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Diffstat (limited to 'Documentation/userspace-api/landlock.rst')
| -rw-r--r-- | Documentation/userspace-api/landlock.rst | 67 |
1 files changed, 60 insertions, 7 deletions
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index cec780c2f497..d8cd8cd9ce25 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -8,7 +8,7 @@ Landlock: unprivileged access control ===================================== :Author: Mickaël Salaün -:Date: September 2022 +: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 @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights. LANDLOCK_ACCESS_FS_MAKE_FIFO | LANDLOCK_ACCESS_FS_MAKE_BLOCK | LANDLOCK_ACCESS_FS_MAKE_SYM | - LANDLOCK_ACCESS_FS_REFER, + LANDLOCK_ACCESS_FS_REFER | + LANDLOCK_ACCESS_FS_TRUNCATE, }; Because we may not know on which kernel version an application will be @@ -69,16 +70,28 @@ 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`` access right which is only supported -starting with the second version of the ABI. +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. .. code-block:: c int abi; abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); - if (abi < 2) { + if (abi < 0) { + /* Degrades gracefully if Landlock is not handled. */ + perror("The running kernel does not enable to use Landlock"); + return 0; + } + switch (abi) { + case 1: + /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; + __attribute__((fallthrough)); + case 2: + /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; } This enables to create an inclusive ruleset that will contain our rules. @@ -127,8 +140,8 @@ descriptor. It may also be required to create rules following the same logic as explained for the ruleset creation, by filtering access rights according to the Landlock -ABI version. In this example, this is not required because -``LANDLOCK_ACCESS_FS_REFER`` is not allowed by any rule. +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 @@ -252,6 +265,37 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target process, a sandboxed process should have a subset of the target process rules, which means the tracee must be in a sub-domain of the tracer. +Truncating files +---------------- + +The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and +``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes +overlap in non-intuitive ways. It is recommended to always specify both of +these together. + +A particularly surprising example is :manpage:`creat(2)`. The name suggests +that this system call requires the rights to create and write files. However, +it also requires the truncate right if an existing file under the same name is +already present. + +It should also be noted that truncating files does not require the +``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)` +system call, this can also be done through :manpage:`open(2)` with the flags +``O_RDONLY | O_TRUNC``. + +When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE`` +right is associated with the newly created file descriptor and will be used for +subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavior is +similar to opening a file for reading or writing, where permissions are checked +during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and +:manpage:`write(2)` calls. + +As a consequence, it is possible to have multiple open file descriptors for the +same file, where one grants the right to truncate the file and the other does +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. + Compatibility ============= @@ -398,6 +442,15 @@ Starting with the Landlock ABI version 2, it is now possible to securely control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER`` access right. +File truncation (ABI < 3) +------------------------- + +File truncation could not be denied before the third Landlock ABI, so it is +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. + .. _kernel_support: Kernel support |