filelock ReadWriteLock: SQLite reopens the lock database by name, so a planted symlink or journal file makes the lock user's process overwrite or unlink its own files

Classification from the external review: conditional security, a cross-user effect that exists only when another user can change names in the lock database's directory. The review notes that the 77/77 inotify result (inotify is a Linux interface that reports file events) is stronger evidence than a theoretical TOCTOU (a time-of-check to time-of-use race). The scanner rated it high. The review accepts the mechanism and narrows it to that precondition.

Scope and preconditions

• The other party is a different local account that can create or rename entries in the directory that holds the lock database. That is a 0777 directory or a group-writable one (including a setgid 2775 project directory shared with that account), in each case without the sticky bit. The lock user's lock must be able to open the database read-write, so a database the other user creates is 0666. filelock itself creates the database 0600.
• Sticky directories (/tmp, /var/tmp, any 1777 directory) narrow the finding rather than exclude it. The redirect and the journal replay do not reach a database the lock user owns there (tested with fs.protected_regular=2). What remains is a planted non-empty <db>-wal (any mode; a directory or FIFO works too) that the lock user cannot unlink. It makes every acquire raise sqlite3.OperationalError: disk I/O error. A hard link (a second name for the same file) to a file of the lock user at the lock path also remains, if fs.protected_hardlinks=0.
• Two kernel settings matter. fs.protected_regular (1 or 2 is the systemd default on most distributions; the kernel's own default is 0) keeps a user from opening a file another user owns in a sticky world-writable directory. Where it is 0, the other user can own the database in /tmp, and SQLite follows a planted symlink itself (not tested). fs.protected_hardlinks=1 (most distributions; kernel default 0) blocks a hard link of a lock user's file onto the lock path. Neither setting affects a non-sticky shared directory.
• Platform and route. The /proc/self/fd redirect is Linux with /proc mounted, 4.0.9 and later. The side-file variants (-journal, -wal, -shm) exist on every POSIX route. On the hard-link route (macOS since 4.1.1 for every connection, Linux without /proc) they need the link-directory swap described under "Mechanism", so a non-sticky parent. Neither the scanner nor the review executed on macOS; the statement rests on the platform branch and the identical code after it. Windows does not take this path.
• Ancestor boundary. Protection against another local user is possible only while no other user can replace the lock's directory or any of its ancestors. That holds when each ancestor belongs to the user or root and is either not writable by others or sticky. A user who can rename an entry 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 b831496b0273: _connect_through_descriptor builds /proc/self/fd/<n> and hands it to sqlite3.connect. The assumption is the comment at line 63, "Descriptor paths reopen the validated inode" (an inode is the kernel's identity for a file, independent of its name). SQLite first reads the file it opened at PRAGMA journal_mode=MEMORY, line 816. The check in _raise_if_database_replaced at line 836 runs only after BEGIN. Both _open_lock_database and _raise_if_wal inspect the descriptor, not the file SQLite is on. The one-level hard link of the macOS route is _link_beside.

Introduced

The /proc/self/fd redirect came with 4d45e83 ("Merge commit from fork", the fix for GHSA-j8f7-rjxc-mr56), first released in 4.0.9 (verified: git tag --contains 4d45e83 gives 4.0.9); 4.0.8 does not have it. The side-file variants are older. They reproduce on cfd4446 ("feat(lock): add SQLite-based ReadWriteLock (#399)"), first released in 3.20.4 (verified), which passed the lock path straight to sqlite3.connect.

Mechanism

On Linux the lock opens the database with O_RDWR|O_CREAT|O_NOFOLLOW|O_NONBLOCK. It checks through the descriptor that the file is a regular file and not a WAL database. It then calls sqlite3.connect("/proc/self/fd/<n>") and expects SQLite to reopen the validated inode. SQLite does not open that name. It runs readlink() on it, which yields the path string the kernel reports for the descriptor at that moment, and opens that string by path, following every symlink. Without interference the trace shows readlink("/proc/534/fd/3") = "/tmp/benign/app.lock.db" and then a second openat of that path. The descriptor contributes only a string. Once the directory entry behind the descriptor is gone, the string is <database> (deleted). A symlink with that exact name sends SQLite wherever its creator chose.

The other user prepares the directory before the lock user runs. They create a database cache.lock.db (0666, so the lock user's lock accepts it) with a second hard link cache.lock.db.keep, and the symlink cache.lock.db (deleted) -> /tmp/victim-notes.txt. The lock user's os.open raises an inotify IN_OPEN event. The other user unlinks cache.lock.db and links cache.lock.db.keep back under the same name. The lock user's descriptor still refers to the unlinked entry, so /proc/self/fd/3 now reads /tmp/shared-cache/cache.lock.db (deleted). sqlite3.connect resolves it, follows the symlink and opens the target O_RDWR|O_CREAT. None of filelock's checks look at that file. _open_lock_database and _raise_if_wal inspect descriptor 3. _raise_if_database_replaced compares an lstat of the lock path with the descriptor's (st_dev, st_ino), which passes because the other user linked the same inode back, and it runs only after BEGIN.

The damage comes from the first statement, PRAGMA journal_mode=MEMORY. SQLite finds <target>-journal, treats it as the rollback journal of an interrupted transaction, and copies its page records into the target before it checks whether the target is a database at all. The journal header supplies the page size and the page count SQLite resizes the file to. Whoever wrote the journal computed the per-page checksum. The content from offset 0 and the final length are therefore the other user's. Strace of the lock user's process (SQLite 3.40.1):

openat("/tmp/shared-cache/cache.lock.db", O_RDWR|O_CREAT|O_NONBLOCK|O_NOFOLLOW|O_CLOEXEC, 0600) = 3
readlink("/proc/3819/fd/3", "/tmp/shared-cache/cache.lock.db (deleted)", 4096) = 41
readlink("/tmp/shared-cache/cache.lock.db (deleted)", "/tmp/victim-notes.txt", 4096) = 21
openat("/tmp/victim-notes.txt", O_RDWR|O_CREAT|O_NOFOLLOW|O_CLOEXEC, 0644) = 4
openat("/tmp/victim-notes.txt-journal", O_RDWR|O_NOFOLLOW|O_CLOEXEC) = 5
pread64(5, "ATTACKER CONTENT WRITTEN BY THE "..., 4096, 516) = 4096
pwrite64(4, "ATTACKER CONTENT WRITTEN BY THE "..., 4096, 0) = 4096
unlink("/tmp/victim-notes.txt-journal") = -1 EPERM            -> "disk I/O error"

The redirect itself needs no journal. The constructor's validation connection already makes SQLite open the target with O_CREAT. With the symlink pointing at a missing path in the lock user's home, constructing the lock created that file (empty, 0644, owned by the lock user) and raised nothing. With the symlink pointing at a 0600 WAL-mode SQLite database of the lock user, write_lock() returned without error and the database's header bytes 18 and 19 changed from 2 2 to 1 1. The pragma converted the lock user's database out of WAL mode, and the redirected acquire held its SQLite lock on that database rather than on the lock database.

Side-file names beside the database. Two comments record the belief that SQLite names its side files after the descriptor path (line 63, line 1095). The path is not synthetic to SQLite. It derives <database>-journal, <database>-wal and <database>-shm from the resolved real path in the shared directory. journal_mode=MEMORY governs only journals SQLite would write, not ones it finds. These variants need no redirect.

A hot -journal makes the lock user unlink a file the journal names; no race. The other user unlinks the lock user's empty lock.db, puts a one-page SQLite database (0666) in its place, and writes a 46-byte lock.db-journal: a path, its 4-byte length, the 4-byte sum of its bytes and SQLite's journal magic. SQLite treats a journal beside a non-empty database as hot (left by an interrupted transaction). The tail names a "super-journal", and recovery ends by unlinking that file. The lock user's next read_lock() or write_lock() returns without error and the named file is gone (unlink("/home/victim/private/notes.txt") = 0, from a 0700 directory of the lock user). The named file must be a non-empty regular file the lock user can read and unlink. A FIFO planted as lock.db-journal instead blocks read_lock(timeout=2) in SQLite's open() of the journal (still blocked when killed after 10 s).

A re-planted -wal puts the holder in WAL mode past _raise_if_wal. _raise_if_wal checks header bytes 18 and 19. Whenever a connection takes its first lock, SQLite stats <db>-wal, and if that file and the database are both non-empty it opens the WAL whatever the header says. The pragma at line 816 converts such a database and unlinks -wal and -shm. The BEGIN at line 833 is a separate statement that starts unlocked and stats <db>-wal again. Re-creating the 32-byte -wal in that gap landed on the first attempt in 20 of 20 runs on a 2-CPU machine.

The lock user's with lock.write_lock(): then holds the -shm byte-120 WAL write lock and only SHARED on the database. Once the other user removes the -wal, a second process of the lock user gets read_lock(timeout=1) granted while the writer is inside its block, with no exception on either side. SQLite also creates <db>-shm with the database's permission bits (0666) and keeps it mapped into memory. After the other user truncates it, the writer dies with SIGBUS (the signal for touching a mapped file region that no longer exists) in connection.rollback() during release().

A -wal in a sticky directory makes every acquire raise; no race, no access to the database. Beside the lock user's own empty 0600 database, a non-empty <db>-wal (one byte, any mode; a directory or FIFO works too) is a leftover SQLite tries to delete. In a 1777 directory the lock user cannot unlink another user's file (unlink(...) = -1 EPERM), so the pragma fails with SQLITE_IOERR_DELETE. The code in _do_acquire_inner translates only "database is locked" into Timeout. sqlite3.OperationalError: disk I/O error therefore leaves every acquire_read() / acquire_write() (and the async variants) within milliseconds, whatever timeout or blocking says, until the file's owner or root removes it.

Further effects observed at this commit in a non-sticky directory. A 0755 script owned by the lock user, renamed to the lock path with a one-record journal beside it, ends up holding the other user's bytes (the acquire raises file is not a database). A hard link to the lock user's 0600 ~/.profile at the lock path does the same in a sticky /tmp when fs.protected_hardlinks=0. When the locking process runs as root, SQLite runs fchown on a -wal or -shm it opens to match the owner of the database, so root's 0600 service.env renamed to app.lock.db-wal belonged to the other user after root's read_lock().

A journal whose super-journal names the database itself makes SQLite open and close a second descriptor on the database, which drops the process's POSIX locks, so acquire_read returns with no lock held in /proc/locks and a writer in another process acquires (two journals in a row, one per statement; write locks are not affected). A one-byte lock.db-wal plus a lock.db-shm on which the other user holds two byte locks makes the acquire take 10 s regardless of timeout and raise locking protocol, or die with SIGBUS if the other user truncates the -shm.

The hard-link route in a directory without the sticky bit. None of the above occurs on the _link_beside branch (macOS, Linux without /proc). There SQLite's main file is .filelock-…/lock.db in the connecting user's 0700 directory. That protection holds only where no other user can rename the .filelock-* entry itself, because 0700 protects the contents, not the entry in the parent. _link_beside checks the link once and returns a pathname, and every later use is by that name.

With /proc hidden in a 0777 directory, the other user staged a directory holding a hard link lock.db to a one-page database and a hot lock.db-journal. They held BEGIN EXCLUSIVE on the database so the lock user's first statement waited in the busy handler (SQLite's retry loop for a locked database), renamed the lock user's .filelock-* directory aside, renamed the staged directory onto its name and rolled back. SQLite got SHARED, found the journal under the swapped name and unlinked the file it named. The lock user's process reported the lock held and exited 0. A polling variant without the write lock also won (in 40 trials the file was gone after 1 to 26 starts of the lock user's program). Under the sticky bit the rename fails with EPERM.

Reproduction

The scanner's reproducer for ANT-2026-KMJ2575J, quoted as received. It ran against commit b831496b0273.

repro

#!/usr/bin/env python3
import os
import shutil
import subprocess
import sys

as_victim = {"user": "victim", "group": "victim", "extra_groups": []}
as_attacker = {"user": "attacker", "group": "attacker", "extra_groups": []}

# set up: the lock database lives in a shared cache directory, 0777 and not sticky
shared_dir = "/tmp/shared-cache"
lock_db = shared_dir + "/cache.lock.db"
shutil.rmtree(shared_dir, ignore_errors=True)
os.mkdir(shared_dir)
os.chmod(shared_dir, 0o777)

# set up: a private 0600 file of the victim's in sticky /tmp, which the attacker cannot write
target = "/tmp/victim-notes.txt"
for leftover in (target, target + "-journal"):
    if os.path.lexists(leftover):
        os.unlink(leftover)
original = b"original victim notes\n"
with open(target, "wb") as notes:
    notes.write(original)
shutil.chown(target, "victim", "victim")
os.chmod(target, 0o600)
direct_write = subprocess.run(
    [sys.executable, "-c", "import sys; open(sys.argv[1], 'r+b')", target], capture_output=True, text=True, **as_attacker
)
print("attacker opening the target for writing:", direct_write.stderr.strip().splitlines()[-1])
assert direct_write.returncode != 0

# attack: a hot journal beside the target, and a redirect of the victim's SQLite from the lock database to the target
content = "ATTACKER CONTENT WRITTEN BY THE VICTIM'S ReadWriteLock\n"
subprocess.run([sys.executable, "/tmp/repro.d/plant_hot_journal.py", target, content], check=True, **as_attacker)
redirect = subprocess.Popen(
    [sys.executable, "/tmp/repro.d/redirect_lock_db.py", lock_db, target], stdout=subprocess.PIPE, text=True, **as_attacker
)
assert redirect.stdout.readline() == "ready\n"

# the victim takes its read lock; the attacker races each acquire, so the victim runs up to 20 times
for run in range(1, 21):
    victim = subprocess.run(
        [sys.executable, "/tmp/repro.d/victim_app.py", lock_db], capture_output=True, text=True, timeout=60, **as_victim
    )
    with open(target, "rb") as notes:
        after = notes.read()
    if after != original:
        break
redirect.kill()
redirect.wait()

# check: taking a lock on cache.lock.db must leave the victim's notes alone
print(f"victim run {run}: exit status {victim.returncode}:", (victim.stderr.strip() or "no error").splitlines()[-1])
status = os.stat(target)
print("target owner, mode, size:", status.st_uid, oct(status.st_mode & 0o7777), status.st_size)
print("target before:", original)
print("target after: ", after[:60])
print("BUG" if after == content.encode().ljust(4096, b"\0") else "OK")

repro.d/plant_hot_journal.py

#!/usr/bin/env python3
"""The attacker: leave a rollba