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

[Aneesh Kumar K.V]

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;

that is explained in later part of the commit.
> >   __remove_pages+0x118/0x170
> >   arch_remove_memory+0x3c/0x150
> >   memunmap_pages+0x1cc/0x2f0
> >   devm_action_release+0x30/0x50
> >   release_nodes+0x2f8/0x3e0
> >   device_release_driver_internal+0x168/0x270
> >   unbind_store+0x130/0x170
> >   drv_attr_store+0x44/0x60
> >   sysfs_kf_write+0x68/0x80
> >   kernfs_fop_write+0x100/0x290
> >   __vfs_write+0x3c/0x70
> >   vfs_write+0xcc/0x240
> >   ksys_write+0x7c/0x140
> >   system_call+0x5c/0x68
> >
> > With commit: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
> > section_mem_map is set to NULL after depopulate_section_mem(). This
> > was done so that pfn_page() can work correctly with kernel config that disables
> > SPARSEMEM_VMEMMAP. With that config pfn_to_page does
> >
> > 	__section_mem_map_addr(__sec) + __pfn;
> > where
> >
> > static inline struct page *__section_mem_map_addr(struct mem_section *section)
> > {
> > 	unsigned long map = section->section_mem_map;
> > 	map &= SECTION_MAP_MASK;
> > 	return (struct page *)map;
> > }
> >
> > Now with SPASEMEM_VMEMAP enabled, mem_section->usage->subsection_map is used to
> > check the pfn validity (pfn_valid()). Since section_deactivate release
> > mem_section->usage if a section is fully deactivated, pfn_valid() check after
> > a subsection_deactivate cause a kernel crash.
> >
> > static inline int pfn_valid(unsigned long pfn)
> > {
> > ...
> > 	return early_section(ms) || pfn_section_valid(ms, pfn);
> > }
> >
> > where
> >
> > static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
> > {
>
> > 	int idx = subsection_map_index(pfn);
> >
> > 	return test_bit(idx, ms->usage->subsection_map);
> > }
> >
> > Avoid this by clearing SECTION_HAS_MEM_MAP when mem_section->usage is freed.
>
> I am sorry, I haven't noticed that during the review of the commit
> mentioned above. This is all subtle as hell, I have to say.
>
> 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.



> > 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.

> 		/*
> 		 * section->usage is gone and VMEMMAP's pfn_valid depens
> 		 * on it (see pfn_section_valid)
> 		 */
> >   	}
> >   
> >   	if (section_is_early && memmap)
> > --
> > 2.25.1