Subject: Re: [PATCH] mm/sparse: Fix kernel crash with pfn_section_valid check

From: Baoquan He <[email-masked]>
Date: Wed, 25 Mar 2020 16:36:37 +0800

On 03/25/20 at 01:42pm, Aneesh Kumar K.V wrote:
> On 3/25/20 1:07 PM, Baoquan He wrote:
> > On 03/25/20 at 03:06pm, Baoquan He wrote:
> > > On 03/25/20 at 08:49am, Aneesh Kumar K.V wrote:
> > 
> > > >   mm/sparse.c | 2 ++
> > > >   1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/mm/sparse.c b/mm/sparse.c
> > > > index aadb7298dcef..3012d1f3771a 100644
> > > > --- a/mm/sparse.c
> > > > +++ b/mm/sparse.c
> > > > @@ -781,6 +781,8 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> > > >   			ms->usage = NULL;
> > > >   		}
> > > >   		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
> > > > +		/* Mark the section invalid */
> > > > +		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
> > > 
> > > Not sure if we should add checking in valid_section() or pfn_valid(),
> > > e.g check ms->usage validation too. Otherwise, this fix looks good to
> > > me.
> > 
> > With SPASEMEM_VMEMAP enabled, we should do validation check on ms->usage
> > before checking any subsection is valid. Since now we do have case
> > in which ms->usage is released, people still try to check it.
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index f0a2c184eb9a..d79bd938852e 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -1306,6 +1306,8 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
> >   {
> >   	int idx = subsection_map_index(pfn);
> > +	if (!ms->usage)
> > +		return 0;
> >   	return test_bit(idx, ms->usage->subsection_map);
> >   }
> >   #else
> > 
> 
> We always check for section valid, before we check if pfn_section_valid().
> 
> static inline int pfn_valid(unsigned long pfn)
> 
> 	struct mem_section *ms;
> 
> 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> 		return 0;
> 	ms = __nr_to_section(pfn_to_section_nr(pfn));
> 	if (!valid_section(ms))
> 		return 0;
> 	/*
> 	 * Traditionally early sections always returned pfn_valid() for
> 	 * the entire section-sized span.
> 	 */
> 	return early_section(ms) || pfn_section_valid(ms, pfn);
> }
> 
> 
> IMHO adding that if (!ms->usage) is redundant.

Yeah, I tend to agree. Consider this happens in the only small window
between ms->usage releasing and ms->section_mem_map releasing when
removing a section. Just thought adding this check to enhance it even
though we have had your fix, because we only check ms->section_mem_map
in valid_section(). Anyway, your fix looks good to me, see if other
people have any comment.

Thanks
Baoquan