Landlock: Abstract Unix Socket Scoping Support

This patch series adds scoping mechanism for abstract unix sockets.
Closes: https://github.com/landlock-lsm/linux/issues/7

Problem
=======

Abstract unix sockets are used for local inter-process communications
independent of the filesystem. Currently, a sandboxed process can
connect to a socket outside of the sandboxed environment, since Landlock
has no restriction for connecting to an abstract socket address(see more
details in [1,2]). Access to such sockets for a sandboxed process should
be scoped the same way ptrace is limited.

[1] https://lore.kernel.org/all/20231023.ahphah4Wii4v@digikod...
[2] https://lore.kernel.org/all/20231102.MaeWaepav8nu@digikod...

Solution
========

To solve this issue, we extend the user space interface by adding a new
"scoped" field to Landlock ruleset attribute structure. This field can
contains different rights to restrict different functionalities. For
abstract unix sockets, we introduce
"LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET" field to specify that a ruleset
will deny any connection from within the sandbox domain to its parent
(i.e. any parent sandbox or non-sandbox processes).

Example
=======

Starting a listening socket with socat(1):
	socat abstract-listen:mysocket -

Starting a sandboxed shell from $HOME with samples/landlock/sandboxer:
	LL_FS_RO=/ LL_FS_RW=. LL_SCOPED="a" ./sandboxer /bin/bash

If we try to connect to the listening socket, the connection would be
refused.
	socat - abstract-connect:mysocket --> fails


Notes of Implementation
=======================

* Using the "scoped" field provides enough compatibility and flexibility
  to extend the scoping mechanism for other IPCs(e.g. signals).

* To access the domain of a socket, we use its credentials of the file's FD
  which point to the credentials of the process that created the socket.
  (see more details in [3]). Cases where the process using the socket has
  a different domain than the process created it are covered in the 
  unix_sock_special_cases test.

[3] https://lore.kernel.org/outreachy/Zmi8Ydz4Z6tYtpY1@tahera...

Thanks to Mickaël Salaün and Paul Moore for guiding me through this
implementation.

Previous Versions
=================
v6: https://lore.kernel.org/all/Zn32CYZiu7pY+rdI@tahera-OptiP...
and https://lore.kernel.org/all/Zn32KKIJrY7Zi51K@tahera-OptiP...
v5: https://lore.kernel.org/all/ZnSZnhGBiprI6FRk@tahera-OptiP...
v4: https://lore.kernel.org/all/ZnNcE3ph2SWi1qmd@tahera-OptiP...
v3: https://lore.kernel.org/all/ZmJJ7lZdQuQop7e5@tahera-OptiP...
v2: https://lore.kernel.org/all/ZgX5TRTrSDPrJFfF@tahera-OptiP...
v1: https://lore.kernel.org/all/ZgXN5fi6A1YQKiAQ@tahera-OptiP...


Tahera Fahimi (4):
  Landlock: Add abstract unix socket connect restriction
  selftests/landlock: Abstract unix socket restriction tests
  samples/landlock: Support abstract unix socket restriction
  documentation/landlock: Adding scoping mechanism documentation

 Documentation/userspace-api/landlock.rst      |  23 +-
 include/uapi/linux/landlock.h                 |  29 +
 samples/landlock/sandboxer.c                  |  25 +-
 security/landlock/limits.h                    |   3 +
 security/landlock/ruleset.c                   |   7 +-
 security/landlock/ruleset.h                   |  23 +-
 security/landlock/syscalls.c                  |  14 +-
 security/landlock/task.c                      | 112 +++
 tools/testing/selftests/landlock/base_test.c  |   2 +-
 .../testing/selftests/landlock/ptrace_test.c  | 867 ++++++++++++++++++
 10 files changed, 1088 insertions(+), 17 deletions(-)

-- 
2.34.1