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 callers FSUID matches the inodes 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.

Affected Software

References

• https://git.kernel.org/stable/c/000bab8753ae29a259feb339b99ee759795a48ac
• https://git.kernel.org/stable/c/32f348ecc149e9ca70a1c424ae8fa9b6919d2713
• https://git.kernel.org/stable/c/4583290898c13c2c2e5eb8773886d153c2c5121d
• https://git.kernel.org/stable/c/4ce87674c3a6b4d3b3d45f85b584ab8618a3cece
• https://git.kernel.org/stable/c/4f5a100f87f32cb65d4bb1ad282a08c92f6f591e
• https://git.kernel.org/stable/c/5e0de753bfe87768ebe6744d869caa92f35e5731
• https://git.kernel.org/stable/c/700f3a7c7fa5764c9f24bbf7c78e0b6e479fa653
• https://git.kernel.org/stable/c/88ff021e1fea2d9b40b2d5efd9013c89f7be04ac
• https://git.kernel.org/stable/c/f3bfac2cabf5333506b263bc0c8497c95302f32d