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After working with the larger SELinux-based distros for several
years, we're finally at a place where we can disable the SELinux
runtime disable functionality. The existing kernel deprecation
notice explains the functionality and why we want to remove it:
The selinuxfs "disable" node allows SELinux to be disabled at
runtime prior to a policy being loaded into the kernel. If
disabled via this mechanism, SELinux will remain disabled until
the system is rebooted.
The preferred method of disabling SELinux is via the "selinux=0"
boot parameter, but the selinuxfs "disable" node was created to
make it easier for systems with primitive bootloaders that did not
allow for easy modification of the kernel command line.
Unfortunately, allowing for SELinux to be disabled at runtime makes
it difficult to secure the kernel's LSM hooks using the
"__ro_after_init" feature.
It is that last sentence, mentioning the '__ro_after_init' hardening,
which is the real motivation for this change, and if you look at the
diffstat you'll see that the impact of this patch reaches across all
the different LSMs, helping prevent tampering at the LSM hook level.
From a SELinux perspective, it is important to note that if you
continue to disable SELinux via "/etc/selinux/config" it may appear
that SELinux is disabled, but it is simply in an uninitialized state.
If you load a policy with `load_policy -i`, you will see SELinux
come alive just as if you had loaded the policy during early-boot.
It is also worth noting that the "/sys/fs/selinux/disable" file is
always writable now, regardless of the Kconfig settings, but writing
to the file has no effect on the system, other than to display an
error on the console if a non-zero/true value is written.
Finally, in the several years where we have been working on
deprecating this functionality, there has only been one instance of
someone mentioning any user visible breakage. In this particular
case it was an individual's kernel test system, and the workaround
documented in the deprecation notice ("selinux=0" on the kernel
command line) resolved the issue without problem.
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
121 lines
3.2 KiB
C
121 lines
3.2 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Landlock LSM - Ptrace hooks
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*
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* Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
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* Copyright © 2019-2020 ANSSI
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*/
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#include <asm/current.h>
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#include <linux/cred.h>
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#include <linux/errno.h>
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#include <linux/kernel.h>
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#include <linux/lsm_hooks.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include "common.h"
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#include "cred.h"
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#include "ptrace.h"
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#include "ruleset.h"
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#include "setup.h"
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/**
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* domain_scope_le - Checks domain ordering for scoped ptrace
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*
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* @parent: Parent domain.
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* @child: Potential child of @parent.
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*
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* Checks if the @parent domain is less or equal to (i.e. an ancestor, which
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* means a subset of) the @child domain.
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*/
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static bool domain_scope_le(const struct landlock_ruleset *const parent,
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const struct landlock_ruleset *const child)
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{
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const struct landlock_hierarchy *walker;
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if (!parent)
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return true;
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if (!child)
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return false;
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for (walker = child->hierarchy; walker; walker = walker->parent) {
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if (walker == parent->hierarchy)
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/* @parent is in the scoped hierarchy of @child. */
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return true;
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}
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/* There is no relationship between @parent and @child. */
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return false;
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}
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static bool task_is_scoped(const struct task_struct *const parent,
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const struct task_struct *const child)
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{
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bool is_scoped;
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const struct landlock_ruleset *dom_parent, *dom_child;
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rcu_read_lock();
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dom_parent = landlock_get_task_domain(parent);
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dom_child = landlock_get_task_domain(child);
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is_scoped = domain_scope_le(dom_parent, dom_child);
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rcu_read_unlock();
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return is_scoped;
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}
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static int task_ptrace(const struct task_struct *const parent,
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const struct task_struct *const child)
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{
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/* Quick return for non-landlocked tasks. */
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if (!landlocked(parent))
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return 0;
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if (task_is_scoped(parent, child))
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return 0;
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return -EPERM;
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}
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/**
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* hook_ptrace_access_check - Determines whether the current process may access
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* another
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*
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* @child: Process to be accessed.
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* @mode: Mode of attachment.
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*
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* If the current task has Landlock rules, then the child must have at least
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* the same rules. Else denied.
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*
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* Determines whether a process may access another, returning 0 if permission
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* granted, -errno if denied.
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*/
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static int hook_ptrace_access_check(struct task_struct *const child,
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const unsigned int mode)
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{
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return task_ptrace(current, child);
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}
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/**
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* hook_ptrace_traceme - Determines whether another process may trace the
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* current one
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*
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* @parent: Task proposed to be the tracer.
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*
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* If the parent has Landlock rules, then the current task must have the same
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* or more rules. Else denied.
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*
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* Determines whether the nominated task is permitted to trace the current
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* process, returning 0 if permission is granted, -errno if denied.
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*/
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static int hook_ptrace_traceme(struct task_struct *const parent)
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{
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return task_ptrace(parent, current);
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}
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static struct security_hook_list landlock_hooks[] __ro_after_init = {
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LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
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LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
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};
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__init void landlock_add_ptrace_hooks(void)
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{
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security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),
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LANDLOCK_NAME);
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}
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