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

Tue Jun 13 19:56:15 UTC 2023

Thanks Günther, I agree with your review.

On 06/06/2023 16:08, Günther Noack wrote:
> 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 <konstantin.meskhidze at huawei.com>
>> ---
>>
>> Changes since v10:
>> * Fixes documentaion as Mickaёl suggested:
>> https://lore.kernel.org/linux-security-module/ec23be77-566e-c8fd-179e-f50e025ac2cf@digikod.net/
>>
>> 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/9f354862-2bc3-39ea-92fd-53803d9bbc21@digikod.net/
>>
>> 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
> 