Subject: [PATCH v3 1/4] security, lsm: Introduce security_create_user_ns()

From: Frederick Lawler <[email-masked]>
Date: Thu, 21 Jul 2022 12:28:05 -0500

Preventing user namespace (privileged or otherwise) creation comes in a
few of forms in order of granularity:

        1. /proc/sys/user/max_user_namespaces sysctl
        2. OS specific patch(es)
        3. CONFIG_USER_NS

To block a task based on its attributes, the LSM hook cred_prepare is a
good candidate for use because it provides more granular control, and
it is called before create_user_ns():

        cred = prepare_creds()
                security_prepare_creds()
                        call_int_hook(cred_prepare, ...
        if (cred)
                create_user_ns(cred)

Since security_prepare_creds() is meant for LSMs to copy and prepare
credentials, access control is an unintended use of the hook. Therefore
introduce a new function security_create_user_ns() with an accompanying
userns_create LSM hook.

This hook takes the prepared creds for LSM authors to write policy
against. On success, the new namespace is applied to credentials,
otherwise an error is returned.

Signed-off-by: Frederick Lawler <[email-masked]>

---
Changes since v2:
- Rename create_user_ns hook to userns_create
Changes since v1:
- Changed commit wording
- Moved execution to be after id mapping check
- Changed signature to only accept a const struct cred *
---
 include/linux/lsm_hook_defs.h | 1 +
 include/linux/lsm_hooks.h     | 4 ++++
 include/linux/security.h      | 6 ++++++
 kernel/user_namespace.c       | 5 +++++
 security/security.c           | 5 +++++
 5 files changed, 21 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index eafa1d2489fd..7ff93cb8ca8d 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -223,6 +223,7 @@ LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2,
 	 unsigned long arg3, unsigned long arg4, unsigned long arg5)
 LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p,
 	 struct inode *inode)
+LSM_HOOK(int, 0, userns_create, const struct cred *cred)
 LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag)
 LSM_HOOK(void, LSM_RET_VOID, ipc_getsecid, struct kern_ipc_perm *ipcp,
 	 u32 *secid)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 91c8146649f5..54fe534d0e01 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -799,6 +799,10 @@
  *	security attributes, e.g. for /proc/pid inodes.
  *	@p contains the task_struct for the task.
  *	@inode contains the inode structure for the inode.
+ * @userns_create:
+ *	Check permission prior to creating a new user namespace.
+ *	@cred points to prepared creds.
+ *	Return 0 if successful, otherwise < 0 error code.
  *
  * Security hooks for Netlink messaging.
  *
diff --git a/include/linux/security.h b/include/linux/security.h
index 7fc4e9f49f54..a195bf33246a 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -435,6 +435,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info,
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			unsigned long arg4, unsigned long arg5);
 void security_task_to_inode(struct task_struct *p, struct inode *inode);
+int security_create_user_ns(const struct cred *cred);
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
 void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
 int security_msg_msg_alloc(struct msg_msg *msg);
@@ -1185,6 +1186,11 @@ static inline int security_task_prctl(int option, unsigned long arg2,
 static inline void security_task_to_inode(struct task_struct *p, struct inode *inode)
 { }
 
+static inline int security_create_user_ns(const struct cred *cred)
+{
+	return 0;
+}
+
 static inline int security_ipc_permission(struct kern_ipc_perm *ipcp,
 					  short flag)
 {
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 5481ba44a8d6..3f464bbda0e9 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -9,6 +9,7 @@
 #include <linux/highuid.h>
 #include <linux/cred.h>
 #include <linux/securebits.h>
+#include <linux/security.h>
 #include <linux/keyctl.h>
 #include <linux/key-type.h>
 #include <keys/user-type.h>
@@ -113,6 +114,10 @@ int create_user_ns(struct cred *new)
 	    !kgid_has_mapping(parent_ns, group))
 		goto fail_dec;
 
+	ret = security_create_user_ns(new);
+	if (ret < 0)
+		goto fail_dec;
+
 	ret = -ENOMEM;
 	ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
 	if (!ns)
diff --git a/security/security.c b/security/security.c
index 188b8f782220..ec9b4696e86c 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1903,6 +1903,11 @@ void security_task_to_inode(struct task_struct *p, struct inode *inode)
 	call_void_hook(task_to_inode, p, inode);
 }
 
+int security_create_user_ns(const struct cred *cred)
+{
+	return call_int_hook(userns_create, 0, cred);
+}
+
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag)
 {
 	return call_int_hook(ipc_permission, 0, ipcp, flag);
-- 
2.30.2