Date: Mon, 4 Nov 2019 18:21:46 +0100 From: Mickaël Salaün <mic@...ikod.net> To: linux-kernel@...r.kernel.org Cc: Mickaël Salaün <mic@...ikod.net>, Alexei Starovoitov <ast@...nel.org>, Andy Lutomirski <luto@...capital.net>, Casey Schaufler <casey@...aufler-ca.com>, Daniel Borkmann <daniel@...earbox.net>, David Drysdale <drysdale@...gle.com>, Florent Revest <revest@...omium.org>, James Morris <jmorris@...ei.org>, Jann Horn <jann@...jh.net>, John Johansen <john.johansen@...onical.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, KP Singh <kpsingh@...omium.org>, Michael Kerrisk <mtk.manpages@...il.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>, Paul Moore <paul@...l-moore.com>, Sargun Dhillon <sargun@...gun.me>, "Serge E . Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>, Stephen Smalley <sds@...ho.nsa.gov>, Tejun Heo <tj@...nel.org>, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, Tycho Andersen <tycho@...ho.ws>, Will Drewry <wad@...omium.org>, bpf@...r.kernel.org, kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-security-module@...r.kernel.org, netdev@...r.kernel.org Subject: [PATCH bpf-next v13 7/7] landlock: Add user and kernel documentation for Landlock This documentation can be built with the Sphinx framework. Signed-off-by: Mickaël Salaün <mic@...ikod.net> Cc: Alexei Starovoitov <ast@...nel.org> Cc: Andy Lutomirski <luto@...capital.net> Cc: Daniel Borkmann <daniel@...earbox.net> Cc: James Morris <jmorris@...ei.org> Cc: Jonathan Corbet <corbet@....net> Cc: Kees Cook <keescook@...omium.org> Cc: Serge E. Hallyn <serge@...lyn.com> Cc: Will Drewry <wad@...omium.org> --- Changes since v12: * enhance the "unprivileged use" explanation * add more explanation about the domain/credentials inheritance * update and add self-reference Sphinx links * more clearly explain the capability-based security principles for program context Changes since v11: * cosmetic improvements Changes since v10: * replace the filesystem hooks with the ptrace one * remove the triggers * update example * add documenation for Landlock domains and seccomp interaction * reference more kernel documenation (e.g. LSM hooks) Changes since v9: * update with expected attach type and expected attach triggers Changes since v8: * remove documentation related to chaining and tagging according to this patch series Changes since v7: * update documentation according to the Landlock revamp Changes since v6: * add a check for ctx->event * rename BPF_PROG_TYPE_LANDLOCK to BPF_PROG_TYPE_LANDLOCK_RULE * rename Landlock version to ABI to better reflect its purpose and add a dedicated changelog section * update tables * relax no_new_privs recommendations * remove ABILITY_WRITE related functions * reword rule "appending" to "prepending" and explain it * cosmetic fixes Changes since v5: * update the rule hierarchy inheritance explanation * briefly explain ctx->arg2 * add ptrace restrictions * explain EPERM * update example (subtype) * use ":manpage:" --- Documentation/security/index.rst | 1 + Documentation/security/landlock/index.rst | 22 +++ Documentation/security/landlock/kernel.rst | 166 +++++++++++++++++++++ Documentation/security/landlock/user.rst | 153 +++++++++++++++++++ 4 files changed, 342 insertions(+) create mode 100644 Documentation/security/landlock/index.rst create mode 100644 Documentation/security/landlock/kernel.rst create mode 100644 Documentation/security/landlock/user.rst diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst index fc503dd689a7..4d213e76ddf4 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -15,3 +15,4 @@ Security Documentation self-protection siphash tpm/index + landlock/index diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst new file mode 100644 index 000000000000..1eced757b05d --- /dev/null +++ b/Documentation/security/landlock/index.rst @@ -0,0 +1,22 @@ +========================================= +Landlock LSM: programmatic access control +========================================= + +:Author: Mickaël Salaün + +Landlock is a stackable Linux Security Module (LSM) that makes it possible to +create security sandboxes, programmable access-controls or safe endpoint +security agents. This kind of sandbox is expected to help mitigate the +security impact of bugs or unexpected/malicious behaviors in user-space +applications. The current version allows only a process with the global +CAP_SYS_ADMIN capability to create such sandboxes but the ultimate goal of +Landlock is to empower any process, including unprivileged ones, to securely +restrict themselves. Landlock is inspired by seccomp-bpf but instead of +filtering syscalls and their raw arguments, a Landlock rule can inspect the use +of kernel objects like processes and hence make a decision according to the +kernel semantic. + +.. toctree:: + + user + kernel diff --git a/Documentation/security/landlock/kernel.rst b/Documentation/security/landlock/kernel.rst new file mode 100644 index 000000000000..ec0109b17e6f --- /dev/null +++ b/Documentation/security/landlock/kernel.rst @@ -0,0 +1,166 @@ +============================== +Landlock: kernel documentation +============================== + +eBPF properties +=============== + +To get an expressive language while still being safe and small, Landlock is +based on eBPF. Landlock should be usable by untrusted processes and must +therefore expose a minimal attack surface. The eBPF bytecode is minimal, +powerful, widely used and designed to be used by untrusted applications. Thus, +reusing the eBPF support in the kernel enables a generic approach while +minimizing new code. + +An eBPF program has access to an eBPF context containing some fields used to +inspect the current object. These arguments may be used directly (e.g. raw +value) or passed to helper functions according to their types (e.g. pointer). +It is then possible to do complex access checks without race conditions or +inconsistent evaluation (i.e. `incorrect mirroring of the OS code and state +<https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_). + +A Landlock hook describes a particular access type. For now, there is one hook +dedicated to ptrace related operations: ``BPF_LANDLOCK_PTRACE``. A Landlock +program is tied to one hook. This makes it possible to statically check +context accesses, potentially performed by such program, and hence prevents +kernel address leaks and ensure the right use of hook arguments with eBPF +functions. Any user can add multiple Landlock programs per Landlock hook. +They are stacked and evaluated one after the other, starting from the most +recent program, as seccomp-bpf does with its filters. Underneath, a hook is an +abstraction over a set of LSM hooks. + + +Guiding principles +================== + +Unprivileged use +---------------- + +* As far as possible, Landlock helpers and contexts should be *designed* to be + usable by unprivileged programs while following the system security policy + enforced by other access control mechanisms (e.g. DAC, LSM). Indeed, a + Landlock program shall not interfere with other access-controls enforced on + the system. + +Because one of the Landlock's goal is to create scoped access-control (i.e. +sandboxing), it makes sense to make it possible to have access-control-safe +programs. This enables to avoid unneeded security risks when writing a +security policy. We should also keep in mind that a Landlock program may be +written and loaded in the kernel by a trusted process, but applied by a +non-root (and possibly malicious) process to sandbox itself e.g., using a +sandboxer service. This sandboxed process must not be able to leverage one of +the Landlock program applied on itself to do a privilege escalation nor to +infer data that should not be accessible otherwise (i.e. side-channels). + +However, when justified, it should be possible to have dedicated +privileged-only program types e.g., to make a security decision based on +properties inaccessible by unprivileged processes, or to log actions with +additional metadata. As explained above, these properties should not be +inferable from the enforced access-control. Care must be taken to not only +focus on these programs' context or helpers to avoid putting everything in a +root-only realm (cf. `CAP_SYS_ADMIN: the new root +<https://lwn.net/Articles/486306/>`_). + +It should be noted that ``CAP_SYS_ADMIN`` is currently required for loading and +for enforcing any Landlock programs, but more fine-grained rights may be +discussed in the future. + + +Landlock hook and context +------------------------- + +* A Landlock hook shall be focused on access control on kernel objects instead + of syscall filtering (i.e. syscall arguments), which is the purpose of + seccomp-bpf. +* A Landlock context provided by a hook shall express the minimal and more + generic interface to control an access for a kernel object. This may be + implemented with kernel pointers used as security capabilities (i.e. + unforgeable token enabling actions on an object according to a set of + rights). +* A hook shall guaranty that all the BPF function calls from a program are + safe. Thus, the related Landlock context arguments shall always be of the + same type for a particular hook. For example, a network hook could share + helpers with a file hook because of UNIX socket. However, the same helpers + may not be compatible for a file system handle and a net handle. +* Multiple hooks may use the same context interface. + + +Landlock helpers +---------------- + +* Landlock helpers shall be as generic as possible while at the same time being + as simple as possible and following the syscall creation principles (cf. + :doc:`/process/adding-syscalls`). +* The only behavior change allowed on a helper is to fix a (logical) bug to + match the initial semantic. +* Helpers shall be reentrant, i.e. only take inputs from arguments (e.g. from + the BPF context), to enable a hook to use a cache. Future program options + might change this cache behavior. +* It is quite easy to add new helpers to extend Landlock. The main concern + should be about the possibility to leak information from the kernel that may + not be accessible otherwise (i.e. side-channel attack). + + +Landlock domain +=============== + +A Landlock domain is a set of eBPF programs. There is a list for each +different program types that can be run on a specific Landlock hook (e.g. +ptrace). A domain is tied to a set of subjects (i.e. tasks). + +A Landlock program should not try (nor be able) to infer which subject is +currently enforced, but to have a unique security policy for all subjects tied +to the same domain. This make the reasoning much easier and help avoid +pitfalls. + +.. kernel-doc:: security/landlock/common.h + :functions: landlock_domain + +.. kernel-doc:: security/landlock/domain_manage.c + :functions: landlock_prepend_prog + + +Adding a Landlock program with seccomp +-------------------------------------- + +The :manpage:`seccomp(2)` syscall can be used with the +``SECCOMP_PREPEND_LANDLOCK_PROG`` operation to prepend a Landlock program to +the current task's domain. + +.. kernel-doc:: security/landlock/domain_syscall.c + :functions: landlock_seccomp_prepend_prog + + +Running a list of Landlock programs +----------------------------------- + +.. kernel-doc:: security/landlock/bpf_run.c + :functions: landlock_access_denied + + +LSM hooks +========= + +.. kernel-doc:: security/landlock/hooks_ptrace.c + :functions: hook_ptrace_access_check + +.. kernel-doc:: security/landlock/hooks_ptrace.c + :functions: hook_ptrace_traceme + + +Questions and answers +===================== + +Why a program does not return an errno or a kill code? +------------------------------------------------------ + +seccomp filters can return multiple kind of code, including an errno value or a +kill signal, which may be convenient for access control. Those return codes +are hardwired in the userland ABI. Instead, Landlock's approach is to return a +bitmask to allow or deny an action, which is much simpler and more generic. +Moreover, we do not really have a choice because, unlike to seccomp, Landlock +programs are not enforced at the syscall entry point but may be executed at any +point in the kernel (through LSM hooks) where an errno return code may not make +sense. However, with this simple ABI and with the ability to call helpers, +Landlock may gain features similar to seccomp-bpf in the future while being +compatible with previous programs. diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst new file mode 100644 index 000000000000..ef48e7752f1b --- /dev/null +++ b/Documentation/security/landlock/user.rst @@ -0,0 +1,153 @@ +================================= +Landlock: userspace documentation +================================= + +Landlock programs +================= + +eBPF programs are used to create security programs. They are contained and can +call only a whitelist of dedicated functions. Moreover, they can only loop +under strict conditions, which protects from denial of service. More +information on BPF can be found in :doc:`/bpf/index`. + + +Writing a program +----------------- + +To enforce a security policy, a thread first needs to create a Landlock +program. The easiest way to write an eBPF program depicting a security program +is to write it in the C language. As described in `samples/bpf/README.rst`_, +LLVM can compile such programs. A simple eBPF program can also be written by +hand has done in `tools/testing/selftests/landlock/`_. + +Once the eBPF program is created, the next step is to create the metadata +describing the Landlock program. This metadata includes an expected attach +type which contains the hook type to which the program is tied. + +A hook is a policy decision point which exposes the same context type for +each program evaluation. + +A Landlock hook describes the kind of kernel object for which a program will be +triggered to allow or deny an action. For example, the hook +``BPF_LANDLOCK_PTRACE`` can be triggered every time a landlocked thread +performs a set of action related to debugging (cf. :manpage:`ptrace(2)`) or if +the kernel needs to know if a process manipulation requested by something else +is legitimate. + +The next step is to fill a :c:type:`struct bpf_load_program_attr +<bpf_load_program_attr>` with ``BPF_PROG_TYPE_LANDLOCK_HOOK``, the expected +attach type and other BPF program metadata. This bpf_attr must then be passed +to the :manpage:`bpf(2)` syscall alongside the ``BPF_PROG_LOAD`` command. If +everything is deemed correct by the kernel, the thread gets a file descriptor +referring to this program. + +In the following code, the `insn` variable is an array of BPF instructions +which can be extracted from an ELF file as is done in bpf_load_file() from +`samples/bpf/bpf_load.c`_. + +.. code-block:: c + + int prog_fd; + struct bpf_load_program_attr load_attr; + + memset(&load_attr, 0, sizeof(struct bpf_load_program_attr)); + load_attr.prog_type = BPF_PROG_TYPE_LANDLOCK_HOOK; + load_attr.expected_attach_type = BPF_LANDLOCK_PTRACE; + load_attr.insns = insns; + load_attr.insns_cnt = sizeof(insn) / sizeof(struct bpf_insn); + load_attr.license = "GPL"; + + prog_fd = bpf_load_program_xattr(&load_attr, log_buf, log_buf_sz); + if (prog_fd == -1) + exit(1); + + +Enforcing a program +------------------- + +Once the Landlock program has been created or received (e.g. through a UNIX +socket), the thread willing to sandbox itself (and its future children) should +perform the following two steps. + +The thread should first request to never be allowed to get new privileges with +a call to :manpage:`prctl(2)` and the ``PR_SET_NO_NEW_PRIVS`` option. More +information can be found in :doc:`/userspace-api/no_new_privs`. + +.. code-block:: c + + if (prctl(PR_SET_NO_NEW_PRIVS, 1, NULL, 0, 0)) + exit(1); + +A thread can apply a program to itself by using the :manpage:`seccomp(2)` +syscall. The operation is ``SECCOMP_PREPEND_LANDLOCK_PROG``, the flags must be +empty and the `args` argument must point to a valid Landlock program file +descriptor. + +.. code-block:: c + + if (seccomp(SECCOMP_PREPEND_LANDLOCK_PROG, 0, &fd)) + exit(1); + +If the syscall succeeds, the program is now enforced on the calling thread and +will be enforced on all its subsequently created children of the thread as +well. Once a thread is landlocked, there is no way to remove this security +policy, only stacking more restrictions is allowed. The program evaluation is +performed from the newest to the oldest. + +When a syscall ask for an action on a kernel object, if this action is denied, +then an ``EACCES`` errno code is returned through the syscall. + + +.. _inherited_programs: + +Inherited programs +------------------ + +Every new thread resulting from a :manpage:`clone(2)` inherits Landlock program +restrictions from its parent. This is similar to the seccomp inheritance (cf. +:doc:`/userspace-api/seccomp_filter`) or any other LSM dealing with task's +:manpage:`credentials(7)`. For instance, one process's thread may apply +Landlock programs to itself, but they will not be automatically applied to +other sibling threads (unlike POSIX thread credential changes, cf. +:manpage:`nptl(7)`). + + +Ptrace restrictions +------------------- + +A sandboxed process has less privileges than a non-sandboxed process and must +then be subject to additional restrictions when manipulating another process. +To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target +process, a sandboxed process should have a subset of the target process +programs. This security policy can easily be implemented like in +`tools/testing/selftests/landlock/test_ptrace.c`_. + + +Landlock structures and constants +================================= + +Contexts +-------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_context_ptrace + + +Return types +------------ + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_ret + + +Additional documentation +======================== + +See https://landlock.io + + +.. Links +.. _samples/bpf/README.rst: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/bpf/README.rst +.. _tools/testing/selftests/landlock/: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/ +.. _samples/bpf/bpf_load.c: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/bpf/bpf_load.c +.. _tools/testing/selftests/landlock/test_ptrace.c: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/test_ptrace.c -- 2.23.0
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