Re: [PATCH v8 12/12] landlock: Document Landlock's network support

["Konstantin Meskhidze (A)" <konstantin.meskhidze@huawei.com>]

11/17/2022 9:44 PM, Mickaël Salaün пишет:
> 
> On 21/10/2022 17:26, Konstantin Meskhidze wrote:
>> Describes network access rules for TCP sockets. Adds network access
>> example in the tutorial. Points out AF_UNSPEC socket family behaviour.
>> Adds kernel configuration support for network.
>> 
>> Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
>> ---
>> 
>> 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 | 72 +++++++++++++++++++-----
>>   1 file changed, 59 insertions(+), 13 deletions(-)
>> 
>> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
>> index d8cd8cd9ce25..d0610ec9ce05 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.
>> 
>> @@ -30,18 +30,20 @@ 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.
>> +rights`_.  Since ABI version 4 a port data appears with related network actions
>> +for TCP socket families.  A set of rules is aggregated in a ruleset, which
>> +can then restrict the thread enforcing it, and its future children.
>> 
>>   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 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.
>> +the need to be explicit about the denied-by-default access rights.  Also ruleset
>> +will have network rules for specific ports, so it should handle network actions.
>> 
>>   .. code-block:: c
>> 
>> @@ -62,6 +64,9 @@ 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 +75,18 @@ 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,
> 
> This is a bad rebase.

   Sorry. Did not get it.
> 
> 
>> +third and fourth version of the ABI.
>> 
>>   .. code-block:: c
>> 
>>       int abi;
>> 
>> +    #define ACCESS_NET_BIND_CONNECT ( \
>> +    LANDLOCK_ACCESS_NET_BIND_TCP | \
>> +    LANDLOCK_ACCESS_NET_CONNECT_TCP)
> 
> Please add a 4-spaces prefix for these two lines.

   Like this??
	#define ACCESS_NET_BIND_CONNECT ( \
             LANDLOCK_ACCESS_NET_BIND_TCP | \
             LANDLOCK_ACCESS_NET_CONNECT_TCP)
> 
> 
>> +
>>       abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
>>       if (abi < 0) {
>>           /* Degrades gracefully if Landlock is not handled. */
>> @@ -92,6 +101,9 @@ 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 &= ~ACCESS_NET_BIND_CONNECT;
>>       }
>> 
>>   This enables to create an inclusive ruleset that will contain our rules.
>> @@ -143,8 +155,22 @@ 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.
>> 
>> +For network part we can add number of rules containing a port number and actions
>> +that a process is allowed to do for certian ports.
> 
> For the network access-control, we can add a set of rules that allow to
> use a port number for a specific action.
> 
   Ok. Will be fixed.
>> +
>> +.. code-block:: c
>> +
>> +    struct landlock_net_service_attr net_service = {
>> +        .allowed_access = LANDLOCK_ACCESS_NET_BIND_TCP,
>> +        .port = 8080,
>> +    };
>> +
>> +    err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_SERVICE,
>> +                            &net_service, 0);
>> +
>>   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

   Ok. Thanks.

>> +denying all other handled accesses for the filesystem.  The ruleset also contains
>> +a rule allowing to bind current proccess to the port 8080.  The next step is to
> 
> 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 TCP binding on port 8080.
> 
  Got it. Thanks.
> 
>>   restrict the current thread from gaining more privileges (e.g. thanks to a SUID
>>   binary).
>> 
>> @@ -296,6 +322,13 @@ not.  It is also possible to pass such file descriptors between processes,
>>   keeping their Landlock properties, even when these processes do not have an
>>   enforced Landlock ruleset.
>> 
>> +AF_UNSPEC socket family
>> +-----------------------
>> +
>> +Sockets of AF_UNSPEC family types are treated as AF_INET(IPv4) socket for bind()
> 
> ``AF_UNSPEC`` sockets are handled as ``AF_INET`` sockets for the TCP
> binding action. However, connecting an ``AF_UNSPEC`` socket is always
> denied if at least one Landlock domain handles
> ``LANDLOCK_NET_CONNECT_TCP``.  This restriction protects against…
> 
> As commented in patch 8/12, I don't think this is the right approach
> anyway. It may then not be worth a dedicated section.
> 
   Ok. Let's discuss it in 8/12 patch.
> 
>> +action.  But connect() one is not allowed by Landlock for AF_UNSPEC sockets. This
>> +logic prevents from disconnecting already connected sockets.
>> +
>>   Compatibility
>>   =============
>> 
>> @@ -355,7 +388,7 @@ Access rights
>>   -------------
>> 
>>   .. kernel-doc:: include/uapi/linux/landlock.h
>> -    :identifiers: fs_access
>> +    :identifiers: fs_access net_access
>> 
>>   Creating a new ruleset
>>   ----------------------
>> @@ -374,6 +407,7 @@ 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 +485,13 @@ 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
>> +sockets' bind() and connect() actions for specific ports allowing processes
>> +to establish restricted connections.
> 
> it is now possible to restrict TCP bind and connect actions to only a
> set of allowed ports.

  Got it. Thanks.
> 
>> +
>>   .. _kernel_support:
>> 
>>   Kernel support
>> @@ -469,6 +510,11 @@ 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`).
> 
> Please use double backquotes everywhere, cf.
> https://git.kernel.org/torvalds/c/2fff00c81d4c
> 
   Ok. I will check.
> 
>> +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
>> 
> .