> Allow kernel services to override LSM settings appropriate to the actions
> performed by a task by duplicating a security record, modifying it and then
> using task_struct::act_as to point to it when performing operations on behalf
> of a task.
>
> This is used, for example, by CacheFiles which has to transparently access the
> cache on behalf of a process that thinks it is doing, say, NFS accesses with a
> potentially inappropriate (with respect to accessing the cache) set of
> security data.
>
> This patch provides two LSM hooks for modifying a task security record:
>
> (*) security_kernel_act_as() which allows modification of the security datum
> with which a task acts on other objects (most notably files).
>
> (*) security_create_files_as() which allows modification of the security
> datum that is used to initialise the security data on a file that a task
> creates.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>
> include/linux/cred.h | 22 ++++++++++++
> include/linux/security.h | 35 +++++++++++++++++++
> kernel/cred.c | 86 ++++++++++++++++++++++++++++++++++++++++++++++
> security/dummy.c | 15 ++++++++
> security/security.c | 13 +++++++
> security/selinux/hooks.c | 45 ++++++++++++++++++++++++
> 6 files changed, 216 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/cred.h b/include/linux/cred.h
> new file mode 100644
> index 0000000..c9f8906
> --- /dev/null
> +++ b/include/linux/cred.h
> @@ -0,0 +1,22 @@
> +/* Credential management
> + *
> + * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (
dhowells@redhat.com)
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public Licence
> + * as published by the Free Software Foundation; either version
> + * 2 of the Licence, or (at your option) any later version.
> + */
> +
> +#ifndef _LINUX_CRED_H
> +#define _LINUX_CRED_H
> +
> +struct task_security;
> +struct inode;
> +
> +extern struct task_security *get_kernel_security(const char *,
> + struct task_struct *);
> +extern int change_create_files_as(struct task_security *, struct inode *);
> +
> +#endif /* _LINUX_CRED_H */
> diff --git a/include/linux/security.h b/include/linux/security.h
> index b7ba073..f0dce11 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -557,6 +557,18 @@ struct request_sock;
> * Duplicate and attach the security structure currently attached to the
> * p->security field.
> * Return 0 if operation was successful.
> + * @task_kernel_act_as:
> + * Set the credentials for a kernel service to act as (subjective context).
> + * @sec points to the task security record to be modified.
> + * @service names the service making the request.
> + * @daemon: A userspace daemon to be used as a reference.
> + * Return 0 if successful.
> + * @task_create_files_as:
> + * Set the file creation context in a task security record to be the same
> + * as the objective context of the specified inode.
> + * @sec points to the task security record to be modified.
> + * @inode points to the inode to use as a reference.
> + * Return 0 if successful.
> * @task_setuid:
> * Check permission before setting one or more of the user identity
> * attributes of the current process. The @flags parameter indicates
> @@ -1321,6 +1333,11 @@ struct security_operations {
> int (*task_alloc_security) (struct task_struct *p);
> void (*task_free_security) (struct task_security *p);
> int (*task_dup_security) (struct task_security *p);
> + int (*task_kernel_act_as)(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon);
> + int (*task_create_files_as)(struct task_security *sec,
> + struct inode *inode);
> int (*task_setuid) (uid_t id0, uid_t id1, uid_t id2, int flags);
> int (*task_post_setuid) (uid_t old_ruid /* or fsuid */ ,
> uid_t old_euid, uid_t old_suid, int flags);
> @@ -1571,6 +1588,11 @@ int security_task_create(unsigned long clone_flags);
> int security_task_alloc(struct task_struct *p);
> void security_task_free(struct task_security *p);
> int security_task_dup(struct task_security *p);
> +int security_task_kernel_act_as(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon);
> +int security_task_create_files_as(struct task_security *sec,
> + struct inode *inode);
> int security_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags);
> int security_task_post_setuid(uid_t old_ruid, uid_t old_euid,
> uid_t old_suid, int flags);
> @@ -2054,6 +2076,19 @@ static inline int security_task_dup(struct task_security *p)
> return 0;
> }
>
> +static inline int security_task_kernel_act_as(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon)
> +{
> + return 0;
> +}
> +
> +static inline int security_task_create_files_as(struct task_security *sec,
> + struct inode *inode)
> +{
> + return 0;
> +}
> +
> static inline int security_task_setuid (uid_t id0, uid_t id1, uid_t id2,
> int flags)
> {
> diff --git a/kernel/cred.c b/kernel/cred.c
> index ddf98c6..eac1d0c 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -11,6 +11,7 @@
> #include <linux/module.h>
> #include <linux/sched.h>
> #include <linux/key.h>
> +#include <linux/keyctl.h>
> #include <linux/init_task.h>
> #include <linux/security.h>
>
> @@ -137,3 +138,88 @@ void put_task_security(struct task_security *sec)
> }
> }
> EXPORT_SYMBOL(put_task_security);
> +
> +/**
> + * get_kernel_security - Get a task security record for a named kernel service
> + * @service: The name of the service
> + * @daemon: A userspace daemon to be used as a reference
> + *
> + * Get a task security record for a specific kernel service. This can then be
> + * used to override a task's own security so that work can be done on behalf of
> + * that task that requires a different security context.
> + *
> + * @daemon is used to provide a base for the security record, but can be NULL.
> + * If @daemon is supplied, then the security data will be derived from that;
> + * otherwise they'll be set to 0 and no groups, full capabilities and no keys.
> + *
> + * @daemon is also passd to the LSM module as a base from which to initialise
> + * any MAC controls.
> + *
> + * The caller may change these controls afterwards if desired.
> + */
> +struct task_security *get_kernel_security(const char *service,
> + struct task_struct *daemon)
> +{
> + const struct task_security *dsec;
> + struct task_security *sec;
> + int ret;
> +
> + sec = kzalloc(sizeof *sec, GFP_KERNEL);
> + if (!sec)
> + return ERR_PTR(-ENOMEM);
> +
> + if (daemon) {
> + rcu_read_lock();
> + dsec = rcu_dereference(daemon->sec);
> + *sec = *dsec;
> + get_group_info(sec->group_info);
> + get_uid(sec->user);
> + rcu_read_unlock();
> +#ifdef CONFIG_KEYS
> + sec->request_key_auth = NULL;
> + sec->thread_keyring = NULL;
> + sec->tgsec = NULL;
> +#endif
> + } else {
> + sec->keep_capabilities = 0;
> + sec->cap_inheritable = CAP_INIT_INH_SET;
> + sec->cap_permitted = CAP_FULL_SET;
> + sec->cap_effective = CAP_INIT_EFF_SET;
> + sec->user = &root_user;
> + get_uid(sec->user);
> + sec->group_info = &init_groups;
> + get_group_info(sec->group_info);
> + }
> +
> + atomic_set(&sec->usage, 1);
> + spin_lock_init(&sec->lock);
> +#ifdef CONFIG_KEYS
> + sec->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
> +#endif
> +
> + ret = security_task_kernel_act_as(sec, service, daemon);
> + if (ret < 0) {
> + put_task_security(sec);
> + return ERR_PTR(ret);
> + }
> +
> + return sec;
> +}
> +EXPORT_SYMBOL(get_kernel_security);
> +
> +/**
> + * change_create_files_as - Change the file create context in a security record
> + * @sec: The security record to alter
> + * @inode: The inode to take the context from
> + *
> + * Change the file creation context in a security record to be the same as the
> + * object context of the specified inode, so that the new inodes have the same
> + * MAC context as that inode.
> + */
> +int change_create_files_as(struct task_security *sec, struct inode *inode)
> +{
> + sec->fsuid = inode->i_uid;
> + sec->fsgid = inode->i_gid;
> + return security_task_create_files_as(sec, inode);
> +}
> +EXPORT_SYMBOL(change_create_files_as);
> diff --git a/security/dummy.c b/security/dummy.c
> index 526549d..1c8bad6 100644
> --- a/security/dummy.c
> +++ b/security/dummy.c
> @@ -495,6 +495,19 @@ static int dummy_task_dup_security(struct task_security *p)
> return 0;
> }
>
> +static int dummy_task_kernel_act_as(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon)
> +{
> + return 0;
> +}
> +
> +static int dummy_task_create_files_as(struct task_security *sec,
> + struct inode *inode)
> +{
> + return 0;
> +}
> +
> static int dummy_task_setuid (uid_t id0, uid_t id1, uid_t id2, int flags)
> {
> return 0;
> @@ -1063,6 +1076,8 @@ void security_fixup_ops (struct security_operations *ops)
> set_to_dummy_if_null(ops, task_alloc_security);
> set_to_dummy_if_null(ops, task_free_security);
> set_to_dummy_if_null(ops, task_dup_security);
> + set_to_dummy_if_null(ops, task_kernel_act_as);
> + set_to_dummy_if_null(ops, task_create_files_as);
> set_to_dummy_if_null(ops, task_setuid);
> set_to_dummy_if_null(ops, task_post_setuid);
> set_to_dummy_if_null(ops, task_setgid);
> diff --git a/security/security.c b/security/security.c
> index 92d66d6..86d94e5 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -583,6 +583,19 @@ int security_task_dup(struct task_security *sec)
> return security_ops->task_dup_security(sec);
> }
>
> +int security_task_kernel_act_as(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon)
> +{
> + return security_ops->task_kernel_act_as(sec, service, daemon);
> +}
> +
> +int security_task_create_files_as(struct task_security *sec,
> + struct inode *inode)
> +{
> + return security_ops->task_create_files_as(sec, inode);
> +}
> +
> int security_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
> {
> return security_ops->task_setuid(id0, id1, id2, flags);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 20a6b55..2416f54 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2846,6 +2846,49 @@ static int selinux_task_dup_security(struct task_security *sec)
> return 0;
> }
>
> +/*
> + * get the security data for a kernel service, deriving the subjective context
> + * from the security data of a userspace daemon if one supplied
> + * - all the creation contexts are set to unlabelled
> + */
> +static int selinux_task_kernel_act_as(struct task_security *sec,
> + const char *service,
> + struct task_struct *daemon)
> +{
> + struct task_security_struct *tsec, *dtsec;
> + u32 ksid;
> + int ret;
> +
> + tsec = sec->security;
> + dtsec = daemon ? daemon->sec->security : init_task.sec->security;
> +
> + ret = security_transition_sid(dtsec->sid, SECINITSID_KERNEL,
> + SECCLASS_PROCESS, &ksid);
> + if (ret < 0)
> + return ret;
> +
> + tsec->sid = ksid;
> + tsec->create_sid = SECINITSID_UNLABELED;
> + tsec->keycreate_sid = SECINITSID_UNLABELED;
> + tsec->sockcreate_sid = SECINITSID_UNLABELED;
