filelock: Remove locks reliably when fcntl/close race is detected

When fcntl_setlk() races with close(), it removes the created lock with
do_lock_file_wait().
However, LSMs can allow the first do_lock_file_wait() that created the lock
while denying the second do_lock_file_wait() that tries to remove the lock.
In theory (but AFAIK not in practice), posix_lock_file() could also fail to
remove a lock due to GFP_KERNEL allocation failure (when splitting a range
in the middle).

After the bug has been triggered, use-after-free reads will occur in
lock_get_status() when userspace reads /proc/locks. This can likely be used
to read arbitrary kernel memory, but can't corrupt kernel memory.
This only affects systems with SELinux / Smack / AppArmor / BPF-LSM in
enforcing mode and only works from some security contexts.

Fix it by calling locks_remove_posix() instead, which is designed to
reliably get rid of POSIX locks associated with the given file and
files_struct and is also used by filp_flush().

Fixes: c293621 ("[PATCH] stale POSIX lock handling")
Cc: stable@kernel.org
Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=2563
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20240702-fs-lock-recover-2-v1-1-edd456f63789@google.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/locks.c

@@ -2448,8 +2448,9 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
  error = do_lock_file_wait(filp, cmd, file_lock);
 
  /*
- * Attempt to detect a close/fcntl race and recover by releasing the
- * lock that was just acquired. There is no need to do that when we're
+ * Detect close/fcntl races and recover by zapping all POSIX locks
+ * associated with this file and our files_struct, just like on
+ * filp_flush(). There is no need to do that when we're
  * unlocking though, or for OFD locks.
  */
  if (!error && file_lock->c.flc_type != F_UNLCK &&
@@ -2464,9 +2465,7 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
  f = files_lookup_fd_locked(files, fd);
  spin_unlock(&files->file_lock);
  if (f != filp) {
- file_lock->c.flc_type = F_UNLCK;
- error = do_lock_file_wait(filp, cmd, file_lock);
- WARN_ON_ONCE(error);
+ locks_remove_posix(filp, files);
  error = -EBADF;
  }
  }