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.