Christian Brauner
075c364273
renameat2() with RENAME_WHITEOUT leaves a whiteout in place of the source. That's a character device owned by the caller's fsuid and fsgid which the file system creates from its ->rename() method, so it never passes the path_mknod hook. Since Linux 5.8 this doesn't require any privileges either. So a managed user namespace that can rename an entry on a file system that outlives it, e.g., in a world-writable directory the user prepared from the outside, can leave a whiteout owned by its transient range behind. The only hook that sees this is path_rename, which gets the flags. Refuse RENAME_WHITEOUT there whenever validate_mount() would refuse a mknod in the source directory.
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