[PATCH 08/28] SECURITY: Allow kernel services to override LSM settings for task actions [try #2]

[David Howells <dhowells@redhat.com>]

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,