CVE-2024-47740

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

f2fs: Require FMODE_WRITE for atomic write ioctls

The F2FS ioctls for starting and committing atomic writes check for inode_owner_or_capable(), but this does not give LSMs like SELinux or Landlock an opportunity to deny the write access - if the caller's FSUID matches the inode's UID, inode_owner_or_capable() immediately returns true.

There are scenarios where LSMs want to deny a process the ability to write particular files, even files that the FSUID of the process owns; but this can currently partially be bypassed using atomic write ioctls in two ways:

• F2FS_IOC_START_ATOMIC_REPLACE + F2FS_IOC_COMMIT_ATOMIC_WRITE can truncate an inode to size 0
• F2FS_IOC_START_ATOMIC_WRITE + F2FS_IOC_ABORT_ATOMIC_WRITE can revert changes another process concurrently made to a file

Fix it by requiring FMODE_WRITE for these operations, just like for F2FS_IOC_MOVE_RANGE. Since any legitimate caller should only be using these ioctls when intending to write into the file, that seems unlikely to break anything.

This vulnerability is rated as Moderate severity because exploitation requires local access and an active F2FS filesystem, limiting the blast radius to unauthorized truncation or reversion of files owned by the user to bypass LSM controls. In Red Hat Enterprise Linux, this flaw does not affect default deployments because the F2FS filesystem driver is not enabled in standard enterprise kernel configurations.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.