From: "Aneesh Kumar K.V" aneesh.kumar@linux.ibm.com Subject: mm/sparse: fix kernel crash with pfn_section_valid check

Fix 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.

[aneesh.kumar@linux.ibm.com: add comment] Link: http://lkml.kernel.org/r/20200326133235.343616-1-aneesh.kumar@linux.ibm.comL...: http://lkml.kernel.org/r/20200325031914.107660-1-aneesh.kumar@linux.ibm.com Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case") Signed-off-by: Aneesh Kumar K.V aneesh.kumar@linux.ibm.com Reported-by: Sachin Sant sachinp@linux.vnet.ibm.com Tested-by: Sachin Sant sachinp@linux.vnet.ibm.com Reviewed-by: Baoquan He bhe@redhat.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Pankaj Gupta pankaj.gupta.linux@gmail.com Reviewed-by: Wei Yang richard.weiyang@gmail.com Cc: Michael Ellerman mpe@ellerman.id.au Cc: Dan Williams dan.j.williams@intel.com Cc: David Hildenbrand david@redhat.com Cc: Oscar Salvador osalvador@suse.de Cc: Mike Rapoport rppt@linux.ibm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/sparse.c | 6 ++++++ 1 file changed, 6 insertions(+)

--- a/mm/sparse.c~mm-sparse-fix-kernel-crash-with-pfn_section_valid-check +++ a/mm/sparse.c @@ -781,6 +781,12 @@ static void section_deactivate(unsigned 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) _