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    SubjectRe: [PATCH] mm/sparse: Fix kernel crash with pfn_section_valid check
    On Wed, 25 Mar 2020 08:49:14 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:

    > Fixes the below crash

    (cc's added)

    > 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
    >
    > 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.
    >
    > Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")

    d41e2f3bd546 had cc:stable, so I shall add cc:stable to this one as well.

    > Cc: Baoquan He <bhe@redhat.com>
    > Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
    > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    > ---
    > mm/sparse.c | 2 ++
    > 1 file changed, 2 insertions(+)
    >
    > diff --git a/mm/sparse.c b/mm/sparse.c
    > index aadb7298dcef..3012d1f3771a 100644
    > --- a/mm/sparse.c
    > +++ b/mm/sparse.c
    > @@ -781,6 +781,8 @@ 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 */
    > + ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
    > }
    >
    > if (section_is_early && memmap)

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