[PATCH v2 1/7] user_ns: 3 new hooks for user namespace operations

[Lukasz Pawelczyk]

This commit implements 3 new LSM hooks that provide the means for LSMs
to embed their own security context within user namespace, effectively
creating some sort of a user_ns related security namespace.

The first one to take advantage of this mechanism is Smack.

The hooks has been documented in the in the security.h below.

Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@xxxxxxxxxxx>
---
 include/linux/lsm_hooks.h      | 28 ++++++++++++++++++++++++++++
 include/linux/security.h       | 23 +++++++++++++++++++++++
 include/linux/user_namespace.h |  4 ++++
 kernel/user.c                  |  3 +++
 kernel/user_namespace.c        | 18 ++++++++++++++++++
 security/security.c            | 28 ++++++++++++++++++++++++++++
 6 files changed, 104 insertions(+)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index f014f25..b6e0c3d 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1260,6 +1260,23 @@
  *	audit_rule_init.
  *	@rule contains the allocated rule
  *
+ * @userns_create:
+ *	Allocates and fills the security part of a new user namespace.
+ *	@ns points to a newly created user namespace.
+ *	Returns 0 or an error code.
+ *
+ * @userns_free:
+ *	Deallocates the security part of a user namespace.
+ *	@ns points to a user namespace about to be destroyed.
+ *
+ * @userns_setns:
+ *	Run during a setns syscall to add a process to an already existing
+ *	user namespace. Returning failure here will block the operation
+ *	requested from userspace (setns() with CLONE_NEWUSER).
+ *	@nsproxy contains a nsproxy to which the user namespace will be assigned.
+ *	@ns contains user namespace that is to be incorporated to the nsproxy.
+ *	Returns 0 or an error code.
+ *
  * @inode_notifysecctx:
  *	Notify the security module of what the security context of an inode
  *	should be.  Initializes the incore security context managed by the
@@ -1611,6 +1628,12 @@ union security_list_options {
 				struct audit_context *actx);
 	void (*audit_rule_free)(void *lsmrule);
 #endif /* CONFIG_AUDIT */
+
+#ifdef CONFIG_SECURITY
+	int (*userns_create)(struct user_namespace *ns);
+	void (*userns_free)(struct user_namespace *ns);
+	int (*userns_setns)(struct nsproxy *nsproxy, struct user_namespace *ns);
+#endif /* CONFIG_SECURITY */
 };
 
 struct security_hook_heads {
@@ -1822,6 +1845,11 @@ struct security_hook_heads {
 	struct list_head audit_rule_match;
 	struct list_head audit_rule_free;
 #endif /* CONFIG_AUDIT */
+#ifdef CONFIG_SECURITY
+	struct list_head userns_create;
+	struct list_head userns_free;
+	struct list_head userns_setns;
+#endif /* CONFIG_SECURITY */
 };
 
 /*
diff --git a/include/linux/security.h b/include/linux/security.h
index 8c8175d..ec17ae1 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1583,6 +1583,29 @@ static inline void security_audit_rule_free(void *lsmrule)
 #endif /* CONFIG_SECURITY */
 #endif /* CONFIG_AUDIT */
 
+#ifdef CONFIG_SECURITY
+int security_userns_create(struct user_namespace *ns);
+void security_userns_free(struct user_namespace *ns);
+int security_userns_setns(struct nsproxy *nsproxy, struct user_namespace *ns);
+
+#else
+
+static inline int security_userns_create(struct user_namespace *ns)
+{
+	return 0;
+}
+
+static inline void security_userns_free(struct user_namespace *ns)
+{ }
+
+static inline int security_userns_setns(struct nsproxy *nsproxy,
+					struct user_namespace *ns)
+{
+	return 0;
+}
+
+#endif /* CONFIG_SECURITY */
+
 #ifdef CONFIG_SECURITYFS
 
 extern struct dentry *securityfs_create_file(const char *name, umode_t mode,
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 8297e5b..a9400cc 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -39,6 +39,10 @@ struct user_namespace {
 	struct key		*persistent_keyring_register;
 	struct rw_semaphore	persistent_keyring_register_sem;
 #endif
+
+#ifdef CONFIG_SECURITY
+	void *security;
+#endif
 };
 
 extern struct user_namespace init_user_ns;
diff --git a/kernel/user.c b/kernel/user.c
index b069ccb..ce5419e 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -59,6 +59,9 @@ struct user_namespace init_user_ns = {
 	.persistent_keyring_register_sem =
 	__RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
 #endif
+#ifdef CONFIG_SECURITY
+	.security = NULL,
+#endif
 };
 EXPORT_SYMBOL_GPL(init_user_ns);
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 4109f83..cadffb6 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -22,6 +22,7 @@
 #include <linux/ctype.h>
 #include <linux/projid.h>
 #include <linux/fs_struct.h>
+#include <linux/security.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 static DEFINE_MUTEX(userns_state_mutex);
@@ -108,6 +109,15 @@ int create_user_ns(struct cred *new)
 
 	set_cred_user_ns(new, ns);
 
+#ifdef CONFIG_SECURITY
+	ret = security_userns_create(ns);
+	if (ret) {
+		ns_free_inum(&ns->ns);
+		kmem_cache_free(user_ns_cachep, ns);
+		return ret;
+	}
+#endif
+
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 	init_rwsem(&ns->persistent_keyring_register_sem);
 #endif
@@ -143,6 +153,9 @@ void free_user_ns(struct user_namespace *ns)
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 		key_put(ns->persistent_keyring_register);
 #endif
+#ifdef CONFIG_SECURITY
+		security_userns_free(ns);
+#endif
 		ns_free_inum(&ns->ns);
 		kmem_cache_free(user_ns_cachep, ns);
 		ns = parent;
@@ -969,6 +982,7 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 {
 	struct user_namespace *user_ns = to_user_ns(ns);
 	struct cred *cred;
+	int err;
 
 	/* Don't allow gaining capabilities by reentering
 	 * the same user namespace.
@@ -986,6 +1000,10 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
+	err = security_userns_setns(nsproxy, user_ns);
+	if (err)
+		return err;
+
 	cred = prepare_creds();
 	if (!cred)
 		return -ENOMEM;
diff --git a/security/security.c b/security/security.c
index bd4c5f6..e902f3b 100644
--- a/security/security.c
+++ b/security/security.c
@@ -25,6 +25,7 @@
 #include <linux/mount.h>
 #include <linux/personality.h>
 #include <linux/backing-dev.h>
+#include <linux/user_namespace.h>
 #include <net/flow.h>
 
 #define MAX_LSM_EVM_XATTR	2
@@ -1541,6 +1542,25 @@ int security_audit_rule_match(u32 secid, u32 field, u32 op, void *lsmrule,
 }
 #endif /* CONFIG_AUDIT */
 
+#ifdef CONFIG_SECURITY
+
+int security_userns_create(struct user_namespace *ns)
+{
+	return call_int_hook(userns_create, 0, ns);
+}
+
+void security_userns_free(struct user_namespace *ns)
+{
+	call_void_hook(userns_free, ns);
+}
+
+int security_userns_setns(struct nsproxy *nsproxy, struct user_namespace *ns)
+{
+	return call_int_hook(userns_setns, 0, nsproxy, ns);
+}
+
+#endif /* CONFIG_SECURITY */
+
 struct security_hook_heads security_hook_heads = {
 	.binder_set_context_mgr =
 		LIST_HEAD_INIT(security_hook_heads.binder_set_context_mgr),
@@ -1885,4 +1905,12 @@ struct security_hook_heads security_hook_heads = {
 	.audit_rule_free =
 		LIST_HEAD_INIT(security_hook_heads.audit_rule_free),
 #endif /* CONFIG_AUDIT */
+#ifdef CONFIG_SECURITY
+	.userns_create =
+		LIST_HEAD_INIT(security_hook_heads.userns_create),
+	.userns_free =
+		LIST_HEAD_INIT(security_hook_heads.userns_free),
+	.userns_setns =
+		LIST_HEAD_INIT(security_hook_heads.userns_setns),
+#endif /* CONFIG_SECURITY */
 };
-- 
2.1.0

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