On Tue, Jun 6, 2023 at 7:09 AM Günther Noack <gnoack@xxxxxxxxxx> 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@xxxxxxxxxx> > > --- > > > > Changes since v10: > > * Fixes documentaion as Mickaёl suggested: > > https://lore.kernel.org/linux-security-module/ec23be77-566e-c8fd-179e-f50e025ac2cf@xxxxxxxxxxx/ > > > > 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@xxxxxxxxxxx/ > > > > 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, Remote port or local 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 > > -- > Sent using Mutt 🐕 Woof Woof
