Subject: Re: [PATCH v11 12/12] landlock: Document Landlock's network support

From: "Günther Noack" <[email-masked]>
Date: Tue, 6 Jun 2023 16:08:13 +0200

On Tue, May 16, 2023 at 12:13:39AM +0800, Konstantin Meskhidze wrote:
> Describe network access rules for TCP sockets. Add network access
> example in the tutorial. Add kernel configuration support for network.
> 
> Signed-off-by: Konstantin Meskhidze <[email-masked]>
> ---
> 
> Changes since v10:
> * Fixes documentaion as Mickaёl suggested:
> https://lore.kernel.org/linux-security-module/[email-masked]/
> 
> Changes since v9:
> * Minor refactoring.
> 
> Changes since v8:
> * Minor refactoring.
> 
> Changes since v7:
> * Fixes documentaion logic errors and typos as Mickaёl suggested:
> https://lore.kernel.org/netdev/[email-masked]/
> 
> Changes since v6:
> * Adds network support documentaion.
> 
> ---
>  Documentation/userspace-api/landlock.rst | 83 ++++++++++++++++++------
>  1 file changed, 62 insertions(+), 21 deletions(-)
> 
> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index f6a7da21708a..f185dbaa726a 100644
> --- a/Documentation/userspace-api/landlock.rst
> +++ b/Documentation/userspace-api/landlock.rst
> @@ -11,10 +11,10 @@ Landlock: unprivileged access control
>  :Date: October 2022
> 
>  The goal of Landlock is to enable to restrict ambient rights (e.g. global
> -filesystem access) for a set of processes.  Because Landlock is a stackable
> -LSM, it makes possible to create safe security sandboxes as new security layers
> -in addition to the existing system-wide access-controls. This kind of sandbox
> -is expected to help mitigate the security impact of bugs or
> +filesystem or network access) for a set of processes.  Because Landlock
> +is a stackable LSM, it makes possible to create safe security sandboxes as new
> +security layers in addition to the existing system-wide access-controls. This
> +kind of sandbox is expected to help mitigate the security impact of bugs or
>  unexpected/malicious behaviors in user space applications.  Landlock empowers
>  any process, including unprivileged ones, to securely restrict themselves.
> 
> @@ -28,20 +28,24 @@ appropriately <kernel_support>`.
>  Landlock rules
>  ==============
> 
> -A Landlock rule describes an action on an object.  An object is currently a
> -file hierarchy, and the related filesystem actions are defined with `access
> -rights`_.  A set of rules is aggregated in a ruleset, which can then restrict
> -the thread enforcing it, and its future children.
> +A Landlock rule describes an action on a kernel object.  Filesystem
> +objects can be defined with a file hierarchy.  Since the fourth ABI
> +version, TCP ports enable to identify inbound or outbound connections.
> +Actions on these kernel objects are defined according to `access
> +rights`_.  A set of rules is aggregated in a ruleset, which
> +can then restrict the thread enforcing it, and its future children.

I feel that this paragraph is a bit long-winded to read when the
additional networking aspect is added on top as well.  Maybe it would
be clearer if we spelled it out in a more structured way, splitting up
the filesystem/networking aspects?

Suggestion:

  A Landlock rule describes an action on an object which the process
  intends to perform.  A set of rules is aggregated in a ruleset,
  which can then restrict the thread enforcing it, and its future
  children.

  The two existing types of rules are:

  Filesystem rules
      For these rules, the object is a file hierarchy,
      and the related filesystem actions are defined with
      `filesystem access rights`.

  Network rules (since ABI v4)
      For these rules, the object is currently a TCP port,
      and the related actions are defined with `network access rights`.

Please note that the landlock(7) man page is in large parts using the
same phrasing as the kernel documentation.  It might be a good idea to
keep them in sync and structured similarly.  (On that mailing list,
the reviews are a bit more focused on good writing style.)

The same reasoning applies to the example below as well.  Explaining
multiple aspects of a thing in a single example can muddy the message,
let's try to avoid that.  But I can also see that if we had two
separate examples, a large part of the example would be duplicated.

>  Defining and enforcing a security policy
>  ----------------------------------------
> 
>  We first need to define the ruleset that will contain our rules.  For this
> -example, the ruleset will contain rules that only allow read actions, but write
> -actions will be denied.  The ruleset then needs to handle both of these kind of
> -actions.  This is required for backward and forward compatibility (i.e. the
> -kernel and user space may not know each other's supported restrictions), hence
> -the need to be explicit about the denied-by-default access rights.
> +example, the ruleset will contain rules that only allow filesystem read actions
> +and establish a specific TCP connection, but filesystem write actions
> +and other TCP actions will be denied.  The ruleset then needs to handle both of
> +these kind of actions.  This is required for backward and forward compatibility
> +(i.e. the kernel and user space may not know each other's supported
> +restrictions), hence the need to be explicit about the denied-by-default access
> +rights.

I think it became a bit long - I'd suggest to split it into multiple
paragraphs, one after "our rules." (in line with landlock(7)), and one
after "will be denied."

Maybe the long sentence "For this example, ..." in the middle
paragraph could also be split up in two, to make it more readable?  I
think the point of that sentence is really just to give a brief
overview over what ruleset we are setting out to write.

> 
>  .. code-block:: c
> 
> @@ -62,6 +66,9 @@ the need to be explicit about the denied-by-default access rights.
>              LANDLOCK_ACCESS_FS_MAKE_SYM |
>              LANDLOCK_ACCESS_FS_REFER |
>              LANDLOCK_ACCESS_FS_TRUNCATE,
> +        .handled_access_net =
> +            LANDLOCK_ACCESS_NET_BIND_TCP |
> +            LANDLOCK_ACCESS_NET_CONNECT_TCP,
>      };
> 
>  Because we may not know on which kernel version an application will be
> @@ -70,14 +77,18 @@ should try to protect users as much as possible whatever the kernel they are
>  using.  To avoid binary enforcement (i.e. either all security features or
>  none), we can leverage a dedicated Landlock command to get the current version
>  of the Landlock ABI and adapt the handled accesses.  Let's check if we should
> -remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE``
> -access rights, which are only supported starting with the second and third
> -version of the ABI.
> +remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE`` or
> +network access rights, which are only supported starting with the second,
> +third and fourth version of the ABI.

At some point it becomes too much to spell it out in one sentence; I'd recommend

  Let's check if we should remove access rights which are only supported
  in higher versions of the ABI.

> 
>  .. code-block:: c
> 
>      int abi;
> 
> +    #define ACCESS_NET_BIND_CONNECT ( \
> +        LANDLOCK_ACCESS_NET_BIND_TCP | \
> +        LANDLOCK_ACCESS_NET_CONNECT_TCP)
> +

This #define does not seem to be used? -- Drop it?

>      abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
>      if (abi < 0) {
>          /* Degrades gracefully if Landlock is not handled. */
> @@ -92,6 +103,11 @@ version of the ABI.
>      case 2:
>          /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
>          ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
> +    case 3:
> +        /* Removes network support for ABI < 4 */
> +        ruleset_attr.handled_access_net &=
> +            ~(LANDLOCK_ACCESS_NET_BIND_TCP |
> +              LANDLOCK_ACCESS_NET_CONNECT_TCP);
>      }
> 
>  This enables to create an inclusive ruleset that will contain our rules.
> @@ -143,10 +159,23 @@ for the ruleset creation, by filtering access rights according to the Landlock
>  ABI version.  In this example, this is not required because all of the requested
>  ``allowed_access`` rights are already available in ABI 1.
> 
> -We now have a ruleset with one rule allowing read access to ``/usr`` while
> -denying all other handled accesses for the filesystem.  The next step is to
> -restrict the current thread from gaining more privileges (e.g. thanks to a SUID
> -binary).
> +For network access-control, we can add a set of rules that allow to use a port
> +number for a specific action: HTTPS connections.
> +
> +.. code-block:: c
> +
> +    struct landlock_net_service_attr net_service = {
> +        .allowed_access = NET_CONNECT_TCP,
> +        .port = 443,
> +    };
> +
> +    err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_SERVICE,
> +                            &net_service, 0);
> +
> +The next step is to restrict the current thread from gaining more privileges
> +(e.g. through a SUID binary). We now have a ruleset with the first rule allowing
> +read access to ``/usr`` while denying all other handled accesses for the filesystem,
> +and a second rule allowing HTTPS connections.
> 
>  .. code-block:: c
> 
> @@ -355,7 +384,7 @@ Access rights
>  -------------
> 
>  .. kernel-doc:: include/uapi/linux/landlock.h
> -    :identifiers: fs_access
> +    :identifiers: fs_access net_access
> 
>  Creating a new ruleset
>  ----------------------
> @@ -374,6 +403,7 @@ Extending a ruleset
> 
>  .. kernel-doc:: include/uapi/linux/landlock.h
>      :identifiers: landlock_rule_type landlock_path_beneath_attr
> +                  landlock_net_service_attr
> 
>  Enforcing a ruleset
>  -------------------
> @@ -451,6 +481,12 @@ always allowed when using a kernel that only supports the first or second ABI.
>  Starting with the Landlock ABI version 3, it is now possible to securely control
>  truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
> 
> +Network support (ABI < 4)
> +-------------------------
> +
> +Starting with the Landlock ABI version 4, it is now possible to restrict TCP
> +bind and connect actions to only a set of allowed ports.
> +
>  .. _kernel_support:
> 
>  Kernel support
> @@ -469,6 +505,11 @@ still enable it by adding ``lsm=landlock,[...]`` to
>  Documentation/admin-guide/kernel-parameters.rst thanks to the bootloader
>  configuration.
> 
> +To be able to explicitly allow TCP operations (e.g., adding a network rule with
> +``LANDLOCK_ACCESS_NET_TCP_BIND``), the kernel must support TCP (``CONFIG_INET=y``).
> +Otherwise, sys_landlock_add_rule() returns an ``EAFNOSUPPORT`` error, which can
> +safely be ignored because this kind of TCP operation is already not possible.
> +
>  Questions and answers
>  =====================
> 
> --
> 2.25.1
> 

—Günther

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