[PATCH v2] mm/sparse: Fix kernel crash with pfn_section_valid check

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

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
 __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;
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.
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 <sachinp@linux.vnet.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
 mm/sparse.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index aadb7298dcef..65599e8bd636 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -781,6 +781,12 @@ 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 so that valid_section()
+		 * return false. This prevents code from dereferencing
+		 * ms->usage array.
+		 */
+		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
 	}
 
 	if (section_is_early && memmap)
-- 
2.25.1