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landlock: Document Landlock's file truncation support

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Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.

Adapt the backwards compatibility example and discussion to remove the
truncation flag where needed.

Point out potential surprising behaviour related to truncate.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-12-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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Günther Noack 2022-10-18 20:22:16 +02:00 committed by Mickaël Salaün
parent faeb919766
commit ede2a34363
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67
Documentation/userspace-api/landlock.rst
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@ -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