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

[Michal Hocko <mhocko@kernel.org>]

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.

>  __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?

> Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
> Cc: Baoquan He <bhe@redhat.com>
> Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@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.
		/*
		 * section->usage is gone and VMEMMAP's pfn_valid depens
		 * on it (see pfn_section_valid)
		 */
>  	}
>  
>  	if (section_is_early && memmap)
> -- 
> 2.25.1
> 

-- 
Michal Hocko
SUSE Labs