[PATCH 2/3] Switch UTS namespace to use shadows

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

The uts sysctl table contains two writable fields
(domainname and nodename), so split the table into
common (read-only) part and writable (shadowed).

This fixes the BUG! You may create a namespace and
then writing to /proc/sys/hostname will cause an
init_uts_ns overwrite.

Signed-off-by: Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>

---

diff --git a/include/linux/utsname.h b/include/linux/utsname.h
index 923db99..7517b36 100644
--- a/include/linux/utsname.h
+++ b/include/linux/utsname.h
@@ -40,6 +40,7 @@ struct new_utsname {
 struct uts_namespace {
 	struct kref kref;
 	struct new_utsname name;
+	struct ctl_table_header *ctl_header;
 };
 extern struct uts_namespace init_uts_ns;
 
@@ -66,6 +67,9 @@ static inline struct new_utsname *init_utsname(void)
 	return &init_uts_ns.name;
 }
 
+int clone_uts_sysctl(struct uts_namespace *ns);
+void free_uts_sysctl(struct uts_namespace *ns);
+
 extern struct rw_semaphore uts_sem;
 
 #endif /* __KERNEL__ */
diff --git a/kernel/utsname.c b/kernel/utsname.c
index 816d7b2..22e40bb 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -26,13 +26,21 @@ static struct uts_namespace *clone_uts_ns(struct uts_namespace *old_ns)
 
 	ns = kmalloc(sizeof(struct uts_namespace), GFP_KERNEL);
 	if (!ns)
-		return ERR_PTR(-ENOMEM);
+		goto err_alloc;
+
+	if (clone_uts_sysctl(ns))
+		goto err_sysctl;
 
 	down_read(&uts_sem);
 	memcpy(&ns->name, &old_ns->name, sizeof(ns->name));
 	up_read(&uts_sem);
 	kref_init(&ns->kref);
 	return ns;
+
+err_sysctl:
+	kfree(ns);
+err_alloc:
+	return ERR_PTR(-ENOMEM);
 }
 
 /*
@@ -62,5 +70,6 @@ void free_uts_ns(struct kref *kref)
 	struct uts_namespace *ns;
 
 	ns = container_of(kref, struct uts_namespace, kref);
+	free_uts_sysctl(ns);
 	kfree(ns);
 }
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index c76c064..8a06f0b 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -75,6 +75,11 @@ static int sysctl_uts_string(ctl_table *table, int __user *name, int nlen,
 #define sysctl_uts_string NULL
 #endif
 
+static struct ctl_table_header *uts_sysctl_shadow(struct ctl_table_header *h)
+{
+	return current->nsproxy->uts_ns->ctl_header;
+}
+
 static struct ctl_table uts_kern_table[] = {
 	{
 		.ctl_name	= KERN_OSTYPE,
@@ -103,6 +108,20 @@ static struct ctl_table uts_kern_table[] = {
 		.proc_handler	= proc_do_uts_string,
 		.strategy	= sysctl_uts_string,
 	},
+	{}
+};
+
+static struct ctl_table uts_root_table[] = {
+	{
+		.ctl_name	= CTL_KERN,
+		.procname	= "kernel",
+		.mode		= 0555,
+		.child		= uts_kern_table,
+	},
+	{}
+};
+
+static struct ctl_table uts_kern_table_sh[] = {
 	{
 		.ctl_name	= KERN_NODENAME,
 		.procname	= "hostname",
@@ -124,19 +143,44 @@ static struct ctl_table uts_kern_table[] = {
 	{}
 };
 
-static struct ctl_table uts_root_table[] = {
+static struct ctl_table uts_root_table_sh[] = {
 	{
 		.ctl_name	= CTL_KERN,
 		.procname	= "kernel",
 		.mode		= 0555,
-		.child		= uts_kern_table,
+		.child		= uts_kern_table_sh,
 	},
 	{}
 };
 
+int clone_uts_sysctl(struct uts_namespace *ns)
+{
+	struct ctl_table_header *h;
+	struct ctl_table *tbl;
+
+	h = create_sysctl_shadow(init_uts_ns.ctl_header);
+	if (h == NULL)
+		return -ENOMEM;
+
+	tbl = h->ctl_table->child;
+
+	tbl[0].data = ns->name.nodename;
+	tbl[1].data = ns->name.domainname;
+
+	ns->ctl_header = h;
+	return 0;
+}
+
+void free_uts_sysctl(struct uts_namespace *ns)
+{
+	free_sysctl_shadow(ns->ctl_header);
+}
+
 static int __init utsname_sysctl_init(void)
 {
 	register_sysctl_table(uts_root_table);
+	init_uts_ns.ctl_header = register_sysctl_table_shadow(uts_root_table_sh,
+			uts_sysctl_shadow);
 	return 0;
 }