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

[Michal Hocko]

On Thu 26-03-20 15:26:22, Aneesh Kumar K.V wrote:
> On 3/26/20 3:10 PM, Michal Hocko wrote:
> > On Wed 25-03-20 08:49:14, Aneesh Kumar K.V wrote:
> > > Fixes the below crash
> > > 
> > > BUG: Kernel NULL pointer dereference on read at 0x00000000
> > > Faulting instruction address: 0xc000000000c3447c
> > > Oops: Kernel access of bad area, sig: 11 [#1]
> > > LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> > > CPU: 11 PID: 7519 Comm: lt-ndctl Not tainted 5.6.0-rc7-autotest #1
> > > ...
> > > NIP [c000000000c3447c] vmemmap_populated+0x98/0xc0
> > > LR [c000000000088354] vmemmap_free+0x144/0x320
> > > Call Trace:
> > >   section_deactivate+0x220/0x240
> > 
> > It would be great to match this to the specific source code.
> 
> The crash is due to NULL dereference at
> 
> test_bit(idx, ms->usage->subsection_map); due to ms->usage = NULL;

It would be nice to call that out here as well

[...]
> > Why do we have to free usage before deactivaing section memmap? Now that
> > we have a late section_mem_map reset shouldn't we tear down the usage in
> > the same branch?
> > 
> 
> We still need to make the section invalid before we call into
> depopulate_section_memmap(). Because architecture like powerpc can share
> vmemmap area across sections (16MB mapping of vmemmap area) and we use
> vmemmap_popluated() to make that decision.

This should be noted in a comment as well.

> > > Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
> > > Cc: Baoquan He <bhe@xxxxxxxxxx>
> > > Reported-by: Sachin Sant <sachinp@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> > > ---
> > >   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;
> > 
> > Btw. this comment is not really helping at all.
> 
> That is marking the section invalid so that
> 
> static inline int valid_section(struct mem_section *section)
> {
> 	return (section && (section->section_mem_map & SECTION_HAS_MEM_MAP));
> }
> 
> 
> returns false.

Yes that is obvious once you are clear where to look. I was really
hoping for a comment that would simply point you to the right
direcection without chasing SECTION_HAS_MEM_MAP usage. This code is
subtle and useful comments, even when they state something that is
obvious to you _right_now_, can be really helpful.

Thanks!
-- 
Michal Hocko
SUSE Labs