[PATCH 6.12 491/567] jbd2: fix the inconsistency between checksum and data in memory for journal sb

Greg Kroah-Hartman
2026-01-06T17:41:50Z

From: Ye Bin <yebin10@huawei.com>

[ Upstream commit 6abfe107894af7e8ce3a2e120c619d81ee764ad5 ]

Copying the file system while it is mounted as read-only results in
a mount failure:
[~]# mkfs.ext4 -F /dev/sdc
[~]# mount /dev/sdc -o ro /mnt/test
[~]# dd if=/dev/sdc of=/dev/sda bs=1M
[~]# mount /dev/sda /mnt/test1
[ 1094.849826] JBD2: journal checksum error
[ 1094.850927] EXT4-fs (sda): Could not load journal inode
mount: mount /dev/sda on /mnt/test1 failed: Bad message

The process described above is just an abstracted way I came up with to
reproduce the issue. In the actual scenario, the file system was mounted
read-only and then copied while it was still mounted. It was found that
the mount operation failed. The user intended to verify the data or use
it as a backup, and this action was performed during a version upgrade.
Above issue may happen as follows:
ext4_fill_super
set_journal_csum_feature_set(sb)
if (ext4_has_metadata_csum(sb))
incompat = JBD2_FEATURE_INCOMPAT_CSUM_V3;
if (test_opt(sb, JOURNAL_CHECKSUM)
jbd2_journal_set_features(sbi->s_journal, compat, 0, incompat);
lock_buffer(journal->j_sb_buffer);
sb->s_feature_incompat |= cpu_to_be32(incompat);
//The data in the journal sb was modified, but the checksum was not
updated, so the data remaining in memory has a mismatch between the
data and the checksum.
unlock_buffer(journal->j_sb_buffer);

In this case, the journal sb copied over is in a state where the checksum
and data are inconsistent, so mounting fails.
To solve the above issue, update the checksum in memory after modifying
the journal sb.

Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20251103010123.3753631-1-yebin@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
[ jbd2_superblock_csum() also takes a journal param ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/jbd2/journal.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2375,6 +2375,12 @@ int jbd2_journal_set_features(journal_t
sb->s_feature_compat |= cpu_to_be32(compat);
sb->s_feature_ro_compat |= cpu_to_be32(ro);
sb->s_feature_incompat |= cpu_to_be32(incompat);
+ /*
+ * Update the checksum now so that it is valid even for read-only
+ * filesystems where jbd2_write_superblock() doesn't get called.
+ */
+ if (jbd2_journal_has_csum_v2or3(journal))
+ sb->s_checksum = jbd2_superblock_csum(journal, sb);
unlock_buffer(journal->j_sb_buffer);
jbd2_journal_init_transaction_limits(journal);

@@ -2404,9 +2410,17 @@ void jbd2_journal_clear_features(journal

sb = journal->j_superblock;

+ lock_buffer(journal->j_sb_buffer);
sb->s_feature_compat &= ~cpu_to_be32(compat);
sb->s_feature_ro_compat &= ~cpu_to_be32(ro);
sb->s_feature_incompat &= ~cpu_to_be32(incompat);
+ /*
+ * Update the checksum now so that it is valid even for read-only
+ * filesystems where jbd2_write_superblock() doesn't get called.
+ */
+ if (jbd2_journal_has_csum_v2or3(journal))
+ sb->s_checksum = jbd2_superblock_csum(journal, sb);
+ unlock_buffer(journal->j_sb_buffer);
jbd2_journal_init_transaction_limits(journal);
}
EXPORT_SYMBOL(jbd2_journal_clear_features);