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Message-ID: <2ced8fc8-79a6-b0fb-70fe-6716fae92aa7@ssi.gouv.fr>
Date: Thu, 1 Aug 2019 19:03:12 +0200
From: Mickaël Salaün <mickael.salaun@....gouv.fr>
To: Randy Dunlap <rdunlap@...radead.org>, 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>, 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
Thanks for this spelling fixes. Some comments:
On 31/07/2019 03:53, Randy Dunlap wrote:
> 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>?
Indeed, a typo was inserted with a rewrite of this part.
>
>> +
>> +.. 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.
What about this:
To be allowed to trace a process (using :manpage:`ptrace(2)`), a
landlocked tracer process must only be constrained by a subset (possibly
empty) of the Landlock programs which are also applied to the tracee.
This ensure that the tracer has less or the same constraints than the
tracee, hence protecting against privilege escalation.
>
>> +
>> +
>> +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
>>
>
>
--
Mickaël Salaün
ANSSI/SDE/ST/LAM
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