apparmor: add user namespace creation mediation

Unprivileged user namespace creation is often used as a first step
in privilege escalation attacks. Instead of disabling it at the
sysrq level, which blocks its legitimate use as for setting up a sandbox,
allow control on a per domain basis.

This allows an admin to quickly lock down a system while also still
allowing legitimate use.

Reviewed-by: Georgia Garcia <[e-mail]>
Signed-off-by: John Johansen <[e-mail]>

security/apparmor/apparmorfs.c

···
 static struct aa_sfs_entry aa_sfs_entry_ns[] = {
 	AA_SFS_FILE_BOOLEAN("profile",		1),
 	AA_SFS_FILE_BOOLEAN("pivot_root",	0),
+	AA_SFS_FILE_STRING("mask", "userns_create"),
 	{ }
 };
 

security/apparmor/audit.c

···
 	"io_uring",
 	"module",
 	"lsm",
-	"unknown",
+	"namespace",
 	"unknown",
 	"unknown",
 	"unknown",

security/apparmor/include/apparmor.h

···
 #define AA_CLASS_IO_URING	18
 #define AA_CLASS_MODULE		19
 #define AA_CLASS_DISPLAY_LSM	20
+#define AA_CLASS_NS		21
 
 #define AA_CLASS_X		31
 #define AA_CLASS_DBUS		32

security/apparmor/include/audit.h

···
 #define OP_PROF_LOAD "profile_load"
 #define OP_PROF_RM "profile_remove"
 
+#define OP_USERNS_CREATE "userns_create"
 
 struct apparmor_audit_data {
 	int error;

security/apparmor/include/task.h

···
 		  u32 request);
 
 
+
+#define AA_USERNS_CREATE	8
+
+int aa_profile_ns_perm(struct aa_profile *profile,
+		       struct apparmor_audit_data *ad, u32 request);
+
 #endif /* __AA_TASK_H */

security/apparmor/lsm.c

···
 	return error;
 }
 
+static int apparmor_userns_create(const struct cred *cred)
+{
+	struct aa_label *label;
+	struct aa_profile *profile;
+	int error = 0;
+	DEFINE_AUDIT_DATA(ad, LSM_AUDIT_DATA_TASK, AA_CLASS_NS,
+			  OP_USERNS_CREATE);
+
+	ad.subj_cred = current_cred();
+
+	label = begin_current_label_crit_section();
+	if (!unconfined(label)) {
+		error = fn_for_each(label, profile,
+				    aa_profile_ns_perm(profile, &ad,
+						       AA_USERNS_CREATE));
+	}
+	end_current_label_crit_section(label);
+
+	return error;
+}
+
 /**
  * apparmor_sk_alloc_security - allocate and attach the sk_security field
  */
···
 	LSM_HOOK_INIT(task_getsecid_obj, apparmor_task_getsecid_obj),
 	LSM_HOOK_INIT(task_setrlimit, apparmor_task_setrlimit),
 	LSM_HOOK_INIT(task_kill, apparmor_task_kill),
+	LSM_HOOK_INIT(userns_create, apparmor_userns_create),
 
 #ifdef CONFIG_AUDIT
 	LSM_HOOK_INIT(audit_rule_init, aa_audit_rule_init),
···
 }
 
 static struct ctl_table apparmor_sysctl_table[] = {
+#ifdef CONFIG_USER_NS
 	{
 		.procname       = "unprivileged_userns_apparmor_policy",
 		.data           = &unprivileged_userns_apparmor_policy,
···
 		.mode           = 0600,
 		.proc_handler   = apparmor_dointvec,
 	},
+#endif /* CONFIG_USER_NS */
 	{
 		.procname       = "apparmor_display_secid_mode",
 		.data           = &apparmor_display_secid_mode,
···
 		.mode           = 0600,
 		.proc_handler   = apparmor_dointvec,
 	},
-
 	{ }
 };
 

security/apparmor/task.c

···
 			profile_tracee_perm(tracee_cred, profile, tracer,
 					    xrequest, &sa));
 }
+
+/* call back to audit ptrace fields */
+static void audit_ns_cb(struct audit_buffer *ab, void *va)
+{
+	struct apparmor_audit_data *ad = aad_of_va(va);
+
+	if (ad->request & AA_USERNS_CREATE)
+		audit_log_format(ab, " requested=\"userns_create\"");
+
+	if (ad->denied & AA_USERNS_CREATE)
+		audit_log_format(ab, " denied=\"userns_create\"");
+}
+
+int aa_profile_ns_perm(struct aa_profile *profile,
+		       struct apparmor_audit_data *ad,
+		       u32 request)
+{
+	struct aa_perms perms = { };
+	int error = 0;
+
+	ad->subj_label = &profile->label;
+	ad->request = request;
+
+	if (!profile_unconfined(profile)) {
+		struct aa_ruleset *rules = list_first_entry(&profile->rules,
+							    typeof(*rules),
+							    list);
+		aa_state_t state;
+
+		state = RULE_MEDIATES(rules, ad->class);
+		if (!state)
+			/* TODO: add flag to complain about unmediated */
+			return 0;
+		perms = *aa_lookup_perms(rules->policy, state);
+		aa_apply_modes_to_perms(profile, &perms);
+		error = aa_check_perms(profile, &perms, request, ad,
+				       audit_ns_cb);
+	}
+
+	return error;
+}