• Aneesh Kumar K.V's avatar
    mm/sparse: fix kernel crash with pfn_section_valid check · b943f045
    Aneesh Kumar K.V authored
    Fix the crash like this:
    
        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 d41e2f3b ("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.comLink: http://lkml.kernel.org/r/20200325031914.107660-1-aneesh.kumar@linux.ibm.com
    Fixes: d41e2f3b ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
    Reported-by: default avatarSachin Sant <sachinp@linux.vnet.ibm.com>
    Signed-off-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Tested-by: default avatarSachin Sant <sachinp@linux.vnet.ibm.com>
    Reviewed-by: default avatarBaoquan He <bhe@redhat.com>
    Reviewed-by: default avatarWei Yang <richard.weiyang@gmail.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Acked-by: default avatarPankaj Gupta <pankaj.gupta.linux@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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    b943f045
sparse.c 25.7 KB