Re: [PATCH v4] memory_hotplug: Add a bounds check to __add_pages

[Michal Hocko <mhocko@kernel.org>]

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.

-- 
Michal Hocko
SUSE Labs