[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 at redhat.com>
>> Reported-by: Sachin Sant <sachinp at linux.vnet.ibm.com>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
>> ---
>>   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
>>
>