CVE-2026-53017: f2fs: fix data loss caused by incorrect use of nat_entry flag

CVE-2026-53017: f2fs: fix data loss caused by incorrect use of nat_entry flag

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix data loss caused by incorrect use of nat_entry flag

Data loss can occur when fsync is performed on a newly created file
(before any checkpoint has been written) concurrently with a checkpoint
operation. The scenario is as follows:

create & write & fsync 'file A'                 write checkpoint
- f2fs_do_sync_file // inline inode
 - f2fs_write_inode // inode folio is dirty
                                                - f2fs_write_checkpoint
                                                 - f2fs_flush_merged_writes
                                                 - f2fs_sync_node_pages
                                                 - f2fs_flush_nat_entries
 - f2fs_fsync_node_pages // no dirty node
 - f2fs_need_inode_block_update // return false
 SPO and lost 'file A'

f2fs_flush_nat_entries() sets the IS_CHECKPOINTED and HAS_LAST_FSYNC
flags for the nat_entry, but this does not mean that the checkpoint has
actually completed successfully. However, f2fs_need_inode_block_update()
checks these flags and incorrectly assumes that the checkpoint has
finished.

The root cause is that the semantics of IS_CHECKPOINTED and
HAS_LAST_FSYNC are only guaranteed after the checkpoint write fully
completes.

This patch modifies f2fs_need_inode_block_update() to acquire the
sbi->node_write lock before reading the nat_entry flags, ensuring that
once IS_CHECKPOINTED and HAS_LAST_FSYNC are observed to be set, the
checkpoint operation has already completed.

The Linux kernel CVE team has assigned CVE-2026-53017 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.8 with commit e05df3b115e7308afbca652769b54e4549fcc723 and fixed in 7.0.10 with commit 20cedb4d9f6b230d0ee469690b8f868f06a07c29
	Issue introduced in 3.8 with commit e05df3b115e7308afbca652769b54e4549fcc723 and fixed in 7.1 with commit 238e14eb7226f883b72caccd2d37bf5707df066b

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-53017
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/f2fs/node.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/20cedb4d9f6b230d0ee469690b8f868f06a07c29
	https://git.kernel.org/stable/c/238e14eb7226f883b72caccd2d37bf5707df066b