[RFC PATCH v2 10/14] lsm: Add security_sb_alloc_userns() hook

[Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>]

Provide a new lsm hook which may be used to allow access to device
nodes for super blocks created in unprivileged namespaces if some
sort of device guard to control access is implemented.

By default this will return -EPERM if no lsm implements the hook.
A first lsm to use this will be the lately converted cgroup_device
module.

Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/security.h      |  5 +++++
 security/security.c           | 26 ++++++++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index f4fa01182910..0f734a0a5ebc 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -278,6 +278,7 @@ LSM_HOOK(int, 0, inode_getsecctx, struct inode *inode, void **ctx,
 LSM_HOOK(int, 0, dev_permission, umode_t mode, dev_t dev, int mask)
 LSM_HOOK(int, -EPERM, inode_mknod_nscap, struct inode *dir, struct dentry *dentry,
 	 umode_t mode, dev_t dev)
+LSM_HOOK(int, -EPERM, sb_alloc_userns, struct super_block *sb)
 
 #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
 LSM_HOOK(int, 0, post_notification, const struct cred *w_cred,
diff --git a/include/linux/security.h b/include/linux/security.h
index bad6992877f4..0f66be1ed1ed 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -487,6 +487,7 @@ int security_locked_down(enum lockdown_reason what);
 int security_dev_permission(umode_t mode, dev_t dev, int mask);
 int security_inode_mknod_nscap(struct inode *dir, struct dentry *dentry,
 			       umode_t mode, dev_t dev);
+int security_sb_alloc_userns(struct super_block *sb);
 #else /* CONFIG_SECURITY */
 
 static inline int call_blocking_lsm_notifier(enum lsm_event event, void *data)
@@ -1408,6 +1409,10 @@ static inline int security_inode_mknod_nscap(struct inode *dir,
 {
 	return -EPERM;
 }
+static inline int security_sb_alloc_userns(struct super_block *sb)
+{
+	return -EPERM;
+}
 #endif	/* CONFIG_SECURITY */
 
 #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
diff --git a/security/security.c b/security/security.c
index 7708374b6d7e..9d5d4ec28e62 100644
--- a/security/security.c
+++ b/security/security.c
@@ -4065,6 +4065,32 @@ int security_inode_mknod_nscap(struct inode *dir, struct dentry *dentry,
 }
 EXPORT_SYMBOL(security_inode_mknod_nscap);
 
+/**
+ * security_sb_alloc_userns() - Grand access to device nodes on sb in userns
+ *
+ * If device access is provided elsewere, this hook will grand access to device nodes
+ * on the allocated sb for unprivileged user namespaces.
+ *
+ * Return: Returns 0 on success, error on failure.
+ */
+int security_sb_alloc_userns(struct super_block *sb)
+{
+	int thisrc;
+	int rc = LSM_RET_DEFAULT(sb_alloc_userns);
+	struct security_hook_list *hp;
+
+	hlist_for_each_entry(hp, &security_hook_heads.sb_alloc_userns, list) {
+		thisrc = hp->hook.sb_alloc_userns(sb);
+		if (thisrc != LSM_RET_DEFAULT(sb_alloc_userns)) {
+			rc = thisrc;
+			if (thisrc != 0)
+				break;
+		}
+	}
+	return rc;
+}
+EXPORT_SYMBOL(security_sb_alloc_userns);
+
 #ifdef CONFIG_WATCH_QUEUE
 /**
  * security_post_notification() - Check if a watch notification can be posted
-- 
2.30.2