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    Subject[PATCH 08/28] SECURITY: Allow kernel services to override LSM settings for task actions [try #2]
    Date
    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;
    + sec->security = tsec;
    + return 0;
    +}
    +
    +/*
    + * set the file creation context in a security record to the same as the
    + * objective context of the specified inode
    + */
    +static int selinux_task_create_files_as(struct task_security *sec,
    + struct inode *inode)
    +{
    + struct task_security_struct *tsec = sec->security;
    + struct inode_security_struct *isec = inode->i_security;
    +
    + tsec->create_sid = isec->sid;
    + return 0;
    +}
    +
    static int selinux_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
    {
    /* Since setuid only affects the current process, and
    @@ -4884,6 +4927,8 @@ static struct security_operations selinux_ops = {
    .task_alloc_security = selinux_task_alloc_security,
    .task_free_security = selinux_task_free_security,
    .task_dup_security = selinux_task_dup_security,
    + .task_kernel_act_as = selinux_task_kernel_act_as,
    + .task_create_files_as = selinux_task_create_files_as,
    .task_setuid = selinux_task_setuid,
    .task_post_setuid = selinux_task_post_setuid,
    .task_setgid = selinux_task_setgid,


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