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[87.106.108.193]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-436296b25casm23040857f8f.4.2026.02.08.15.11.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 08 Feb 2026 15:11:15 -0800 (PST) From: =?UTF-8?q?G=C3=BCnther=20Noack?= To: =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= , "John Johansen" , "Paul Moore" , James Morris , "Serge E . Hallyn" Cc: =?UTF-8?q?G=C3=BCnther=20Noack?= , linux-security-module@vger.kernel.org, "Tingmao Wang" , "Justin Suess" , "Samasth Norway Ananda" , "Matthieu Buffet" , "Mikhail Ivanov" , konstantin.meskhidze@huawei.com, "Demi Marie Obenour" , "Alyssa Ross" , "Jann Horn" , "Tahera Fahimi" , Simon Horman , netdev@vger.kernel.org, Alexander Viro , Christian Brauner Subject: [PATCH v4 0/6] landlock: UNIX connect() control by pathname and scope Date: Mon, 9 Feb 2026 00:10:10 +0100 Message-ID: <20260208231017.114343-1-gnoack3000@gmail.com> X-Mailer: git-send-email 2.52.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hello! This patch set introduces a filesystem-based Landlock restriction mechanism for connecting to UNIX domain sockets (or addressing them with sendmsg(2)). It introduces the filesystem access right LANDLOCK_ACCESS_FS_RESOLVE_UNIX. For the connection-oriented SOCK_STREAM and SOCK_SEQPACKET type sockets, the access right makes the connect(2) operation fail with EACCES, if denied. SOCK_DGRAM-type UNIX sockets can be used both with connect(2), or by passing an explicit recipient address with every sendmsg(2) invocation. In the latter case, the Landlock check is done when an explicit recipient address is passed to sendmsg(2) and can make sendmsg(2) return EACCES. When UNIX datagram sockets are connected with connect(2), a fixed recipient address is associated with the socket and the check happens during connect(2) and may return EACCES. When LANDLOCK_ACCESS_FS_RESOLVE_UNIX is handled within a Landlock domain, this domain will only allow connect(2) and sendmsg(2) to server sockets that were created within the same domain. Or, to phrase it the other way around: Unless it is allow-listed with a LANDLOCK_PATH_BENEATH rule, the newly created domain denies connect(2) and sendmsg(2) actions that are directed *outwards* of that domain. In that regard, LANDLOCK_ACCESS_FS_RESOLVE_UNIX has the same semantics as one of the "scoped" access rights. == Motivation Currently, landlocked processes can connect to named UNIX sockets through the BSD socket API described in unix(7), by invoking socket(2) followed by connect(2) with a suitable struct sockname_un holding the socket's filename. This is a surprising gap in Landlock's sandboxing capabilities for users (e.g. in [1]) and it can be used to escape a sandbox when a Unix service offers command execution (various such scenarios were listed by Tingmao Wang in [2]). The original feature request is at [4]. == Alternatives and Related Work === Alternative: Use existing LSM hooks We have carefully and seriously considered the use of existing LSM hooks, but still came to the conclusion that a new LSM hook is better suited in this case: The existing hooks security_unix_stream_connect(), security_unix_may_send() and security_socket_connect() do not give access to the resolved filesystem path. * Resolving the filesystem path in the struct sockaddr_un again within a Landlock would produce a TOCTOU race, so this is not an option. * We would therefore need to wire through the resolved struct path from unix_find_bsd() to one of the existing LSM hooks which get called later. This would be a more substantial change to af_unix.c. The struct path that is available in the listening-side struct sock is can be read through the existing hooks, but it is not an option to use this information: As the listening socket may have been bound from within a different namespace, the path that was used for that can is in the general case not meaningful for a sandboxed process. In particular, it is not possible to use this path (or prefixes thereof) when constructing a sandbox policy in the client-side process. Paul Moore also chimed in in support of adding a new hook, with the rationale that the simplest change to the LSM hook interface has traditionally proven to be the most robust. [11] More details are on the Github issue at [6] and on the LKML at [9]. In a the discussion of the V2 review, started by Christian Brauner [10], we have further explored the approach of reusing the existing LSM hooks but still ended up leaning on the side of introducing a new hook, with Paul Moore and me (gnoack) arguing for that option. Further insights about the LSM hook were shared in the V3 review by Tingmao Wang [12], who spotted additional requirements due to the two approaches being merged into one patch set. The summary of that discussion is in [13]. === Related work: Scope Control for Pathname Unix Sockets The motivation for this patch is the same as in Tingmao Wang's patch set for "scoped" control for pathname Unix sockets [2], originally proposed in the Github feature request [5]. In [14], we have settled on the decision to merge the two patch sets into this one, whose primary way of controlling connect(2) is LANDLOCK_ACCESS_FS_RESOLVE_UNIX, but where this flag additionally has the semantics of only restricting this unix(7) IPC *outwards* of the created Landlock domain, in line with the logic that exists for the existing "scoped" flags already. By having LANDLOCK_ACCESS_FS_RESOLVE_UNIX implement "scoping" semantics, we can avoid introducing two separate interacting flags for now, but we retain the option of introducing LANDLOCK_SCOPE_PATHNAME_UNIX_SOCKET at a later point in time, should such a flag be needed to express additional rules. == Credits The feature was originally suggested by Jann Horn in [7]. Tingmao Wang and Demi Marie Obenour have taken the initiative to revive this discussion again in [1], [4] and [5]. Tingmao Wang has sent the patch set for the scoped access control for pathname Unix sockets [2] and has contributed substantial insights during the code review, shaping the form of the LSM hook and agreeing to merge the pathname and scoped-flag patch sets. Justin Suess has sent the patch for the LSM hook in [8] and subsequently through this patch set. Christian Brauner and Paul Moore have contributed to the design of the new LSM hook, discussing the tradeoffs in [10]. Ryan Sullivan has started on an initial implementation and has brought up relevant discussion points on the Github issue at [4]. As maintainer of Landlock, Mickaël Salaün has done the main review so far and particularly pointed out ways in which the UNIX connect() patch sets interact with each other and what we need to look for with regards to UAPI consistency as Landlock evolves. [1] https://lore.kernel.org/landlock/515ff0f4-2ab3-46de-8d1e-5c66a93c6ede@gmail.com/ [2] Tingmao Wang's "Implement scope control for pathname Unix sockets" https://lore.kernel.org/all/cover.1767115163.git.m@maowtm.org/ [3] https://lore.kernel.org/all/20251230.bcae69888454@gnoack.org/ [4] Github issue for FS-based control for named Unix sockets: https://github.com/landlock-lsm/linux/issues/36 [5] Github issue for scope-based restriction of named Unix sockets: https://github.com/landlock-lsm/linux/issues/51 [6] https://github.com/landlock-lsm/linux/issues/36#issuecomment-2950632277 [7] https://lore.kernel.org/linux-security-module/CAG48ez3NvVnonOqKH4oRwRqbSOLO0p9djBqgvxVwn6gtGQBPcw@mail.gmail.com/ [8] Patch for the LSM hook: https://lore.kernel.org/all/20251231213314.2979118-1-utilityemal77@gmail.com/ [9] https://lore.kernel.org/all/20260108.64bd7391e1ae@gnoack.org/ [10] https://lore.kernel.org/all/20260113-kerngesund-etage-86de4a21da24@brauner/ [11] https://lore.kernel.org/all/CAHC9VhQHZCe0LMx4xzSo-h1SWY489U4frKYnxu4YVrcJN3x7nA@mail.gmail.com/ [12] https://lore.kernel.org/all/e6b6b069-384c-4c45-a56b-fa54b26bc72a@maowtm.org/ [13] https://lore.kernel.org/all/aYMenaSmBkAsFowd@google.com/ [14] https://lore.kernel.org/all/20260205.Kiech3gupee1@digikod.net/ --- == Older versions of this patch set V1: https://lore.kernel.org/all/20260101134102.25938-1-gnoack3000@gmail.com/ V2: https://lore.kernel.org/all/20260110143300.71048-2-gnoack3000@gmail.com/ V3: https://lore.kernel.org/all/20260119203457.97676-2-gnoack3000@gmail.com/ Changes in V4: Since this version, this patch set subsumes the scoping semantics from Tingmao Wang's "Scope Control" patch set [2], per discussion with Tingmao Wang and Mickaël Salaün in [14] and in the thread leading up to it. Now, LANDLOCK_SCOPE_PATHNAME_UNIX_SOCKET only restricts connect(2) and sendmsg(2) *outwards* of the domain where it is restricted, *with the same semantics as a "scoped" flag*. * Implement a layer-mask based version of domain_is_scoped(): unmask_scoped_access(). Rationale: domain_is_scoped() returns early, which we can't do in the layer masks based variant. The two variants are similar enough. * LSM hook: Replace 'type' argument with 'sk' argument, per discussion in [12] and [13]. * Bump ABI version to 9 (pessimistically assuming that we won't make it for 7.0) * Documentation fixes in header file and in Documentation/ * selftests: more test variants, now also parameterizing whether the server socket gets created within the Landlock domain or before that * selftests: use EXPECT_EQ() for test cleanup Changes in V3: * LSM hook: rename it to security_unix_find() (Justin Suess) (resolving the previously open question about the LSM hook name) Related discussions: https://lore.kernel.org/all/20260112.Wufar9coosoo@digikod.net/ https://lore.kernel.org/all/CAHC9VhSRiHwLEWfFkQdPEwgB4AXKbXzw_+3u=9hPpvUTnu02Bg@mail.gmail.com/ * Reunite the three UNIX resolving access rights back into one (resolving the previously open question about the access right structuring) Related discussion: https://lore.kernel.org/all/20260112.Wufar9coosoo@digikod.net/) * Sample tool: Add new UNIX lookup access rights to ACCESS_FILE Changes in V2: * Send Justin Suess's LSM hook patch together with the Landlock implementation * LSM hook: Pass type and flags parameters to the hook, to make the access right more generally usable across LSMs, per suggestion from Paul Moore (Implemented by Justin) * Split the access right into the three types of UNIX domain sockets: SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET. * selftests: More exhaustive tests. * Removed a minor commit from V1 which adds a missing close(fd) to a test (it is already in the mic-next branch) Günther Noack (5): landlock: Control pathname UNIX domain socket resolution by path samples/landlock: Add support for named UNIX domain socket restrictions landlock/selftests: Test named UNIX domain socket restrictions landlock: Document FS access right for pathname UNIX sockets landlock: Document design rationale for scoped access rights Justin Suess (1): lsm: Add LSM hook security_unix_find Documentation/security/landlock.rst | 38 ++ Documentation/userspace-api/landlock.rst | 16 +- include/linux/lsm_hook_defs.h | 5 + include/linux/security.h | 11 + include/uapi/linux/landlock.h | 10 + net/unix/af_unix.c | 9 + samples/landlock/sandboxer.c | 15 +- security/landlock/access.h | 11 +- security/landlock/audit.c | 1 + security/landlock/fs.c | 107 ++++- security/landlock/limits.h | 2 +- security/landlock/syscalls.c | 2 +- security/security.c | 20 + tools/testing/selftests/landlock/base_test.c | 2 +- tools/testing/selftests/landlock/fs_test.c | 386 ++++++++++++++++++- 15 files changed, 608 insertions(+), 27 deletions(-) -- 2.52.0