[PATCH v2 4/4] selinux: Implement create_user_ns hook

[Frederick Lawler <fred@cloudflare.com>]

Unprivileged user namespace creation is an intended feature to enable
sandboxing, however this feature is often used to as an initial step to
perform a privilege escalation attack.

This patch implements a new namespace { userns_create } access control
permission to restrict which domains allow or deny user namespace
creation. This is necessary for system administrators to quickly protect
their systems while waiting for vulnerability patches to be applied.

This permission can be used in the following way:

        allow domA_t domB_t : namespace { userns_create };

Signed-off-by: Frederick Lawler <fred@cloudflare.com>

---
Changes since v1:
- Introduce this patch
---
 security/selinux/hooks.c            | 9 +++++++++
 security/selinux/include/classmap.h | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index beceb89f68d9..73fbcb434fe0 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4227,6 +4227,14 @@ static void selinux_task_to_inode(struct task_struct *p,
 	spin_unlock(&isec->lock);
 }
 
+static int selinux_userns_create(const struct cred *cred)
+{
+	u32 sid = current_sid();
+
+	return avc_has_perm(&selinux_state, sid, sid, SECCLASS_NAMESPACE,
+						NAMESPACE__USERNS_CREATE, NULL);
+}
+
 /* Returns error only if unable to parse addresses */
 static int selinux_parse_skb_ipv4(struct sk_buff *skb,
 			struct common_audit_data *ad, u8 *proto)
@@ -7117,6 +7125,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
 	LSM_HOOK_INIT(task_kill, selinux_task_kill),
 	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
+	LSM_HOOK_INIT(create_user_ns, selinux_userns_create),
 
 	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
 	LSM_HOOK_INIT(ipc_getsecid, selinux_ipc_getsecid),
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index ff757ae5f253..9943e85c6b3e 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -254,6 +254,8 @@ const struct security_class_mapping secclass_map[] = {
 	  { COMMON_FILE_PERMS, NULL } },
 	{ "io_uring",
 	  { "override_creds", "sqpoll", NULL } },
+	{ "namespace",
+	  { "userns_create", NULL } },
 	{ NULL }
   };
 
-- 
2.30.2