release/5.2.46/nilfs2-fix-null-pointer-dereference-at-nilfs_segctor.patch

From 0f7d77a58e0b9d9a861aa02fdb74111663a85a62 Mon Sep 17 00:00:00 2001
From: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Date: Wed, 10 Jun 2020 18:41:35 -0700
Subject: [PATCH] nilfs2: fix null pointer dereference at
 nilfs_segctor_do_construct()

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: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 445eef41bfaf..91b58c897f92 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2780,6 +2780,8 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 	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);
-- 
2.7.4