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SubjectRe: [PATCH v2] mm/sparse: Fix kernel crash with pfn_section_valid check
On Thu, Mar 26, 2020 at 07:02:35PM +0530, 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]
>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
> __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
>The crash is due to NULL dereference at
>test_bit(idx, ms->usage->subsection_map); due to ms->usage = NULL; in pfn_section_valid()
>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;
>static inline struct page *__section_mem_map_addr(struct mem_section *section)
> unsigned long map = section->section_mem_map;
> 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);
>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.
>For architectures like ppc64 where large pages are used for vmmemap mapping (16MB),
>a specific vmemmap mapping can cover multiple sections. Hence before a vmemmap
>mapping page can be freed, the kernel needs to make sure there are no valid sections
>within that mapping. Clearing the section valid bit before
>depopulate_section_memap enables this.
>Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
>Reported-by: Sachin Sant <>
>Tested-by: Sachin Sant <>
>Cc: Baoquan He <>
>Cc: Michael Ellerman <>
>Cc: Dan Williams <>
>Cc: Pankaj Gupta <>
>Cc: David Hildenbrand <>
>Cc: Michal Hocko <>
>Cc: Wei Yang <>
>Cc: Oscar Salvador <>
>Cc: Mike Rapoport <>
>Cc: <>
>Signed-off-by: Aneesh Kumar K.V <>

Reviewed-by: Wei Yang <>

Wei Yang
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