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Subject[PATCH 11/26] Security: Allow kernel services to override LSM settings for task actions
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 | 23 +++++++
include/linux/security.h | 42 ++++++++++++-
kernel/cred.c | 111 +++++++++++++++++++++++++++++++++++
security/dummy.c | 17 +++++
security/security.c | 15 ++++-
security/selinux/hooks.c | 51 ++++++++++++++++
security/selinux/include/security.h | 2 -
security/selinux/ss/services.c | 5 +-
8 files changed, 257 insertions(+), 9 deletions(-)
create mode 100644 include/linux/cred.h

diff --git a/include/linux/cred.h b/include/linux/cred.h
new file mode 100644
index 0000000..497af5b
--- /dev/null
+++ b/include/linux/cred.h
@@ -0,0 +1,23 @@
+/* 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(struct task_struct *);
+extern int set_security_override(struct task_security *, u32);
+extern int set_security_override_from_ctx(struct task_security *, const char *);
+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 e8f2f2d..5a219ba 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -557,6 +557,19 @@ 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).
+ * @p points to the task that nominated @secid.
+ * @sec points to the task security record to be modified.
+ * @secid specifies the security ID to be set
+ * 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.
+ * @p points to the task that nominated @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
@@ -1325,6 +1338,11 @@ 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_struct *p,
+ struct task_security *sec, u32 secid);
+ int (*task_create_files_as)(struct task_struct *p,
+ 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);
@@ -1393,7 +1411,7 @@ struct security_operations {
int (*getprocattr)(struct task_struct *p, char *name, char **value);
int (*setprocattr)(struct task_struct *p, char *name, void *value, size_t size);
int (*secid_to_secctx)(u32 secid, char **secdata, u32 *seclen);
- int (*secctx_to_secid)(char *secdata, u32 seclen, u32 *secid);
+ int (*secctx_to_secid)(const char *secdata, u32 seclen, u32 *secid);
void (*release_secctx)(char *secdata, u32 seclen);

#ifdef CONFIG_SECURITY_NETWORK
@@ -1576,6 +1594,11 @@ 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_struct *p,
+ struct task_security *sec, u32 secid);
+int security_task_create_files_as(struct task_struct *p,
+ 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);
@@ -1628,7 +1651,7 @@ int security_setprocattr(struct task_struct *p, char *name, void *value, size_t
int security_netlink_send(struct sock *sk, struct sk_buff *skb);
int security_netlink_recv(struct sk_buff *skb, int cap);
int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen);
-int security_secctx_to_secid(char *secdata, u32 seclen, u32 *secid);
+int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid);
void security_release_secctx(char *secdata, u32 seclen);

#else /* CONFIG_SECURITY */
@@ -2060,6 +2083,19 @@ static inline int security_task_dup(struct task_security *p)
return 0;
}

+static inline int security_task_kernel_act_as(struct task_struct *p,
+ struct task_security *sec,
+ u32 secid)
+{
+ 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)
{
@@ -2311,7 +2347,7 @@ static inline int security_secid_to_secctx(u32 secid, char **secdata, u32 *secle
return -EOPNOTSUPP;
}

-static inline int security_secctx_to_secid(char *secdata,
+static inline int security_secctx_to_secid(const char *secdata,
u32 seclen,
u32 *secid)
{
diff --git a/kernel/cred.c b/kernel/cred.c
index ddf98c6..d740d47 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 +138,113 @@ void put_task_security(struct task_security *sec)
}
}
EXPORT_SYMBOL(put_task_security);
+
+/**
+ * get_kernel_security - Get a task security record for a kernel service
+ * @daemon: A userspace daemon to be used as a reference
+ *
+ * Get a task security record for a 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.
+ *
+ * The caller may change these controls afterwards if desired.
+ */
+struct task_security *get_kernel_security(struct task_struct *daemon)
+{
+ const struct task_security *dsec;
+ struct task_security *sec;
+
+ 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
+
+ return sec;
+}
+EXPORT_SYMBOL(get_kernel_security);
+
+/**
+ * set_security_override - Set the security ID in a security record
+ * @sec: The security record to alter
+ * @secid: The LSM security ID to set
+ *
+ * Set the LSM security ID in a security record so that the subjective security
+ * is overridden when the act_as pointer of a task is overridden.
+ */
+int set_security_override(struct task_security *sec, u32 secid)
+{
+ return security_task_kernel_act_as(current, sec, secid);
+}
+EXPORT_SYMBOL(set_security_override);
+
+/**
+ * set_security_override_from_ctx - Set the security ID in a security record
+ * @sec: The security record to alter
+ * @secctx: The LSM security context to generate the security ID from.
+ *
+ * Set the LSM security ID in a security record so that the subjective security
+ * is overridden when the act_as pointer of a task is overridden. The security
+ * ID is specified in string form as a security context to be interpreted by
+ * the LSM.
+ */
+int set_security_override_from_ctx(struct task_security *sec, const char *secctx)
+{
+ u32 secid;
+ int ret;
+
+ ret = security_secctx_to_secid(secctx, strlen(secctx), &secid);
+ if (ret < 0)
+ return ret;
+
+ return set_security_override(sec, secid);
+}
+EXPORT_SYMBOL(set_security_override_from_ctx);
+
+/**
+ * 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(current, sec, inode);
+}
+EXPORT_SYMBOL(change_create_files_as);
diff --git a/security/dummy.c b/security/dummy.c
index 72f1666..139b9de 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_struct *p,
+ struct task_security *sec, u32 secid)
+{
+ return 0;
+}
+
+static int dummy_task_create_files_as(struct task_struct *p,
+ 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;
@@ -943,7 +956,7 @@ static int dummy_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
return -EOPNOTSUPP;
}

-static int dummy_secctx_to_secid(char *secdata, u32 seclen, u32 *secid)
+static int dummy_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
{
return -EOPNOTSUPP;
}
@@ -1068,6 +1081,8 @@ 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 1ef4908..a950909 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_struct *p,
+ struct task_security *sec, u32 secid)
+{
+ return security_ops->task_kernel_act_as(p, sec, secid);
+}
+
+int security_task_create_files_as(struct task_struct *p,
+ struct task_security *sec,
+ struct inode *inode)
+{
+ return security_ops->task_create_files_as(p, 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);
@@ -821,7 +834,7 @@ int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
}
EXPORT_SYMBOL(security_secid_to_secctx);

-int security_secctx_to_secid(char *secdata, u32 seclen, u32 *secid)
+int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
{
return security_ops->secctx_to_secid(secdata, seclen, secid);
}
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1d3eab7..781ed64 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2846,6 +2846,53 @@ static int selinux_task_dup_security(struct task_security *sec)
return 0;
}

+/*
+ * set the security data for a kernel service
+ * - all the creation contexts are set to unlabelled
+ */
+static int selinux_task_kernel_act_as(struct task_struct *p,
+ struct task_security *sec, u32 secid)
+{
+ struct task_security_struct *tsec = sec->security;
+ struct task_security_struct *ptsec = p->act_as->security;
+ int ret;
+
+ ret = avc_has_perm(ptsec->sid, secid,
+ SECCLASS_KERNEL_SERVICE,
+ KERNEL_SERVICE__USE_AS_OVERRIDE,
+ NULL);
+ if (ret == 0) {
+ tsec->sid = secid;
+ tsec->create_sid = 0;
+ tsec->keycreate_sid = 0;
+ tsec->sockcreate_sid = 0;
+ }
+ return ret;
+}
+
+/*
+ * 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_struct *p,
+ struct task_security *sec,
+ struct inode *inode)
+{
+ struct inode_security_struct *isec = inode->i_security;
+ struct task_security_struct *tsec = sec->security;
+ struct task_security_struct *ptsec = p->act_as->security;
+ int ret;
+
+ ret = avc_has_perm(ptsec->sid, isec->sid,
+ SECCLASS_KERNEL_SERVICE,
+ KERNEL_SERVICE__CREATE_FILES_AS,
+ NULL);
+
+ if (ret == 0)
+ 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
@@ -4734,7 +4781,7 @@ static int selinux_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
return security_sid_to_context(secid, secdata, seclen);
}

-static int selinux_secctx_to_secid(char *secdata, u32 seclen, u32 *secid)
+static int selinux_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
{
return security_context_to_sid(secdata, seclen, secid);
}
@@ -4889,6 +4936,8 @@ 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,
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index 39337af..727b29e 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -66,7 +66,7 @@ int security_change_sid(u32 ssid, u32 tsid,
int security_sid_to_context(u32 sid, char **scontext,
u32 *scontext_len);

-int security_context_to_sid(char *scontext, u32 scontext_len,
+int security_context_to_sid(const char *scontext, u32 scontext_len,
u32 *out_sid);

int security_context_to_sid_default(char *scontext, u32 scontext_len, u32 *out_sid, u32 def_sid);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index d572dc9..be4bed3 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -677,7 +677,8 @@ out:

}

-static int security_context_to_sid_core(char *scontext, u32 scontext_len, u32 *sid, u32 def_sid)
+static int security_context_to_sid_core(const char *scontext, u32 scontext_len,
+ u32 *sid, u32 def_sid)
{
char *scontext2;
struct context context;
@@ -803,7 +804,7 @@ out:
* Returns -%EINVAL if the context is invalid, -%ENOMEM if insufficient
* memory is available, or 0 on success.
*/
-int security_context_to_sid(char *scontext, u32 scontext_len, u32 *sid)
+int security_context_to_sid(const char *scontext, u32 scontext_len, u32 *sid)
{
return security_context_to_sid_core(scontext, scontext_len,
sid, SECSID_NULL);


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