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From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     David Hildenbrand <david@redhat.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
Date:   Fri, 27 Sep 2019 16:33:54 +1000
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         <8e00cf16-76b9-6655-86b6-288b454d6fe5@redhat.com>
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         <10237d54-f182-be5d-1b83-3d0780d71671@redhat.com>
Organization: IBM Australia
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Subject: RE: [PATCH v4] memory_hotplug: Add a bounds check to __add_pages
Sender: linux-kernel-owner@vger.kernel.org
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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?

> -- 
> 
> Thanks,
> 
> David / dhildenb

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819