[PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls
Kees Cook
keescook at chromium.org
Tue Jan 15 19:44:15 UTC 2019
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-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
Given the refactoring of the LSM enabling logic, you'll need to do
some minor merging with the linux-next tree to get this to apply to
security-next. That would make James's life easier, I think, though
maybe James can speak to that, since I'm not sure how the trees are
split right now.
>
> # Object integrity file lists
> diff --git a/security/safesetid/Kconfig b/security/safesetid/Kconfig
> new file mode 100644
> index 000000000000..bf89a47ffcc8
> --- /dev/null
> +++ b/security/safesetid/Kconfig
> @@ -0,0 +1,12 @@
> +config SECURITY_SAFESETID
> + bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilities"
> + default n
> + help
> + SafeSetID is an LSM module that gates the setid family of syscalls to
> + restrict UID/GID transitions from a given UID/GID to only those
> + approved by a system-wide whitelist. These restrictions also prohibit
> + the given UIDs/GIDs from obtaining auxiliary privileges associated
> + with CAP_SET{U/G}ID, such as allowing a user to set up user namespace
> + UID mappings.
> +
> + If you are unsure how to answer this question, answer N.
> diff --git a/security/safesetid/Makefile b/security/safesetid/Makefile
> new file mode 100644
> index 000000000000..6b0660321164
> --- /dev/null
> +++ b/security/safesetid/Makefile
> @@ -0,0 +1,7 @@
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# Makefile for the safesetid LSM.
> +#
> +
> +obj-$(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.
> +
> +/*
> + * 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;
> + }
> + 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?
> + 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;
> + }
> +
> + /* 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
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