CRED: Allow kernel services to override LSM settings for task actions
Allow kernel services to override LSM settings appropriate to the actions
performed by a task by duplicating a set of credentials, modifying it and then
using task_struct::cred 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
credentials.
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_kernel_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.
The patch also provides four new credentials handling functions, which wrap the
LSM functions:
(1) prepare_kernel_cred()
Prepare a set of credentials for a kernel service to use, based either on
a daemon's credentials or on init_cred. All the keyrings are cleared.
(2) set_security_override()
Set the LSM security ID in a set of credentials to a specific security
context, assuming permission from the LSM policy.
(3) set_security_override_from_ctx()
As (2), but takes the security context as a string.
(4) set_create_files_as()
Set the file creation LSM security ID in a set of credentials to be the
same as that on a particular inode.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com> [Smack changes]
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 55a9c99..26c1ab1 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -18,6 +18,7 @@
struct user_struct;
struct cred;
+struct inode;
/*
* COW Supplementary groups list
@@ -148,6 +149,11 @@
extern void abort_creds(struct cred *);
extern const struct cred *override_creds(const struct cred *);
extern void revert_creds(const struct cred *);
+extern struct cred *prepare_kernel_cred(struct task_struct *);
+extern int change_create_files_as(struct cred *, struct inode *);
+extern int set_security_override(struct cred *, u32);
+extern int set_security_override_from_ctx(struct cred *, const char *);
+extern int set_create_files_as(struct cred *, struct inode *);
extern void __init cred_init(void);
/**
diff --git a/include/linux/security.h b/include/linux/security.h
index 56a0eed..59a11e1 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -587,6 +587,19 @@
* @new points to the new credentials.
* @old points to the original credentials.
* Install a new set of credentials.
+ * @kernel_act_as:
+ * Set the credentials for a kernel service to act as (subjective context).
+ * @new points to the credentials to be modified.
+ * @secid specifies the security ID to be set
+ * The current task must be the one that nominated @secid.
+ * Return 0 if successful.
+ * @kernel_create_files_as:
+ * Set the file creation context in a set of credentials to be the same as
+ * the objective context of the specified inode.
+ * @new points to the credentials to be modified.
+ * @inode points to the inode to use as a reference.
+ * The current task must be the one that nominated @inode.
+ * 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
@@ -1381,6 +1394,8 @@
int (*cred_prepare)(struct cred *new, const struct cred *old,
gfp_t gfp);
void (*cred_commit)(struct cred *new, const struct cred *old);
+ int (*kernel_act_as)(struct cred *new, u32 secid);
+ int (*kernel_create_files_as)(struct cred *new, struct inode *inode);
int (*task_setuid) (uid_t id0, uid_t id1, uid_t id2, int flags);
int (*task_fix_setuid) (struct cred *new, const struct cred *old,
int flags);
@@ -1632,6 +1647,8 @@
void security_cred_free(struct cred *cred);
int security_prepare_creds(struct cred *new, const struct cred *old, gfp_t gfp);
void security_commit_creds(struct cred *new, const struct cred *old);
+int security_kernel_act_as(struct cred *new, u32 secid);
+int security_kernel_create_files_as(struct cred *new, struct inode *inode);
int security_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags);
int security_task_fix_setuid(struct cred *new, const struct cred *old,
int flags);
@@ -2151,6 +2168,17 @@
{
}
+static inline int security_kernel_act_as(struct cred *cred, u32 secid)
+{
+ return 0;
+}
+
+static inline int security_kernel_create_files_as(struct cred *cred,
+ 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 f3ca106..13697ca 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -462,3 +462,116 @@
cred_jar = kmem_cache_create("cred_jar", sizeof(struct cred),
0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
}
+
+/**
+ * prepare_kernel_cred - Prepare a set of credentials for a kernel service
+ * @daemon: A userspace daemon to be used as a reference
+ *
+ * Prepare a set of credentials for a kernel service. This can then be used to
+ * override a task's own credentials so that work can be done on behalf of that
+ * task that requires a different subjective 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.
+ *
+ * Returns the new credentials or NULL if out of memory.
+ *
+ * Does not take, and does not return holding current->cred_replace_mutex.
+ */
+struct cred *prepare_kernel_cred(struct task_struct *daemon)
+{
+ const struct cred *old;
+ struct cred *new;
+
+ new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
+ if (!new)
+ return NULL;
+
+ if (daemon)
+ old = get_task_cred(daemon);
+ else
+ old = get_cred(&init_cred);
+
+ get_uid(new->user);
+ get_group_info(new->group_info);
+
+#ifdef CONFIG_KEYS
+ atomic_inc(&init_tgcred.usage);
+ new->tgcred = &init_tgcred;
+ new->request_key_auth = NULL;
+ new->thread_keyring = NULL;
+ new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
+#endif
+
+#ifdef CONFIG_SECURITY
+ new->security = NULL;
+#endif
+ if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
+ goto error;
+
+ atomic_set(&new->usage, 1);
+ put_cred(old);
+ return new;
+
+error:
+ put_cred(new);
+ return NULL;
+}
+EXPORT_SYMBOL(prepare_kernel_cred);
+
+/**
+ * set_security_override - Set the security ID in a set of credentials
+ * @new: The credentials to alter
+ * @secid: The LSM security ID to set
+ *
+ * Set the LSM security ID in a set of credentials so that the subjective
+ * security is overridden when an alternative set of credentials is used.
+ */
+int set_security_override(struct cred *new, u32 secid)
+{
+ return security_kernel_act_as(new, secid);
+}
+EXPORT_SYMBOL(set_security_override);
+
+/**
+ * set_security_override_from_ctx - Set the security ID in a set of credentials
+ * @new: The credentials to alter
+ * @secctx: The LSM security context to generate the security ID from.
+ *
+ * Set the LSM security ID in a set of credentials so that the subjective
+ * security is overridden when an alternative set of credentials is used. 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 cred *new, 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(new, secid);
+}
+EXPORT_SYMBOL(set_security_override_from_ctx);
+
+/**
+ * set_create_files_as - Set the LSM file create context in a set of credentials
+ * @new: The credentials to alter
+ * @inode: The inode to take the context from
+ *
+ * Change the LSM file creation context in a set of credentials 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 set_create_files_as(struct cred *new, struct inode *inode)
+{
+ new->fsuid = inode->i_uid;
+ new->fsgid = inode->i_gid;
+ return security_kernel_create_files_as(new, inode);
+}
+EXPORT_SYMBOL(set_create_files_as);
diff --git a/security/capability.c b/security/capability.c
index 185804f..b9e3914 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -348,6 +348,16 @@
{
}
+static int cap_kernel_act_as(struct cred *new, u32 secid)
+{
+ return 0;
+}
+
+static int cap_kernel_create_files_as(struct cred *new, struct inode *inode)
+{
+ return 0;
+}
+
static int cap_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
{
return 0;
@@ -889,6 +899,8 @@
set_to_cap_if_null(ops, cred_free);
set_to_cap_if_null(ops, cred_prepare);
set_to_cap_if_null(ops, cred_commit);
+ set_to_cap_if_null(ops, kernel_act_as);
+ set_to_cap_if_null(ops, kernel_create_files_as);
set_to_cap_if_null(ops, task_setuid);
set_to_cap_if_null(ops, task_fix_setuid);
set_to_cap_if_null(ops, task_setgid);
diff --git a/security/security.c b/security/security.c
index dc5babb2..038ef04 100644
--- a/security/security.c
+++ b/security/security.c
@@ -616,6 +616,16 @@
return security_ops->cred_commit(new, old);
}
+int security_kernel_act_as(struct cred *new, u32 secid)
+{
+ return security_ops->kernel_act_as(new, secid);
+}
+
+int security_kernel_create_files_as(struct cred *new, struct inode *inode)
+{
+ return security_ops->kernel_create_files_as(new, 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 91b06f2..520f82a 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3277,6 +3277,50 @@
secondary_ops->cred_commit(new, old);
}
+/*
+ * set the security data for a kernel service
+ * - all the creation contexts are set to unlabelled
+ */
+static int selinux_kernel_act_as(struct cred *new, u32 secid)
+{
+ struct task_security_struct *tsec = new->security;
+ u32 sid = current_sid();
+ int ret;
+
+ ret = avc_has_perm(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_kernel_create_files_as(struct cred *new, struct inode *inode)
+{
+ struct inode_security_struct *isec = inode->i_security;
+ struct task_security_struct *tsec = new->security;
+ u32 sid = current_sid();
+ int ret;
+
+ ret = avc_has_perm(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
@@ -5593,6 +5637,8 @@
.cred_free = selinux_cred_free,
.cred_prepare = selinux_cred_prepare,
.cred_commit = selinux_cred_commit,
+ .kernel_act_as = selinux_kernel_act_as,
+ .kernel_create_files_as = selinux_kernel_create_files_as,
.task_setuid = selinux_task_setuid,
.task_fix_setuid = selinux_task_fix_setuid,
.task_setgid = selinux_task_setgid,
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index de39674..8ad48161 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -1012,6 +1012,41 @@
}
/**
+ * smack_kernel_act_as - Set the subjective context in a set of credentials
+ * @new points to the set of credentials to be modified.
+ * @secid specifies the security ID to be set
+ *
+ * Set the security data for a kernel service.
+ */
+static int smack_kernel_act_as(struct cred *new, u32 secid)
+{
+ char *smack = smack_from_secid(secid);
+
+ if (smack == NULL)
+ return -EINVAL;
+
+ new->security = smack;
+ return 0;
+}
+
+/**
+ * smack_kernel_create_files_as - Set the file creation label in a set of creds
+ * @new points to the set of credentials to be modified
+ * @inode points to the inode to use as a reference
+ *
+ * Set the file creation context in a set of credentials to the same
+ * as the objective context of the specified inode
+ */
+static int smack_kernel_create_files_as(struct cred *new,
+ struct inode *inode)
+{
+ struct inode_smack *isp = inode->i_security;
+
+ new->security = isp->smk_inode;
+ return 0;
+}
+
+/**
* smack_task_setpgid - Smack check on setting pgid
* @p: the task object
* @pgid: unused
@@ -2641,6 +2676,8 @@
.cred_free = smack_cred_free,
.cred_prepare = smack_cred_prepare,
.cred_commit = smack_cred_commit,
+ .kernel_act_as = smack_kernel_act_as,
+ .kernel_create_files_as = smack_kernel_create_files_as,
.task_fix_setuid = cap_task_fix_setuid,
.task_setpgid = smack_task_setpgid,
.task_getpgid = smack_task_getpgid,
