filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse

SoftFileLock and MarkerSoftFileLock evict an unparsable marker (the lock file at <path>, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. SoftFileLease expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

Another local user (the other user below) who can create files in the lock's directory, a sticky /tmp included (the sticky bit is a directory flag that lets only a file's owner remove it), parks an empty file of its own at the lock path. It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values. It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.

Two of the lock user's processes then run the critical section together with no error. A SoftFileLease waiter takes a live lease that its owner wrote moments before, within milliseconds instead of after lease_duration. Affected versions: filelock >= 4.1.0 and <= 4.2.0 (finding ANT-2026-RKC48J5B).

Severity

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N, 4.7 (Medium).

AV:L (local): the other user needs an account on the same host. AC:H (high attack complexity): a race the other user cannot steer. It needs ext4-style inode reuse, a coarse-clock timestamp tick the other user can predict, and two polls of the lock user inside a 4 ms tick and a ~75 µs create-to-write gap. The scanner measured 2 to 10 % of attempts; none of our own 250 attempts fired. PR:L (low privileges): an unprivileged account is enough. UI:N (no user interaction): the lock user's processes act on their own. C:N (no confidentiality impact): the other user reads nothing. I:H (high integrity impact): two holders write the data the lock protects, and neither learns of the other. A:N (no availability impact): the lock keeps working.

Classification from the review: security, conditional, by mechanism. It is the same cross-UID primitive (a UID is a user account's numeric id) as the stale-break inode-reuse race reported in a separate advisory: an inode number reused after the other user's unlink in a sticky directory, which our review reproduced across UIDs, plus a forged future mtime. The scanner rated it high (CWE-694) and demonstrated it across UIDs in its own runs. It did not fire in 250 attempts of ours over ext4 with inode reuse confirmed, nor in the scanner-side triage, so its cross-user claim rests on the report's own runs and on the mechanism.

Scope and preconditions

• The other user can create files in the lock's directory; a root-owned 1777 directory is enough. It creates, utimes (sets the timestamps of) and unlinks only its own file. In the sticky directory its unlink, rename, open(..., "w") and utime on a marker of the lock user all fail, and deleting its own file only frees the lock.
• The directory is on a filesystem that reuses a freed inode number at once: ext4, and overlayfs on it (thousands of create/unlink cycles returned one number). tmpfs does not reuse one, so the pair cannot repeat there; btrfs does not either.
• File timestamps come from a coarse clock whose future values the other user can compute (CLOCK_REALTIME_COARSE, 4 ms steps on Linux 6.1; 20,000 file creations all carried that clock's value).
• At least two processes of the lock user are waiting on the lock while the other user's file is there. The other user does not choose which processes overlap or when; each attempt blocks the lock for 2.2 s (for a lease, lease_duration).
• No symbolic link, hard link or FIFO takes part, so the fs.protected_* sysctls (Linux settings that restrict following links and opening pipes in sticky world-writable directories) play no part. StrictSoftFileLock keys its watch the same way but only raises SoftFileLockProtocolError, so it breaks no lock there.

filelock's checks against another local user hold only when no other user can replace the lock's directory or any of its ancestors. That is the case when each ancestor belongs to the lock user or root and is either not writable by others or sticky. A user who can rename entries in a non-sticky directory above the lock controls every name below it. No file-based lock can defend against that.

Affected code

At the scanned commit b831496, the watch is SoftFileLock._marker_unchanged_for, with its caller in _try_break_stale_lock. The MarkerSoftFileLock and SoftFileLease paths hand an unparsable marker to the same code. The re-check in break_lock_file compares the values the contender just read.

Introduced

0f7c4d3794dc ("fix(soft): expire leases on the contender's clock", #796, 2026-10-07), first released in 4.1.0 (verified against the tags). Earlier releases compared the marker's mtime with the wall clock, and this does not affect them.

Mechanism

SoftFileLock._acquire creates the marker with O_CREAT | O_EXCL (create the file, fail if it exists) and only then writes the holder record. In between it calls fstat and uname, stats /proc/self/ns/pid and reads /proc/<pid>/stat, so for about 75 µs (measured) the marker exists and is empty. A contender that polls in that gap must not take the marker for garbage, and the check that protects it is the two-second watch. _try_break_stale_lock reads the marker, finds no holder, and calls break_lock_file(lock_file, mtime, ino) only if _marker_unchanged_for(mtime, ino) reaches MALFORMED_LOCK_AGE_THRESHOLD (2.0 s). That function keeps self._marker_seen = ((mtime, ino), first seen) across polls and returns the elapsed monotonic time (time from a clock that only moves forward) while the pair repeats.

Neither half of the pair belongs to one file. A file's owner sets its mtime to any value with utime, a future time included, and ext4 gives the inode number of an unlinked file to the next file created in the directory.

1. While the lock is free, the other user creates an empty file at the lock path (inode I) and sets its mtime to T, a tick of the coarse clock about 2.2 s in the future.
2. Every waiting process of the lock user polls each 50 ms, reads an empty file and records _marker_seen = ((T, I), first seen). After 2 s each poll calls break_lock_file, whose rename fails with EPERM (operation not permitted; seen with strace): the directory is sticky and the file belongs to the other user. suppress(OSError, ...) swallows the error, the file stays, and nothing resets _marker_seen.
3. When the coarse clock reaches T, the other user unlinks its file. Inode I is free.
4. Process F of the lock user polls during tick T. Its O_EXCL create succeeds, and its marker is inode I with mtime T.
5. Process S polls before F has written its record. It reads an empty file with pair (T, I), returns the 2.2 s that S spent watching the other user's file, and S calls .