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landlock: Fix grammar issues in documentation

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Improve user space and kernel documentation.

Signed-off-by: Daniel Burgener <dburgener@linux.microsoft.com>
Link: https://lore.kernel.org/r/20241015172647.2007644-1-dburgener@linux.microsoft.com
[mic: Extend commit message, reword ptrace restriction as discussed in
the thread]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
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Daniel Burgener 2024-10-15 13:26:46 -04:00 committed by Mickaël Salaün
parent 9803787a23
commit dad2f20715
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2 changed files with 41 additions and 42 deletions
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14
Documentation/security/landlock.rst
View File

@ -11,18 +11,18 @@ Landlock LSM: kernel documentation
Landlock's goal is to create scoped access-control (i.e. sandboxing). To
harden a whole system, this feature should be available to any process,
including unprivileged ones. Because such process may be compromised or
including unprivileged ones. Because such a process may be compromised or
backdoored (i.e. untrusted), Landlock's features must be safe to use from the
kernel and other processes point of view. Landlock's interface must therefore
expose a minimal attack surface.
Landlock is designed to be usable by unprivileged processes while following the
system security policy enforced by other access control mechanisms (e.g. DAC,
LSM). Indeed, a Landlock rule shall not interfere with other access-controls
enforced on the system, only add more restrictions.
LSM). A Landlock rule shall not interfere with other access-controls enforced
on the system, only add more restrictions.
Any user can enforce Landlock rulesets on their processes. They are merged and
evaluated according to the inherited ones in a way that ensures that only more
evaluated against inherited rulesets in a way that ensures that only more
constraints can be added.
User space documentation can be found here:
@ -43,7 +43,7 @@ Guiding principles for safe access controls
only impact the processes requesting them.
* Resources (e.g. file descriptors) directly obtained from the kernel by a
sandboxed process shall retain their scoped accesses (at the time of resource
acquisition) whatever process use them.
acquisition) whatever process uses them.
Cf. `File descriptor access rights`_.
Design choices
@ -71,7 +71,7 @@ the same results, when they are executed under the same Landlock domain.
Taking the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right as an example, it may be
allowed to open a file for writing without being allowed to
:manpage:`ftruncate` the resulting file descriptor if the related file
hierarchy doesn't grant such access right. The following sequences of
hierarchy doesn't grant that access right. The following sequences of
operations have the same semantic and should then have the same result:
* ``truncate(path);``
@ -81,7 +81,7 @@ Similarly to file access modes (e.g. ``O_RDWR``), Landlock access rights
attached to file descriptors are retained even if they are passed between
processes (e.g. through a Unix domain socket). Such access rights will then be
enforced even if the receiving process is not sandboxed by Landlock. Indeed,
this is required to keep a consistent access control over the whole system, and
this is required to keep access controls consistent over the whole system, and
this avoids unattended bypasses through file descriptor passing (i.e. confused
deputy attack).

69
Documentation/userspace-api/landlock.rst
View File

@ -10,11 +10,11 @@ Landlock: unprivileged access control
:Author: Mickaël Salaün
:Date: October 2024
The goal of Landlock is to enable to restrict ambient rights (e.g. global
The goal of Landlock is to enable restriction of ambient rights (e.g. global
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
is a stackable LSM, it makes it 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.
@ -86,8 +86,8 @@ to be explicit about the denied-by-default access rights.
LANDLOCK_SCOPE_SIGNAL,
};
Because we may not know on which kernel version an application will be
executed, it is safer to follow a best-effort security approach. Indeed, we
Because we may not know which kernel version an application will be executed
on, it is safer to follow a best-effort security approach. Indeed, we
should try to protect users as much as possible whatever the kernel they are
using.
@ -129,7 +129,7 @@ version, and only use the available subset of access rights:
LANDLOCK_SCOPE_SIGNAL);
}
This enables to create an inclusive ruleset that will contain our rules.
This enables the creation of an inclusive ruleset that will contain our rules.
.. code-block:: c
@ -219,42 +219,41 @@ If the ``landlock_restrict_self`` system call succeeds, the current thread is
now restricted and this policy will be enforced on all its subsequently created
children as well. Once a thread is landlocked, there is no way to remove its
security policy; only adding more restrictions is allowed. These threads are
now in a new Landlock domain, merge of their parent one (if any) with the new
ruleset.
now in a new Landlock domain, which is a merger of their parent one (if any)
with the new ruleset.
Full working code can be found in `samples/landlock/sandboxer.c`_.
Good practices
--------------
It is recommended setting access rights to file hierarchy leaves as much as
It is recommended to set access rights to file hierarchy leaves as much as
possible. For instance, it is better to be able to have ``~/doc/`` as a
read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to
``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy.
Following this good practice leads to self-sufficient hierarchies that do not
depend on their location (i.e. parent directories). This is particularly
relevant when we want to allow linking or renaming. Indeed, having consistent
access rights per directory enables to change the location of such directory
access rights per directory enables changing the location of such directories
without relying on the destination directory access rights (except those that
are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER``
documentation).
Having self-sufficient hierarchies also helps to tighten the required access
rights to the minimal set of data. This also helps avoid sinkhole directories,
i.e. directories where data can be linked to but not linked from. However,
i.e. directories where data can be linked to but not linked from. However,
this depends on data organization, which might not be controlled by developers.
In this case, granting read-write access to ``~/tmp/``, instead of write-only
access, would potentially allow to move ``~/tmp/`` to a non-readable directory
access, would potentially allow moving ``~/tmp/`` to a non-readable directory
and still keep the ability to list the content of ``~/tmp/``.
Layers of file path access rights
---------------------------------
Each time a thread enforces a ruleset on itself, it updates its Landlock domain
with a new layer of policy. Indeed, this complementary policy is stacked with
the potentially other rulesets already restricting this thread. A sandboxed
thread can then safely add more constraints to itself with a new enforced
ruleset.
with a new layer of policy. This complementary policy is stacked with any
other rulesets potentially already restricting this thread. A sandboxed thread
can then safely add more constraints to itself with a new enforced ruleset.
One policy layer grants access to a file path if at least one of its rules
encountered on the path grants the access. A sandboxed thread can only access
@ -265,7 +264,7 @@ etc.).
Bind mounts and OverlayFS
-------------------------
Landlock enables to restrict access to file hierarchies, which means that these
Landlock enables restricting access to file hierarchies, which means that these
access rights can be propagated with bind mounts (cf.
Documentation/filesystems/sharedsubtree.rst) but not with
Documentation/filesystems/overlayfs.rst.
@ -278,21 +277,21 @@ access to multiple file hierarchies at the same time, whether these hierarchies
are the result of bind mounts or not.
An OverlayFS mount point consists of upper and lower layers. These layers are
combined in a merge directory, result of the mount point. This merge hierarchy
may include files from the upper and lower layers, but modifications performed
on the merge hierarchy only reflects on the upper layer. From a Landlock
policy point of view, each OverlayFS layers and merge hierarchies are
standalone and contains their own set of files and directories, which is
different from bind mounts. A policy restricting an OverlayFS layer will not
restrict the resulted merged hierarchy, and vice versa. Landlock users should
then only think about file hierarchies they want to allow access to, regardless
of the underlying filesystem.
combined in a merge directory, and that merged directory becomes available at
the mount point. This merge hierarchy may include files from the upper and
lower layers, but modifications performed on the merge hierarchy only reflect
on the upper layer. From a Landlock policy point of view, all OverlayFS layers
and merge hierarchies are standalone and each contains their own set of files
and directories, which is different from bind mounts. A policy restricting an
OverlayFS layer will not restrict the resulted merged hierarchy, and vice versa.
Landlock users should then only think about file hierarchies they want to allow
access to, regardless of the underlying filesystem.
Inheritance
-----------
Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain
restrictions from its parent. This is similar to the seccomp inheritance (cf.
restrictions from its parent. This is similar to seccomp inheritance (cf.
Documentation/userspace-api/seccomp_filter.rst) or any other LSM dealing with
task's :manpage:`credentials(7)`. For instance, one process's thread may apply
Landlock rules to itself, but they will not be automatically applied to other
@ -311,8 +310,8 @@ Ptrace restrictions
A sandboxed process has less privileges than a non-sandboxed process and must
then be subject to additional restrictions when manipulating another process.
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.
process, a sandboxed process should have a superset of the target process's
access rights, which means the tracee must be in a sub-domain of the tracer.
IPC scoping
-----------
@ -322,7 +321,7 @@ interactions between sandboxes. Each Landlock domain can be explicitly scoped
for a set of actions by specifying it on a ruleset. For example, if a
sandboxed process should not be able to :manpage:`connect(2)` to a
non-sandboxed process through abstract :manpage:`unix(7)` sockets, we can
specify such restriction with ``LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET``.
specify such a restriction with ``LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET``.
Moreover, if a sandboxed process should not be able to send a signal to a
non-sandboxed process, we can specify this restriction with
``LANDLOCK_SCOPE_SIGNAL``.
@ -394,7 +393,7 @@ Backward and forward compatibility
Landlock is designed to be compatible with past and future versions of the
kernel. This is achieved thanks to the system call attributes and the
associated bitflags, particularly the ruleset's ``handled_access_fs``. Making
handled access right explicit enables the kernel and user space to have a clear
handled access rights explicit enables the kernel and user space to have a clear
contract with each other. This is required to make sure sandboxing will not
get stricter with a system update, which could break applications.
@ -606,9 +605,9 @@ Build time configuration
Landlock was first introduced in Linux 5.13 but it must be configured at build
time with ``CONFIG_SECURITY_LANDLOCK=y``. Landlock must also be enabled at boot
time as the other security modules. The list of security modules enabled by
time like other security modules. The list of security modules enabled by
default is set with ``CONFIG_LSM``. The kernel configuration should then
contains ``CONFIG_LSM=landlock,[...]`` with ``[...]`` as the list of other
contain ``CONFIG_LSM=landlock,[...]`` with ``[...]`` as the list of other
potentially useful security modules for the running system (see the
``CONFIG_LSM`` help).
@ -670,7 +669,7 @@ Questions and answers
What about user space sandbox managers?
---------------------------------------
Using user space process to enforce restrictions on kernel resources can lead
Using user space processes to enforce restrictions on kernel resources can lead
to race conditions or inconsistent evaluations (i.e. `Incorrect mirroring of
the OS code and state
<https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_).