[PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls
Micah Morton
mortonm at chromium.org
Tue Jan 15 21:58:37 UTC 2019
On Tue, Jan 15, 2019 at 11:44 AM Kees Cook <keescook at chromium.org> wrote:
>
> On Tue, Jan 15, 2019 at 10:04 AM <mortonm at chromium.org> wrote:
> >
> > From: Micah Morton <mortonm at chromium.org>
> >
> > SafeSetID gates the setid family of syscalls to restrict UID/GID
> > transitions from a given UID/GID to only those approved by a
> > system-wide whitelist. These restrictions also prohibit the given
> > UIDs/GIDs from obtaining auxiliary privileges associated with
> > CAP_SET{U/G}ID, such as allowing a user to set up user namespace UID
> > mappings. For now, only gating the set*uid family of syscalls is
> > supported, with support for set*gid coming in a future patch set.
> >
> > Signed-off-by: Micah Morton <mortonm at chromium.org>
> > ---
> > Changes since the last patch set: Pulled out the "no-op" changes that
> > mark setid call sites in kernel/sys.c into a separate patch, and made
> > other small mods proposed by Kees Cook. NOTE: this patch is still using
> > do_exit(SIGKILL) to kill the process in check_uid_transition in lsm.c.
> > This may need to change, pending further discussion.
> > Documentation/admin-guide/LSM/SafeSetID.rst | 107 ++++++++
> > Documentation/admin-guide/LSM/index.rst | 1 +
> > security/Kconfig | 1 +
> > security/Makefile | 2 +
> > security/safesetid/Kconfig | 12 +
> > security/safesetid/Makefile | 7 +
> > security/safesetid/lsm.c | 266 ++++++++++++++++++++
> > security/safesetid/lsm.h | 30 +++
> > security/safesetid/securityfs.c | 185 ++++++++++++++
> > 9 files changed, 611 insertions(+)
> > create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst
> > create mode 100644 security/safesetid/Kconfig
> > create mode 100644 security/safesetid/Makefile
> > create mode 100644 security/safesetid/lsm.c
> > create mode 100644 security/safesetid/lsm.h
> > create mode 100644 security/safesetid/securityfs.c
> >
> > diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst
> > new file mode 100644
> > index 000000000000..ffb64be67f7a
> > --- /dev/null
> > +++ b/Documentation/admin-guide/LSM/SafeSetID.rst
> > @@ -0,0 +1,107 @@
> > +=========
> > +SafeSetID
> > +=========
> > +SafeSetID is an LSM module that gates the setid family of syscalls to restrict
> > +UID/GID transitions from a given UID/GID to only those approved by a
> > +system-wide whitelist. These restrictions also prohibit the given UIDs/GIDs
> > +from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
> > +allowing a user to set up user namespace UID mappings.
> > +
> > +
> > +Background
> > +==========
> > +In absence of file capabilities, processes spawned on a Linux system that need
> > +to switch to a different user must be spawned with CAP_SETUID privileges.
> > +CAP_SETUID is granted to programs running as root or those running as a non-root
> > +user that have been explicitly given the CAP_SETUID runtime capability. It is
> > +often preferable to use Linux runtime capabilities rather than file
> > +capabilities, since using file capabilities to run a program with elevated
> > +privileges opens up possible security holes since any user with access to the
> > +file can exec() that program to gain the elevated privileges.
> > +
> > +While it is possible to implement a tree of processes by giving full
> > +CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a
> > +tree of processes under non-root user(s) in the first place. Specifically,
> > +since CAP_SETUID allows changing to any user on the system, including the root
> > +user, it is an overpowered capability for what is needed in this scenario,
> > +especially since programs often only call setuid() to drop privileges to a
> > +lesser-privileged user -- not elevate privileges. Unfortunately, there is no
> > +generally feasible way in Linux to restrict the potential UIDs that a user can
> > +switch to through setuid() beyond allowing a switch to any user on the system.
> > +This SafeSetID LSM seeks to provide a solution for restricting setid
> > +capabilities in such a way.
> > +
> > +The main use case for this LSM is to allow a non-root program to transition to
> > +other untrusted uids without full blown CAP_SETUID capabilities. The non-root
> > +program would still need CAP_SETUID to do any kind of transition, but the
> > +additional restrictions imposed by this LSM would mean it is a "safer" version
> > +of CAP_SETUID since the non-root program cannot take advantage of CAP_SETUID to
> > +do any unapproved actions (e.g. setuid to uid 0 or create/enter new user
> > +namespace). The higher level goal is to allow for uid-based sandboxing of system
> > +services without having to give out CAP_SETUID all over the place just so that
> > +non-root programs can drop to even-lesser-privileged uids. This is especially
> > +relevant when one non-root daemon on the system should be allowed to spawn other
> > +processes as different uids, but its undesirable to give the daemon a
> > +basically-root-equivalent CAP_SETUID.
> > +
> > +
> > +Other Approaches Considered
> > +===========================
> > +
> > +Solve this problem in userspace
> > +-------------------------------
> > +For candidate applications that would like to have restricted setid capabilities
> > +as implemented in this LSM, an alternative option would be to simply take away
> > +setid capabilities from the application completely and refactor the process
> > +spawning semantics in the application (e.g. by using a privileged helper program
> > +to do process spawning and UID/GID transitions). Unfortunately, there are a
> > +number of semantics around process spawning that would be affected by this, such
> > +as fork() calls where the program doesn’t immediately call exec() after the
> > +fork(), parent processes specifying custom environment variables or command line
> > +args for spawned child processes, or inheritance of file handles across a
> > +fork()/exec(). Because of this, as solution that uses a privileged helper in
> > +userspace would likely be less appealing to incorporate into existing projects
> > +that rely on certain process-spawning semantics in Linux.
> > +
> > +Use user namespaces
> > +-------------------
> > +Another possible approach would be to run a given process tree in its own user
> > +namespace and give programs in the tree setid capabilities. In this way,
> > +programs in the tree could change to any desired UID/GID in the context of their
> > +own user namespace, and only approved UIDs/GIDs could be mapped back to the
> > +initial system user namespace, affectively preventing privilege escalation.
> > +Unfortunately, it is not generally feasible to use user namespaces in isolation,
> > +without pairing them with other namespace types, which is not always an option.
> > +Linux checks for capabilities based off of the user namespace that “owns” some
> > +entity. For example, Linux has the notion that network namespaces are owned by
> > +the user namespace in which they were created. A consequence of this is that
> > +capability checks for access to a given network namespace are done by checking
> > +whether a task has the given capability in the context of the user namespace
> > +that owns the network namespace -- not necessarily the user namespace under
> > +which the given task runs. Therefore spawning a process in a new user namespace
> > +effectively prevents it from accessing the network namespace owned by the
> > +initial namespace. This is a deal-breaker for any application that expects to
> > +retain the CAP_NET_ADMIN capability for the purpose of adjusting network
> > +configurations. Using user namespaces in isolation causes problems regarding
> > +other system interactions, including use of pid namespaces and device creation.
> > +
> > +Use an existing LSM
> > +-------------------
> > +None of the other in-tree LSMs have the capability to gate setid transitions, or
> > +even employ the security_task_fix_setuid hook at all. SELinux says of that hook:
> > +"Since setuid only affects the current process, and since the SELinux controls
> > +are not based on the Linux identity attributes, SELinux does not need to control
> > +this operation."
> > +
> > +
> > +Directions for use
> > +==================
> > +This LSM hooks the setid syscalls to make sure transitions are allowed if an
> > +applicable restriction policy is in place. Policies are configured through
> > +securityfs by writing to the safesetid/add_whitelist_policy and
> > +safesetid/flush_whitelist_policies files at the location where securityfs is
> > +mounted. The format for adding a policy is '<UID>:<UID>', using literal
> > +numbers, such as '123:456'. To flush the policies, any write to the file is
> > +sufficient. Again, configuring a policy for a UID will prevent that UID from
> > +obtaining auxiliary setid privileges, such as allowing a user to set up user
> > +namespace UID mappings.
> > diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst
> > index 9842e21afd4a..a6ba95fbaa9f 100644
> > --- a/Documentation/admin-guide/LSM/index.rst
> > +++ b/Documentation/admin-guide/LSM/index.rst
> > @@ -46,3 +46,4 @@ subdirectories.
> > Smack
> > tomoyo
> > Yama
> > + SafeSetID
> > diff --git a/security/Kconfig b/security/Kconfig
> > index 78dc12b7eeb3..9efc7a5e3280 100644
> > --- a/security/Kconfig
> > +++ b/security/Kconfig
> > @@ -236,6 +236,7 @@ source "security/tomoyo/Kconfig"
> > source "security/apparmor/Kconfig"
> > source "security/loadpin/Kconfig"
> > source "security/yama/Kconfig"
> > +source "security/safesetid/Kconfig"
> >
> > source "security/integrity/Kconfig"
> >
> > diff --git a/security/Makefile b/security/Makefile
> > index 4d2d3782ddef..c598b904938f 100644
> > --- a/security/Makefile
> > +++ b/security/Makefile
> > @@ -10,6 +10,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
> > subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
> > subdir-$(CONFIG_SECURITY_YAMA) += yama
> > subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
> > +subdir-$(CONFIG_SECURITY_SAFESETID) += safesetid
> >
> > # always enable default capabilities
> > obj-y += commoncap.o
> > @@ -25,6 +26,7 @@ obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/
> > obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/
> > obj-$(CONFIG_SECURITY_YAMA) += yama/
> > obj-$(CONFIG_SECURITY_LOADPIN) += loadpin/
> > +obj-$(CONFIG_SECURITY_SAFESETID) += safesetid/
> > obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
>
> Given the refactoring of the LSM enabling logic, you'll need to do
> some minor merging with the linux-next tree to get this to apply to
> security-next. That would make James's life easier, I think, though
> maybe James can speak to that, since I'm not sure how the trees are
> split right now.
These patches apply cleanly to security-next at the moment (unless I'm
doing something weird -- the last commit I see in the git log is mine
from last week: c1a85a00ea66cb6f0bd0f14e47c28c2b0999799f)
>
> >
> > # Object integrity file lists
> > diff --git a/security/safesetid/Kconfig b/security/safesetid/Kconfig
> > new file mode 100644
> > index 000000000000..bf89a47ffcc8
> > --- /dev/null
> > +++ b/security/safesetid/Kconfig
> > @@ -0,0 +1,12 @@
> > +config SECURITY_SAFESETID
> > + bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilities"
> > + default n
> > + help
> > + SafeSetID is an LSM module that gates the setid family of syscalls to
> > + restrict UID/GID transitions from a given UID/GID to only those
> > + approved by a system-wide whitelist. These restrictions also prohibit
> > + the given UIDs/GIDs from obtaining auxiliary privileges associated
> > + with CAP_SET{U/G}ID, such as allowing a user to set up user namespace
> > + UID mappings.
> > +
> > + If you are unsure how to answer this question, answer N.
> > diff --git a/security/safesetid/Makefile b/security/safesetid/Makefile
> > new file mode 100644
> > index 000000000000..6b0660321164
> > --- /dev/null
> > +++ b/security/safesetid/Makefile
> > @@ -0,0 +1,7 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +#
> > +# Makefile for the safesetid LSM.
> > +#
> > +
> > +obj-$(CONFIG_SECURITY_SAFESETID) := safesetid.o
> > +safesetid-y := lsm.o securityfs.o
> > diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
> > new file mode 100644
> > index 000000000000..aa7bd3323751
> > --- /dev/null
> > +++ b/security/safesetid/lsm.c
> > @@ -0,0 +1,266 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * SafeSetID Linux Security Module
> > + *
> > + * Author: Micah Morton <mortonm at chromium.org>
> > + *
> > + * Copyright (C) 2018 The Chromium OS Authors.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2, as
> > + * published by the Free Software Foundation.
> > + *
> > + */
> > +
> > +#define pr_fmt(fmt) "SafeSetID: " fmt
> > +
> > +#include <asm/syscall.h>
> > +#include <linux/hashtable.h>
> > +#include <linux/lsm_hooks.h>
> > +#include <linux/module.h>
> > +#include <linux/ptrace.h>
> > +#include <linux/sched/task_stack.h>
> > +#include <linux/security.h>
> > +
> > +#define NUM_BITS 8 /* 128 buckets in hash table */
> > +
> > +static DEFINE_HASHTABLE(safesetid_whitelist_hashtable, NUM_BITS);
> > +
> > +/*
> > + * Hash table entry to store safesetid policy signifying that 'parent' user
> > + * can setid to 'child' user.
> > + */
> > +struct entry {
> > + struct hlist_node next;
> > + struct hlist_node dlist; /* for deletion cleanup */
> > + uint64_t parent_kuid;
> > + uint64_t child_kuid;
> > +};
> > +
> > +static DEFINE_SPINLOCK(safesetid_whitelist_hashtable_spinlock);
> > +
> > +static bool check_setuid_policy_hashtable_key(kuid_t parent)
> > +{
> > + struct entry *entry;
> > +
> > + rcu_read_lock();
> > + hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
> > + entry, next, __kuid_val(parent)) {
> > + if (entry->parent_kuid == __kuid_val(parent)) {
> > + rcu_read_unlock();
> > + return true;
> > + }
> > + }
> > + rcu_read_unlock();
> > +
> > + return false;
> > +}
> > +
> > +static bool check_setuid_policy_hashtable_key_value(kuid_t parent,
> > + kuid_t child)
> > +{
> > + struct entry *entry;
> > +
> > + rcu_read_lock();
> > + hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
> > + entry, next, __kuid_val(parent)) {
> > + if (entry->parent_kuid == __kuid_val(parent) &&
> > + entry->child_kuid == __kuid_val(child)) {
> > + rcu_read_unlock();
> > + return true;
> > + }
> > + }
> > + rcu_read_unlock();
> > +
> > + return false;
> > +}
> > +
> > +static int safesetid_security_capable(const struct cred *cred,
> > + struct user_namespace *ns,
> > + int cap,
> > + unsigned int opts)
> > +{
> > + if (cap == CAP_SETUID &&
> > + check_setuid_policy_hashtable_key(cred->uid)) {
> > + if (!(opts & CAP_OPT_INSETID)) {
> > + /*
> > + * Deny if we're not in a set*uid() syscall to avoid
> > + * giving powers gated by CAP_SETUID that are related
> > + * to functionality other than calling set*uid() (e.g.
> > + * allowing user to set up userns uid mappings).
> > + */
> > + pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions",
> > + __kuid_val(cred->uid));
> > + return -1;
> > + }
> > + }
> > + return 0;
> > +}
> > +
> > +static int check_uid_transition(kuid_t parent, kuid_t child)
> > +{
> > + if (check_setuid_policy_hashtable_key_value(parent, child))
> > + return 0;
> > + pr_warn("UID transition (%d -> %d) blocked",
> > + __kuid_val(parent),
> > + __kuid_val(child));
> > + /*
> > + * Kill this process to avoid potential security vulnerabilities
> > + * that could arise from a missing whitelist entry preventing a
> > + * privileged process from dropping to a lesser-privileged one.
> > + */
> > + do_exit(SIGKILL);
> > +}
>
> This needs double-checking, but I think you want this, to avoid
> missing various process clean-up steps (like performing a core dump if
> desired, etc):
>
> force_sig(SIGKILL, current);
> return -EACCES;
>
> But please double-check that a rejected setuid() syscall never
> completes and the process does die with SIGKILL.
Yep, this looks good. I changed those lines and see the following
strace output from a process that isn't allowed to setuid to root per
the whitelist policies:
...
setgid(0) = 0
setuid(0) = ?
+++ killed by SIGKILL +++
FWIW, I checked this with the following command on a ChromeOS device
in dev mode:
localhost ~ # strace -ff -o /tmp/strace /sbin/minijail0 -u shill -g
shill -c 0xc0 -- /sbin/capsh --user=root -- -c /usr/bin/whoami
>
> > +
> > +/*
> > + * Check whether there is either an exception for user under old cred struct to
> > + * set*uid to user under new cred struct, or the UID transition is allowed (by
> > + * Linux set*uid rules) even without CAP_SETUID.
> > + */
> > +static int safesetid_task_fix_setuid(struct cred *new,
> > + const struct cred *old,
> > + int flags)
> > +{
> > +
> > + /* Do nothing if there are no setuid restrictions for this UID. */
> > + if (!check_setuid_policy_hashtable_key(old->uid))
> > + return 0;
> > +
> > + switch (flags) {
> > + case LSM_SETID_RE:
> > + /*
> > + * Users for which setuid restrictions exist can only set the
> > + * real UID to the real UID or the effective UID, unless an
> > + * explicit whitelist policy allows the transition.
> > + */
> > + if (!uid_eq(old->uid, new->uid) &&
> > + !uid_eq(old->euid, new->uid)) {
> > + return check_uid_transition(old->uid, new->uid);
> > + }
> > + /*
> > + * Users for which setuid restrictions exist can only set the
> > + * effective UID to the real UID, the effective UID, or the
> > + * saved set-UID, unless an explicit whitelist policy allows
> > + * the transition.
> > + */
> > + if (!uid_eq(old->uid, new->euid) &&
> > + !uid_eq(old->euid, new->euid) &&
> > + !uid_eq(old->suid, new->euid)) {
> > + return check_uid_transition(old->euid, new->euid);
> > + }
> > + break;
> > + case LSM_SETID_ID:
> > + /*
> > + * Users for which setuid restrictions exist cannot change the
> > + * real UID or saved set-UID unless an explicit whitelist
> > + * policy allows the transition.
> > + */
> > + if (!uid_eq(old->uid, new->uid))
> > + return check_uid_transition(old->uid, new->uid);
> > + if (!uid_eq(old->suid, new->suid))
> > + return check_uid_transition(old->suid, new->suid);
> > + break;
> > + case LSM_SETID_RES:
> > + /*
> > + * Users for which setuid restrictions exist cannot change the
> > + * real UID, effective UID, or saved set-UID to anything but
> > + * one of: the current real UID, the current effective UID or
> > + * the current saved set-user-ID unless an explicit whitelist
> > + * policy allows the transition.
> > + */
> > + if (!uid_eq(new->uid, old->uid) &&
> > + !uid_eq(new->uid, old->euid) &&
> > + !uid_eq(new->uid, old->suid)) {
> > + return check_uid_transition(old->uid, new->uid);
> > + }
> > + if (!uid_eq(new->euid, old->uid) &&
> > + !uid_eq(new->euid, old->euid) &&
> > + !uid_eq(new->euid, old->suid)) {
> > + return check_uid_transition(old->euid, new->euid);
> > + }
> > + if (!uid_eq(new->suid, old->uid) &&
> > + !uid_eq(new->suid, old->euid) &&
> > + !uid_eq(new->suid, old->suid)) {
> > + return check_uid_transition(old->suid, new->suid);
> > + }
> > + break;
> > + case LSM_SETID_FS:
> > + /*
> > + * Users for which setuid restrictions exist cannot change the
> > + * filesystem UID to anything but one of: the current real UID,
> > + * the current effective UID or the current saved set-UID
> > + * unless an explicit whitelist policy allows the transition.
> > + */
> > + if (!uid_eq(new->fsuid, old->uid) &&
> > + !uid_eq(new->fsuid, old->euid) &&
> > + !uid_eq(new->fsuid, old->suid) &&
> > + !uid_eq(new->fsuid, old->fsuid)) {
> > + return check_uid_transition(old->fsuid, new->fsuid);
> > + }
> > + break;
>
> As a robustness measure can you add a default case here that will
> "fail closed"? Something like:
>
> default:
> WARN_ON_ONCE("Unknown setid state %d\n", flags);
> force_sig(SIGKILL, current);
> return -EINVAL;
Done.
>
> > + }
> > + return 0;
> > +}
> > +
> > +int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)
> > +{
> > + struct entry *new;
> > +
> > + /* Return if entry already exists */
> > + if (check_setuid_policy_hashtable_key_value(parent, child))
> > + return 0;
> > +
> > + new = kzalloc(sizeof(struct entry), GFP_KERNEL);
> > + if (!new)
> > + return -ENOMEM;
> > + new->parent_kuid = __kuid_val(parent);
> > + new->child_kuid = __kuid_val(child);
> > + spin_lock(&safesetid_whitelist_hashtable_spinlock);
> > + hash_add_rcu(safesetid_whitelist_hashtable,
> > + &new->next,
> > + __kuid_val(parent));
> > + spin_unlock(&safesetid_whitelist_hashtable_spinlock);
> > + return 0;
> > +}
> > +
> > +void flush_safesetid_whitelist_entries(void)
> > +{
> > + struct entry *entry;
> > + struct hlist_node *hlist_node;
> > + unsigned int bkt_loop_cursor;
> > + HLIST_HEAD(free_list);
> > +
> > + /*
> > + * Could probably use hash_for_each_rcu here instead, but this should
> > + * be fine as well.
> > + */
> > + spin_lock(&safesetid_whitelist_hashtable_spinlock);
> > + hash_for_each_safe(safesetid_whitelist_hashtable, bkt_loop_cursor,
> > + hlist_node, entry, next) {
> > + hash_del_rcu(&entry->next);
> > + hlist_add_head(&entry->dlist, &free_list);
> > + }
> > + spin_unlock(&safesetid_whitelist_hashtable_spinlock);
> > + synchronize_rcu();
> > + hlist_for_each_entry_safe(entry, hlist_node, &free_list, dlist) {
> > + hlist_del(&entry->dlist);
> > + kfree(entry);
> > + }
> > +}
> > +
> > +static struct security_hook_list safesetid_security_hooks[] = {
> > + LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
> > + LSM_HOOK_INIT(capable, safesetid_security_capable)
> > +};
> > +
> > +static int __init safesetid_security_init(void)
> > +{
> > + security_add_hooks(safesetid_security_hooks,
> > + ARRAY_SIZE(safesetid_security_hooks), "safesetid");
> > +
> > + return 0;
> > +}
> > +
> > +DEFINE_LSM(safesetid_security_init) = {
> > + .init = safesetid_security_init,
> > +};
> > diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h
> > new file mode 100644
> > index 000000000000..bf78af9bf314
> > --- /dev/null
> > +++ b/security/safesetid/lsm.h
> > @@ -0,0 +1,30 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * SafeSetID Linux Security Module
> > + *
> > + * Author: Micah Morton <mortonm at chromium.org>
> > + *
> > + * Copyright (C) 2018 The Chromium OS Authors.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2, as
> > + * published by the Free Software Foundation.
> > + *
> > + */
> > +#ifndef _SAFESETID_H
> > +#define _SAFESETID_H
> > +
> > +#include <linux/types.h>
> > +
> > +/* Function type. */
> > +enum safesetid_whitelist_file_write_type {
> > + SAFESETID_WHITELIST_ADD, /* Add whitelist policy. */
> > + SAFESETID_WHITELIST_FLUSH, /* Flush whitelist policies. */
> > +};
> > +
> > +/* Add entry to safesetid whitelist to allow 'parent' to setid to 'child'. */
> > +int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child);
> > +
> > +void flush_safesetid_whitelist_entries(void);
> > +
> > +#endif /* _SAFESETID_H */
> > diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
> > new file mode 100644
> > index 000000000000..c3ce7b63b4af
> > --- /dev/null
> > +++ b/security/safesetid/securityfs.c
> > @@ -0,0 +1,185 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * SafeSetID Linux Security Module
> > + *
> > + * Author: Micah Morton <mortonm at chromium.org>
> > + *
> > + * Copyright (C) 2018 The Chromium OS Authors.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2, as
> > + * published by the Free Software Foundation.
> > + *
> > + */
> > +#include <linux/security.h>
> > +#include <linux/cred.h>
> > +
> > +#include "lsm.h"
> > +
> > +static struct dentry *safesetid_policy_dir;
> > +
> > +struct safesetid_file_entry {
> > + const char *name;
> > + enum safesetid_whitelist_file_write_type type;
> > + struct dentry *dentry;
> > +};
> > +
> > +static struct safesetid_file_entry safesetid_files[] = {
> > + {.name = "add_whitelist_policy",
> > + .type = SAFESETID_WHITELIST_ADD},
> > + {.name = "flush_whitelist_policies",
> > + .type = SAFESETID_WHITELIST_FLUSH},
> > +};
> > +
> > +/*
> > + * In the case the input buffer contains one or more invalid UIDs, the kuid_t
> > + * variables pointed to by 'parent' and 'child' will get updated but this
> > + * function will return an error.
> > + */
> > +static int parse_safesetid_whitelist_policy(const char __user *buf,
> > + size_t len,
> > + kuid_t *parent,
> > + kuid_t *child)
> > +{
> > + char *kern_buf;
> > + char *parent_buf;
> > + char *child_buf;
> > + const char separator[] = ":";
> > + int ret;
> > + size_t first_substring_length;
> > + long parsed_parent;
> > + long parsed_child;
> > +
> > + /* Duplicate string from user memory and NULL-terminate */
> > + kern_buf = memdup_user_nul(buf, len);
> > + if (IS_ERR(kern_buf))
> > + return PTR_ERR(kern_buf);
> > +
> > + /*
> > + * Format of |buf| string should be <UID>:<UID>.
> > + * Find location of ":" in kern_buf (copied from |buf|).
> > + */
> > + first_substring_length = strcspn(kern_buf, separator);
> > + if (first_substring_length == 0 || first_substring_length == len) {
> > + ret = -EINVAL;
> > + goto free_kern;
> > + }
> > +
> > + parent_buf = kmemdup_nul(kern_buf, first_substring_length, GFP_KERNEL);
> > + if (!parent_buf) {
> > + ret = -ENOMEM;
> > + goto free_kern;
> > + }
> > +
> > + ret = kstrtol(parent_buf, 0, &parsed_parent);
> > + if (ret)
> > + goto free_both;
> > +
> > + child_buf = kern_buf + first_substring_length + 1;
> > + ret = kstrtol(child_buf, 0, &parsed_child);
> > + if (ret)
> > + goto free_both;
> > +
> > + *parent = make_kuid(current_user_ns(), parsed_parent);
> > + if (!uid_valid(*parent)) {
> > + ret = -EINVAL;
> > + goto free_both;
> > + }
> > +
> > + *child = make_kuid(current_user_ns(), parsed_child);
> > + if (!uid_valid(*child)) {
> > + ret = -EINVAL;
> > + goto free_both;
> > + }
> > +
> > +free_both:
> > + kfree(parent_buf);
> > +free_kern:
> > + kfree(kern_buf);
> > + return ret;
> > +}
> > +
> > +static ssize_t safesetid_file_write(struct file *file,
> > + const char __user *buf,
> > + size_t len,
> > + loff_t *ppos)
> > +{
> > + struct safesetid_file_entry *file_entry =
> > + file->f_inode->i_private;
> > + kuid_t parent;
> > + kuid_t child;
> > + int ret;
> > +
> > + if (!ns_capable(current_user_ns(), CAP_MAC_ADMIN))
> > + return -EPERM;
> > +
> > + if (*ppos != 0)
> > + return -EINVAL;
> > +
> > + switch (file_entry->type) {
> > + case SAFESETID_WHITELIST_FLUSH:
> > + flush_safesetid_whitelist_entries();
>
> missing break?
Thanks.
>
> > + case SAFESETID_WHITELIST_ADD:
> > + ret = parse_safesetid_whitelist_policy(buf, len, &parent,
> > + &child);
> > + if (ret)
> > + return ret;
> > +
> > + ret = add_safesetid_whitelist_entry(parent, child);
> > + if (ret)
> > + return ret;
>
> And add a default here too, something like:
>
> default:
> WARN_ON_ONCE("Unknown securityfs file %d!?\n", file_entry->type);
> break;
>
Done.
> > + }
> > +
> > + /* Return len on success so caller won't keep trying to write */
> > + return len;
> > +}
> > +
> > +static const struct file_operations safesetid_file_fops = {
> > + .write = safesetid_file_write,
> > +};
> > +
> > +static void safesetid_shutdown_securityfs(void)
> > +{
> > + int i;
> > +
> > + for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
> > + struct safesetid_file_entry *entry =
> > + &safesetid_files[i];
> > + securityfs_remove(entry->dentry);
> > + entry->dentry = NULL;
> > + }
> > +
> > + securityfs_remove(safesetid_policy_dir);
> > + safesetid_policy_dir = NULL;
> > +}
> > +
> > +static int __init safesetid_init_securityfs(void)
> > +{
> > + int i;
> > + int ret;
> > +
> > + safesetid_policy_dir = securityfs_create_dir("safesetid", NULL);
> > + if (!safesetid_policy_dir) {
> > + ret = PTR_ERR(safesetid_policy_dir);
> > + goto error;
> > + }
> > +
> > + for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
> > + struct safesetid_file_entry *entry =
> > + &safesetid_files[i];
> > + entry->dentry = securityfs_create_file(
> > + entry->name, 0200, safesetid_policy_dir,
> > + entry, &safesetid_file_fops);
> > + if (IS_ERR(entry->dentry)) {
> > + ret = PTR_ERR(entry->dentry);
> > + goto error;
> > + }
> > + }
> > +
> > + return 0;
> > +
> > +error:
> > + safesetid_shutdown_securityfs();
> > + return ret;
> > +}
> > +fs_initcall(safesetid_init_securityfs);
> > --
> > 2.20.1.97.g81188d93c3-goog
> >
>
> And if I didn't say it before, thank you for the docs on this too! :)
>
> --
> Kees Cook
More information about the Linux-security-module-archive
mailing list