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

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