Re: [RFC v2 19/19] ima: Setup securityfs for IMA namespace

[James Bottomley]

On Mon, 2021-12-06 at 16:44 +0100, Christian Brauner wrote:
> On Mon, Dec 06, 2021 at 08:38:29AM -0500, James Bottomley wrote:
> > On Mon, 2021-12-06 at 13:08 +0100, Christian Brauner wrote:
[...]
> > >  Instead subsequents mounts resurface the same superblock. There
> > > might be an inherent design reason why this needs to be this way
> > > but I would advise against these semantics for anything that
> > > wants to be namespaced. Probably the first securityfs mount in
> > > init_user_ns can follow these semantics but ones tied to a non-
> > > initial user namespace should not as the userns can go away. In
> > > that case the pinning logic seems strange as conceptually the
> > > userns pins the securityfs mount as evidenced by the fact that we
> > > key by it in get_tree_keyed().
> > 
> > Yes, that's basically what I did: pin if ns == &init_user_ns but
> > don't pin if not.  However, I'm still not sure I got the triggers
> > right.  We have to trigger the notifier call (which adds the
> > namespaced file entries) from context free, because that's the
> > first place the superblock mount is fully set up ... I can't do it
> > in fill_super because the mount isn't fully initialized (and the
> > locking prevents it).  I did manage to get the notifier for
> > teardown triggered from kill_super, though.
> 
> I don't think you need a vfsmount at all to be honest. I think this
> can all be done without much ceremony. Here's a brutalist completely
> untested patch outlining one approach:

This is what I did (incremental to Stefan's series + my previous
patch): it avoids superblock threading by switching to a root dentry in
the securityfs user namespace area ... or am I being too simple again
... ?

I'm still a bit unhappy about triggering a blocking notifier under the
umount semaphore ...

James

---
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 6b8bd060d8c4..03a0879376a0 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -104,8 +104,7 @@ struct user_namespace {
 	struct ima_namespace	*ima_ns;
 #endif
 #ifdef CONFIG_SECURITYFS
-	struct vfsmount		*securityfs_mount;
-	bool			securityfs_notifier_sent;
+	struct dentry		*securityfs_root;
 #endif
 } __randomize_layout;
 
diff --git a/security/inode.c b/security/inode.c
index 62ab4630dc31..863fccfd3687 100644
--- a/security/inode.c
+++ b/security/inode.c
@@ -25,6 +25,7 @@
 #include <linux/user_namespace.h>
 #include <linux/ima.h>
 
+static struct vfsmount *securityfs_mount;
 static int securityfs_mount_count;
 
 static BLOCKING_NOTIFIER_HEAD(securityfs_ns_notifier);
@@ -41,42 +42,22 @@ static const struct super_operations securityfs_super_operations = {
 	.free_inode	= securityfs_free_inode,
 };
 
-static struct file_system_type fs_type;
-
-static void securityfs_free_context(struct fs_context *fc)
-{
-	struct user_namespace *ns = fc->user_ns;
-	if (ns == &init_user_ns ||
-	    ns->securityfs_notifier_sent)
-		return;
-
-	ns->securityfs_notifier_sent = true;
-
-	ns->securityfs_mount = vfs_kern_mount(&fs_type, SB_KERNMOUNT,
-					      fs_type.name, NULL);
-	if (IS_ERR(ns->securityfs_mount)) {
-		printk(KERN_ERR "kern mount on securityfs ERROR: %ld\n",
-		       PTR_ERR(ns->securityfs_mount));
-		ns->securityfs_mount = NULL;
-		return;
-	}
-
-	blocking_notifier_call_chain(&securityfs_ns_notifier,
-				     SECURITYFS_NS_ADD, fc->user_ns);
-	mntput(ns->securityfs_mount);
-}
-
 static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	static const struct tree_descr files[] = {{""}};
 	int error;
+	struct user_namespace *ns = fc->user_ns;
 
 	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
 	if (error)
 		return error;
 
+	ns->securityfs_root = sb->s_root;
+
 	sb->s_op = &securityfs_super_operations;
 
+	blocking_notifier_call_chain(&securityfs_ns_notifier,
+				     SECURITYFS_NS_ADD, ns);
 	return 0;
 }
 
@@ -87,7 +68,6 @@ static int securityfs_get_tree(struct fs_context *fc)
 
 static const struct fs_context_operations securityfs_context_ops = {
 	.get_tree	= securityfs_get_tree,
-	.free		= securityfs_free_context,
 };
 
 static int securityfs_init_fs_context(struct fs_context *fc)
@@ -104,8 +84,7 @@ static void securityfs_kill_super(struct super_block *sb)
 		blocking_notifier_call_chain(&securityfs_ns_notifier,
 					     SECURITYFS_NS_REMOVE,
 					     sb->s_fs_info);
-	ns->securityfs_notifier_sent = false;
-	ns->securityfs_mount = NULL;
+	ns->securityfs_root = NULL;
 	kill_litter_super(sb);
 }
 
@@ -179,14 +158,18 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
 	pr_debug("securityfs: creating file '%s', ns=%u\n",name, ns->ns.inum);
 
 	if (ns == &init_user_ns) {
-		error = simple_pin_fs(&fs_type, &ns->securityfs_mount,
+		error = simple_pin_fs(&fs_type, &securityfs_mount,
 				      &securityfs_mount_count);
 		if (error)
 			return ERR_PTR(error);
 	}
 
-	if (!parent)
-		parent = ns->securityfs_mount->mnt_root;
+	if (!parent) {
+		if (ns == &init_user_ns)
+			parent = securityfs_mount->mnt_root;
+		else
+			parent = ns->securityfs_root;
+	}
 
 	dir = d_inode(parent);
 
@@ -232,7 +215,7 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
 out:
 	inode_unlock(dir);
 	if (ns == &init_user_ns)
-		simple_release_fs(&ns->securityfs_mount,
+		simple_release_fs(&securityfs_mount,
 				  &securityfs_mount_count);
 	return dentry;
 }
@@ -376,7 +359,7 @@ void securityfs_remove(struct dentry *dentry)
 	}
 	inode_unlock(dir);
 	if (ns == &init_user_ns)
-		simple_release_fs(&ns->securityfs_mount,
+		simple_release_fs(&securityfs_mount,
 				  &securityfs_mount_count);
 }
 EXPORT_SYMBOL(securityfs_remove);
@@ -405,8 +388,6 @@ static int __init securityfs_init(void)
 	if (retval)
 		return retval;
 
-	init_user_ns.securityfs_mount = NULL;
-
 	retval = register_filesystem(&fs_type);
 	if (retval) {
 		sysfs_remove_mount_point(kernel_kobj, "security");