[PATCH 4.14 031/190] nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()

Greg Kroah-Hartman
2020-06-19T14:47:38Z

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
@@ -2794,6 +2794,8 @@ 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);