Date: Sun, 21 Jul 2019 23:31:16 +0200 From: Mickaël Salaün <mic@...ikod.net> To: linux-kernel@...r.kernel.org Cc: Mickaël Salaün <mic@...ikod.net>, Alexander Viro <viro@...iv.linux.org.uk>, Alexei Starovoitov <ast@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...capital.net>, Arnaldo Carvalho de Melo <acme@...nel.org>, Casey Schaufler <casey@...aufler-ca.com>, Daniel Borkmann <daniel@...earbox.net>, David Drysdale <drysdale@...gle.com>, "David S . Miller" <davem@...emloft.net>, "Eric W . Biederman" <ebiederm@...ssion.com>, 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>, 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>, Thomas Graf <tgraf@...g.ch>, Tycho Andersen <tycho@...ho.ws>, Will Drewry <wad@...omium.org>, kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-security-module@...r.kernel.org, netdev@...r.kernel.org Subject: [PATCH bpf-next v10 10/10] 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: David S. Miller <davem@...emloft.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> --- 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 | 20 +++ Documentation/security/landlock/kernel.rst | 99 ++++++++++++++ Documentation/security/landlock/user.rst | 147 +++++++++++++++++++++ 4 files changed, 267 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 aad6d92ffe31..32b4c1db2325 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -12,3 +12,4 @@ Security Documentation SCTP self-protection tpm/index + landlock/index diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst new file mode 100644 index 000000000000..d0af868d1582 --- /dev/null +++ b/Documentation/security/landlock/index.rst @@ -0,0 +1,20 @@ +========================================= +Landlock LSM: programmatic access control +========================================= + +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 files 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..7d1e06d544bf --- /dev/null +++ b/Documentation/security/landlock/kernel.rst @@ -0,0 +1,99 @@ +============================== +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 can be used directly (e.g. cookie) +or passed to helper functions according to their types (e.g. inode 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 two +hooks dedicated to filesystem related operations: LANDLOCK_HOOK_FS_PICK and +LANDLOCK_HOOK_FS_WALK. 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 +---------------- + +* Landlock helpers and context should be usable by any unprivileged and + untrusted program while following the system security policy enforced by + other access control mechanisms (e.g. DAC, LSM). + + +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. +* 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. + *Documentation/adding-syscalls.txt*). +* 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). + + +Questions and answers +===================== + +Why not create a custom hook for each kind of action? +----------------------------------------------------- + +Landlock programs can handle these checks. Adding more exceptions to the +kernel code would lead to more code complexity. A decision to ignore a kind of +action can and should be done at the beginning of a Landlock program. + + +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 +boolean 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..14c4f3b377bd --- /dev/null +++ b/Documentation/security/landlock/user.rst @@ -0,0 +1,147 @@ +================================ +Landlock: userland 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 *Documentation/networking/filter.txt*. + + +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. Files *samples/bpf/landlock1_kern.c* and those in +*tools/testing/selftests/landlock/* can be used as examples. + +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, and expected attach +triggers which identify the actions for which the program should be run. + +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_FS_PICK can be triggered every time a landlocked thread performs a +set of action related to the filesystem (e.g. open, read, write, mount...). +This actions are identified by the `triggers` bitfield. + +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_FS_PICK; + load_attr.expected_attach_triggers = LANDLOCK_TRIGGER_FS_PICK_OPEN; + 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 *Documentation/prctl/no_new_privs.txt*. + +.. 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 as +described in *Documentation/prctl/seccomp_filter.txt*. + + +Ptrace restrictions +------------------- + +A landlocked process has less privileges than a non-landlocked 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 landlocked process must have a subset of the target process programs. + + +Landlock structures and constants +================================= + +Hook types +---------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_hook_type + + +Contexts +-------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_ctx_fs_pick landlock_ctx_fs_walk landlock_ctx_fs_get + + +Triggers for fs_pick +-------------------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_triggers + + +Additional documentation +======================== + +See https://landlock.io -- 2.22.0
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