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    Subject[PATCH 4.19 042/267] nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()
    Date
    From: Ryusuke Konishi <konishi.ryusuke@gmail.com>

    commit 8301c719a2bd131436438e49130ee381d30933f5 upstream.

    After commit c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if
    mapping has no dirty pages"), the following null pointer dereference has
    been reported on nilfs2:

    BUG: kernel NULL pointer dereference, address: 00000000000000a8
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 0 P4D 0
    Oops: 0000 [#1] SMP PTI
    ...
    RIP: 0010:percpu_counter_add_batch+0xa/0x60
    ...
    Call Trace:
    __test_set_page_writeback+0x2d3/0x330
    nilfs_segctor_do_construct+0x10d3/0x2110 [nilfs2]
    nilfs_segctor_construct+0x168/0x260 [nilfs2]
    nilfs_segctor_thread+0x127/0x3b0 [nilfs2]
    kthread+0xf8/0x130
    ...

    This crash turned out to be caused by set_page_writeback() call for
    segment summary buffers at nilfs_segctor_prepare_write().

    set_page_writeback() can call inc_wb_stat(inode_to_wb(inode),
    WB_WRITEBACK) where inode_to_wb(inode) is NULL if the inode of
    underlying block device does not have an associated wb.

    This fixes the issue by calling inode_attach_wb() in advance to ensure
    to associate the bdev inode with its wb.

    Fixes: c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if mapping has no dirty pages")
    Reported-by: Walton Hoops <me@waltonhoops.com>
    Reported-by: Tomas Hlavaty <tom@logand.com>
    Reported-by: ARAI Shun-ichi <hermes@ceres.dti.ne.jp>
    Reported-by: Hideki EIRAKU <hdk1983@gmail.com>
    Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
    Cc: <stable@vger.kernel.org> [5.4+]
    Link: http://lkml.kernel.org/r/20200608.011819.1399059588922299158.konishi.ryusuke@gmail.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/nilfs2/segment.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/fs/nilfs2/segment.c
    +++ b/fs/nilfs2/segment.c
    @@ -2780,6 +2780,8 @@ int nilfs_attach_log_writer(struct super
    if (!nilfs->ns_writer)
    return -ENOMEM;

    + inode_attach_wb(nilfs->ns_bdev->bd_inode, NULL);
    +
    err = nilfs_segctor_start_thread(nilfs->ns_writer);
    if (err) {
    kfree(nilfs->ns_writer);

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