selinux: teach SELinux about anonymous inodes

This change uses the anon_inodes and LSM infrastructure introduced in
the previous patches to give SELinux the ability to control
anonymous-inode files that are created using the new
anon_inode_getfd_secure() function.

A SELinux policy author detects and controls these anonymous inodes by
adding a name-based type_transition rule that assigns a new security
type to anonymous-inode files created in some domain. The name used
for the name-based transition is the name associated with the
anonymous inode for file listings --- e.g., "[userfaultfd]" or
"[perf_event]".

Example:

type uffd_t;
type_transition sysadm_t sysadm_t : anon_inode uffd_t "[userfaultfd]";
allow sysadm_t uffd_t:anon_inode { create };

(The next patch in this series is necessary for making userfaultfd
support this new interface.  The example above is just
for exposition.)
Signed-off-by: Daniel Colascione <dancol@google.com>
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -2962,6 +2962,62 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 	return 0;
 }
 
+static int selinux_inode_init_security_anon(struct inode *inode,
+					    const struct qstr *name,
+					    const struct inode *context_inode)
+{
+	const struct task_security_struct *tsec = selinux_cred(current_cred());
+	struct common_audit_data ad;
+	struct inode_security_struct *isec;
+	int rc;
+
+	if (unlikely(!selinux_initialized(&selinux_state)))
+		return 0;
+
+	isec = selinux_inode(inode);
+
+	/*
+	 * We only get here once per ephemeral inode.  The inode has
+	 * been initialized via inode_alloc_security but is otherwise
+	 * untouched.
+	 */
+
+	if (context_inode) {
+		struct inode_security_struct *context_isec =
+			selinux_inode(context_inode);
+		if (context_isec->initialized != LABEL_INITIALIZED) {
+			pr_err("SELinux:  context_inode is not initialized");
+			return -EACCES;
+		}
+
+		isec->sclass = context_isec->sclass;
+		isec->sid = context_isec->sid;
+	} else {
+		isec->sclass = SECCLASS_ANON_INODE;
+		rc = security_transition_sid(
+			&selinux_state, tsec->sid, tsec->sid,
+			isec->sclass, name, &isec->sid);
+		if (rc)
+			return rc;
+	}
+
+	isec->initialized = LABEL_INITIALIZED;
+	/*
+	 * Now that we've initialized security, check whether we're
+	 * allowed to actually create this type of anonymous inode.
+	 */
+
+	ad.type = LSM_AUDIT_DATA_INODE;
+	ad.u.inode = inode;
+
+	return avc_has_perm(&selinux_state,
+			    tsec->sid,
+			    isec->sid,
+			    isec->sclass,
+			    FILE__CREATE,
+			    &ad);
+}
+
 static int selinux_inode_create(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	return may_create(dir, dentry, SECCLASS_FILE);
@@ -7032,6 +7088,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 
 	LSM_HOOK_INIT(inode_free_security, selinux_inode_free_security),
 	LSM_HOOK_INIT(inode_init_security, selinux_inode_init_security),
+	LSM_HOOK_INIT(inode_init_security_anon, selinux_inode_init_security_anon),
 	LSM_HOOK_INIT(inode_create, selinux_inode_create),
 	LSM_HOOK_INIT(inode_link, selinux_inode_link),
 	LSM_HOOK_INIT(inode_unlink, selinux_inode_unlink),

security/selinux/include/classmap.h

@@ -249,6 +249,8 @@ struct security_class_mapping secclass_map[] = {
 	  {"open", "cpu", "kernel", "tracepoint", "read", "write"} },
 	{ "lockdown",
 	  { "integrity", "confidentiality", NULL } },
+	{ "anon_inode",
+	  { COMMON_FILE_PERMS, NULL } },
 	{ NULL }
   };