Date: Tue, 30 Jul 2019 18:53:02 -0700 From: Randy Dunlap <rdunlap@...radead.org> To: Mickaël Salaün <mic@...ikod.net>, linux-kernel@...r.kernel.org Cc: 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: Re: [PATCH bpf-next v10 10/10] landlock: Add user and kernel documentation for Landlock On 7/21/19 2:31 PM, Mickaël Salaün wrote: > 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/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 there are 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 ensures > +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 kinds > +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...). actions > +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*. A little confusing. Is there a mixup of <insn> and <insns>? > + > +.. 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, asks > +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. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Maybe that last statement is correct, but it seems to me that it is missing something. > + > + > +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 > -- ~Randy
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