From: Ryusuke Konishi konishi.ryusuke@gmail.com Subject: nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()

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.

Link: http://lkml.kernel.org/r/20200608.011819.1399059588922299158.konishi.ryusuke... Fixes: c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if mapping has no dirty pages") Signed-off-by: Ryusuke Konishi konishi.ryusuke@gmail.com Tested-by: Ryusuke Konishi konishi.ryusuke@gmail.com 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 Cc: stable@vger.kernel.org [5.4+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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

--- a/fs/nilfs2/segment.c~nilfs2-fix-null-pointer-dereference-at-nilfs_segctor_do_construct +++ a/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); _