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Subject: Re: [PATCH] powerpc: Perform a bounds check in arch_add_memory
From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Wed, 04 Sep 2019 15:25:55 +1000
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On Mon, 2019-09-02 at 09:28 +0200, David Hildenbrand wrote:
> On 02.09.19 01:54, Alastair D'Silva wrote:
> > On Tue, 2019-08-27 at 09:13 +0200, David Hildenbrand wrote:
> > > On 27.08.19 08:39, Alastair D'Silva wrote:
> > > > On Tue, 2019-08-27 at 08:28 +0200, Michal Hocko wrote:
> > > > > On Tue 27-08-19 15:20:46, Alastair D'Silva wrote:
> > > > > > From: Alastair D'Silva <alastair@d-silva.org>
> > > > > > 
> > > > > > It is possible for firmware to allocate memory ranges
> > > > > > outside
> > > > > > the range of physical memory that we support
> > > > > > (MAX_PHYSMEM_BITS).
> > > > > 
> > > > > Doesn't that count as a FW bug? Do you have any evidence of
> > > > > that
> > > > > in
> > > > > the
> > > > > field? Just wondering...
> > > > > 
> > > > 
> > > > Not outside our lab, but OpenCAPI attached LPC memory is
> > > > assigned
> > > > addresses based on the slot/NPU it is connected to. These
> > > > addresses
> > > > prior to:
> > > > 4ffe713b7587 ("powerpc/mm: Increase the max addressable memory
> > > > to
> > > > 2PB")
> > > > were inaccessible and resulted in bogus sections - see our
> > > > discussion
> > > > on 'mm: Trigger bug on if a section is not found in
> > > > __section_nr'.
> > > > Doing this check here was your suggestion :)
> > > > 
> > > > It's entirely possible that a similar problem will occur in the
> > > > future,
> > > > and it's cheap to guard against, which is why I've added this.
> > > > 
> > > 
> > > If you keep it here, I guess this should be wrapped by a
> > > WARN_ON_ONCE().
> > > 
> > > If we move it to common code (e.g., __add_pages() or
> > > add_memory()),
> > > then
> > > probably not. I can see that s390x allows to configure
> > > MAX_PHYSMEM_BITS,
> > > so the check could actually make sense.
> > > 
> > 
> > I couldn't see a nice platform indepedent way to determine the
> > allowable address range, but if there is, then I'll move this to
> > the
> > generic code instead.
> > 
> 
> At least on the !ZONE_DEVICE path we have
> 
> __add_memory() -> register_memory_resource() ...
> 
> return ERR_PTR(-E2BIG);
> 
> 
> I was thinking about something like
> 
> int add_pages()
> {
> 	if ((start + size - 1) >> MAX_PHYSMEM_BITS)
> 		return -E2BIG;	
> 
> 	return arch_add_memory(...)
> }
> 
> And switching users of arch_add_memory() to add_pages(). However, x86
> already has an add_pages() function, so that would need some more
> thought.
> 
> Maybe simply renaming the existing add_pages() to arch_add_pages().
> 
> add_pages(): Create virtual mapping
> __add_pages(): Don't create virtual mapping
> 
> arch_add_memory(): Arch backend for add_pages()
> arch_add_pages(): Arch backend for __add_pages()
> 
> It would be even more consistent if we would have arch_add_pages()
> vs.
> __arch_add_pages().

Looking a bit further, I think a good course of action would be to add
the check to memory_hotplug.c:check_hotplug_memory_range().

This would be the least invasive, and could check both
MAX_POSSIBLE_PHYSMEM_BITS and MAX_PHYSMEM_BITS.

With that in mind, we can drop this patch.

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