounds check to
 check_hotplug_memory_range()
To:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.com>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Wei Yang <richard.weiyang@gmail.com>, Qian Cai <cai@lca.pw>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Logan Gunthorpe <logang@deltatee.com>,
        Ira Weiny <ira.weiny@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190917010752.28395-1-alastair@au1.ibm.com>
 <20190917010752.28395-2-alastair@au1.ibm.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <d4fbabd0-19ed-a70e-48cc-3fdfcab44841@redhat.com>
Date:   Tue, 17 Sep 2019 09:26:54 +0200
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On 17.09.19 03:07, 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").
> 
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>  include/linux/memory_hotplug.h |  1 +
>  mm/memory_hotplug.c            | 13 ++++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index f46ea71b4ffd..bc477e98a310 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -110,6 +110,7 @@ extern void __online_page_increment_counters(struct page *page);
>  extern void __online_page_free(struct page *page);
>  
>  extern int try_online_node(int nid);
> +int check_hotplug_memory_addressable(u64 start, u64 size);
>  
>  extern int arch_add_memory(int nid, u64 start, u64 size,
>  			struct mhp_restrictions *restrictions);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c73f09913165..02cb9a74f561 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1030,6 +1030,17 @@ int try_online_node(int nid)
>  	return ret;
>  }
>  
> +int check_hotplug_memory_addressable(u64 start, u64 size)
> +{
> +#ifdef MAX_PHYSMEM_BITS
> +	if ((start + size - 1) >> MAX_PHYSMEM_BITS)
> +		return -E2BIG;
> +#endif
> +
> +	return 0;
> +}

I guess checking for address space wrapping would be overkill. This
change makes sense for architecture-independent device drivers that make
use of the add/remove memory infrastructure (e.g., virtio-mem I am
working on).

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

-- 

Thanks,

David / dhildenb