Re: [PATCH v28 04/12] landlock: Add ptrace restrictions

["Serge E. Hallyn" <serge@hallyn.com>]

On Tue, Feb 02, 2021 at 05:27:02PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@linux.microsoft.com>
> 
> Using ptrace(2) and related debug features on a target process can lead
> to a privilege escalation.  Indeed, ptrace(2) can be used by an attacker
> to impersonate another task and to remain undetected while performing
> malicious activities.  Thanks to  ptrace_may_access(), various part of
> the kernel can check if a tracer is more privileged than a tracee.
> 
> A landlocked process has fewer privileges than a non-landlocked process
> and must then be subject to additional restrictions when manipulating
> processes. To be allowed to use ptrace(2) and related syscalls on a
> target process, a landlocked process must have a subset of the target
> process's rules (i.e. the tracee must be in a sub-domain of the tracer).
> 
> Cc: James Morris <jmorris@namei.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

Thanks, I appreciate that things are well named and easy to reason
about.

> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> Reviewed-by: Jann Horn <jannh@google.com>
> ---
> 
> Changes since v25:
> * Rename function to landlock_add_ptrace_hooks().
> 
> Changes since v22:
> * Add Reviewed-by: Jann Horn <jannh@google.com>
> 
> Changes since v21:
> * Fix copyright dates.
> 
> Changes since v14:
> * Constify variables.
> 
> Changes since v13:
> * Make the ptrace restriction mandatory, like in the v10.
> * Remove the eBPF dependency.
> 
> Previous changes:
> https://lore.kernel.org/lkml/20191104172146.30797-5-mic@digikod.net/
> ---
>  security/landlock/Makefile |   2 +-
>  security/landlock/ptrace.c | 120 +++++++++++++++++++++++++++++++++++++
>  security/landlock/ptrace.h |  14 +++++
>  security/landlock/setup.c  |   2 +
>  4 files changed, 137 insertions(+), 1 deletion(-)
>  create mode 100644 security/landlock/ptrace.c
>  create mode 100644 security/landlock/ptrace.h
> 
> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
> index 041ea242e627..f1d1eb72fa76 100644
> --- a/security/landlock/Makefile
> +++ b/security/landlock/Makefile
> @@ -1,4 +1,4 @@
>  obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
>  
>  landlock-y := setup.o object.o ruleset.o \
> -	cred.o
> +	cred.o ptrace.o
> diff --git a/security/landlock/ptrace.c b/security/landlock/ptrace.c
> new file mode 100644
> index 000000000000..f55b82446de2
> --- /dev/null
> +++ b/security/landlock/ptrace.c
> @@ -0,0 +1,120 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Landlock LSM - Ptrace hooks
> + *
> + * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
> + * Copyright © 2019-2020 ANSSI
> + */
> +
> +#include <asm/current.h>
> +#include <linux/cred.h>
> +#include <linux/errno.h>
> +#include <linux/kernel.h>
> +#include <linux/lsm_hooks.h>
> +#include <linux/rcupdate.h>
> +#include <linux/sched.h>
> +
> +#include "common.h"
> +#include "cred.h"
> +#include "ptrace.h"
> +#include "ruleset.h"
> +#include "setup.h"
> +
> +/**
> + * domain_scope_le - Checks domain ordering for scoped ptrace
> + *
> + * @parent: Parent domain.
> + * @child: Potential child of @parent.
> + *
> + * Checks if the @parent domain is less or equal to (i.e. an ancestor, which
> + * means a subset of) the @child domain.
> + */
> +static bool domain_scope_le(const struct landlock_ruleset *const parent,
> +		const struct landlock_ruleset *const child)
> +{
> +	const struct landlock_hierarchy *walker;
> +
> +	if (!parent)
> +		return true;
> +	if (!child)
> +		return false;
> +	for (walker = child->hierarchy; walker; walker = walker->parent) {
> +		if (walker == parent->hierarchy)
> +			/* @parent is in the scoped hierarchy of @child. */
> +			return true;
> +	}
> +	/* There is no relationship between @parent and @child. */
> +	return false;
> +}
> +
> +static bool task_is_scoped(const struct task_struct *const parent,
> +		const struct task_struct *const child)
> +{
> +	bool is_scoped;
> +	const struct landlock_ruleset *dom_parent, *dom_child;
> +
> +	rcu_read_lock();
> +	dom_parent = landlock_get_task_domain(parent);
> +	dom_child = landlock_get_task_domain(child);
> +	is_scoped = domain_scope_le(dom_parent, dom_child);
> +	rcu_read_unlock();
> +	return is_scoped;
> +}
> +
> +static int task_ptrace(const struct task_struct *const parent,
> +		const struct task_struct *const child)
> +{
> +	/* Quick return for non-landlocked tasks. */
> +	if (!landlocked(parent))
> +		return 0;
> +	if (task_is_scoped(parent, child))
> +		return 0;
> +	return -EPERM;
> +}
> +
> +/**
> + * hook_ptrace_access_check - Determines whether the current process may access
> + *			      another
> + *
> + * @child: Process to be accessed.
> + * @mode: Mode of attachment.
> + *
> + * If the current task has Landlock rules, then the child must have at least
> + * the same rules.  Else denied.
> + *
> + * Determines whether a process may access another, returning 0 if permission
> + * granted, -errno if denied.
> + */
> +static int hook_ptrace_access_check(struct task_struct *const child,
> +		const unsigned int mode)
> +{
> +	return task_ptrace(current, child);
> +}
> +
> +/**
> + * hook_ptrace_traceme - Determines whether another process may trace the
> + *			 current one
> + *
> + * @parent: Task proposed to be the tracer.
> + *
> + * If the parent has Landlock rules, then the current task must have the same
> + * or more rules.  Else denied.
> + *
> + * Determines whether the nominated task is permitted to trace the current
> + * process, returning 0 if permission is granted, -errno if denied.
> + */
> +static int hook_ptrace_traceme(struct task_struct *const parent)
> +{
> +	return task_ptrace(parent, current);
> +}
> +
> +static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
> +	LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
> +	LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
> +};
> +
> +__init void landlock_add_ptrace_hooks(void)
> +{
> +	security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),
> +			LANDLOCK_NAME);
> +}
> diff --git a/security/landlock/ptrace.h b/security/landlock/ptrace.h
> new file mode 100644
> index 000000000000..265b220ae3bf
> --- /dev/null
> +++ b/security/landlock/ptrace.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Landlock LSM - Ptrace hooks
> + *
> + * Copyright © 2017-2019 Mickaël Salaün <mic@digikod.net>
> + * Copyright © 2019 ANSSI
> + */
> +
> +#ifndef _SECURITY_LANDLOCK_PTRACE_H
> +#define _SECURITY_LANDLOCK_PTRACE_H
> +
> +__init void landlock_add_ptrace_hooks(void);
> +
> +#endif /* _SECURITY_LANDLOCK_PTRACE_H */
> diff --git a/security/landlock/setup.c b/security/landlock/setup.c
> index 8661112fb238..a5d6ef334991 100644
> --- a/security/landlock/setup.c
> +++ b/security/landlock/setup.c
> @@ -11,6 +11,7 @@
>  
>  #include "common.h"
>  #include "cred.h"
> +#include "ptrace.h"
>  #include "setup.h"
>  
>  struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
> @@ -20,6 +21,7 @@ struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
>  static int __init landlock_init(void)
>  {
>  	landlock_add_cred_hooks();
> +	landlock_add_ptrace_hooks();
>  	pr_info("Up and running.\n");
>  	return 0;
>  }
> -- 
> 2.30.0