[PATCH v11 12/12] landlock: Document Landlock's network support
Konstantin Meskhidze (A)
konstantin.meskhidze at huawei.com
Mon Jun 19 14:25:20 UTC 2023
6/13/2023 10:56 PM, Mickaël Salaün пишет:
> Thanks Günther, I agree with your review.
Thanks Günther, I will fix documentation.
>
> 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
>>
> .
More information about the Linux-security-module-archive
mailing list