[kernel-hardening] Re: [PATCH v5 10/10] landlock: Add user and kernel documentation for Landlock

["Mickaël Salaün" <mic@digikod.net>]

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On 22/02/2017 06:21, Andy Lutomirski wrote:
> On Tue, Feb 21, 2017 at 5:26 PM, Mickaël Salaün <mic@digikod.net> wrote:
>> This documentation can be built with the Sphinx framework.
>>
>> Signed-off-by: Mickaël Salaün <mic@digikod.net>
>> Cc: Alexei Starovoitov <ast@kernel.org>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Cc: David S. Miller <davem@davemloft.net>
>> Cc: James Morris <james.l.morris@oracle.com>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Serge E. Hallyn <serge@hallyn.com>
> 
> 
>> +
>> +Writing a rule
>> +--------------
>> +
>> +To enforce a security policy, a thread first needs to create a Landlock rule.
>> +The easiest way to write an eBPF program depicting a security rule 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/rules/* can be used as examples.  The
>> +following example is a simple rule to forbid file creation, whatever syscall
>> +may be used (e.g. open, mkdir, link...).
>> +
>> +.. code-block:: c
>> +
>> +    static int deny_file_creation(struct landlock_context *ctx)
>> +    {
>> +        if (ctx->arg2 & LANDLOCK_ACTION_FS_NEW)
>> +            return 1;
>> +        return 0;
>> +    }
>> +
> 
> Would it make sense to define landlock_context (or at least a prefix
> thereof) in here?  Also, can't "arg2" have a better name?

arg2 is a generic name. Its meaning depends on the Landlock event, here
it is an action bitfield (FS event).

> 
> Can you specify what the return value means?  Are 0 and 1 the only
> choices?  Would "KILL" be useful?  How about "COREDUMP"?

This is explained thereafter and in the kernel Q&A section. I need to
briefly introduce that here.

> 
>> +File system action types
>> +------------------------
>> +
>> +Flags are used to express actions.  This makes it possible to compose actions
>> +and leaves room for future improvements to add more fine-grained action types.
>> +
>> +.. kernel-doc:: include/uapi/linux/bpf.h
>> +    :doc: landlock_action_fs
>> +
>> +.. flat-table:: FS action types availability
>> +
>> +    * - flags
>> +      - since
>> +
>> +    * - LANDLOCK_ACTION_FS_EXEC
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_WRITE
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_READ
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_NEW
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_GET
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_REMOVE
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_IOCTL
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_LOCK
>> +      - v1
>> +
>> +    * - LANDLOCK_ACTION_FS_FCNTL
>> +      - v1
> 
> What happens if you run an old program on a new kernel?  Can you get
> unexpected action types?

The old flags will still make sense, the new ones should be ignored by
the rule.

> 
>> +
>> +
>> +Ability types
>> +-------------
>> +
>> +The ability of a Landlock rule describes the available features (i.e. context
>> +fields and helpers).  This is useful to abstract user-space privileges for
>> +Landlock rules, which may not need all abilities (e.g. debug).  Only the
>> +minimal set of abilities should be used (e.g. disable debug once in
>> +production).
>> +
>> +
>> +.. kernel-doc:: include/uapi/linux/bpf.h
>> +    :doc: landlock_subtype_ability
>> +
>> +.. flat-table:: Ability types availability
>> +
>> +    * - flags
>> +      - since
>> +      - capability
>> +
>> +    * - LANDLOCK_SUBTYPE_ABILITY_WRITE
>> +      - v1
>> +      - CAP_SYS_ADMIN
>> +
>> +    * - LANDLOCK_SUBTYPE_ABILITY_DEBUG
>> +      - v1
>> +      - CAP_SYS_ADMIN
>> +
> 
> What do "WRITE" and "DEBUG" mean in this context?  I'm totally lost.
> 
> Hmm.  Reading below, "WRITE" seems to mean "modify state".  Would that
> be accurate?

That is correct, but handling a state in a safe way imply more than only
the ability to "write" outside bpfland (e.g. sequential execution).

> 
>> +
>> +Helper functions
>> +----------------
>> +
>> +See *include/uapi/linux/bpf.h* for functions documentation.
>> +
>> +.. flat-table:: Generic functions availability
>> +
> 
>> +
>> +    * - bpf_get_current_comm
>> +      - v1
>> +      - LANDLOCK_SUBTYPE_ABILITY_DEBUG
> 
> What would this be used for?

To get more information about the process which trigger an action?

> 
>> +    * - bpf_get_trace_printk
>> +      - v1
>> +      - LANDLOCK_SUBTYPE_ABILITY_DEBUG
>> +
> 
> This is different from the other DEBUG stuff in that it has side
> effects.  I wonder if it should have a different flag.

I think the debug flag is a clear warning to not ship a rule using this
ability. Maybe a sub-flag LANDLOCK_SUBTYPE_ABILITY_DEBUG_PRINT would fit
here?

 Mickaël


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