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    Subject[PATCH 5.4 102/155] mm/sparse: fix kernel crash with pfn_section_valid check
    Date
    From: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

    commit b943f045a9af9fd02f923e43fe8d7517e9961701 upstream.

    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 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.comLink: 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")
    Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
    Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
    Reviewed-by: Baoquan He <bhe@redhat.com>
    Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Acked-by: Pankaj 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: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/mm/sparse.c
    +++ b/mm/sparse.c
    @@ -789,6 +789,12 @@ 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)

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