Re: [PATCH] namespace.c: fix bind mount from foreign namespace

[Miklos Szeredi]

> Ok. less restriction without compromising security is a good idea.
> 
> Under the premise that bind mounts across namespace should be allowed;
> any insight why the "founding fathers" :) allowed only bind
> and not recursive bind?  What issue would that create? One can
> easily workaround that restriction by manually binding recursively.
> So does the recursive bind restriction serve any purpose?
> 
> I remember Miklos saying its not a security issue but a
> implementation/locking issue. That can be fixed aswell.

Yes, as pointed out by Jamie, both namespaces need to be locked for
this to work.  Something like the attached should do it.

Miklos


Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2005-05-16 22:02:36.000000000 +0200
+++ linux/fs/namespace.c	2005-05-16 22:13:30.000000000 +0200
@@ -622,6 +622,8 @@ out_unlock:
 static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
 {
 	struct nameidata old_nd;
+	struct namespace *ns1 = current->namespace;
+	struct namespace *ns2 = NULL;
 	struct vfsmount *mnt = NULL;
 	int err = mount_is_safe(nd);
 	if (err)
@@ -632,15 +634,30 @@ static int do_loopback(struct nameidata 
 	if (err)
 		return err;
 
-	down_write(&current->namespace->sem);
 	err = -EINVAL;
-	if (check_mnt(nd->mnt) && (!recurse || check_mnt(old_nd.mnt))) {
-		err = -ENOMEM;
-		if (recurse)
-			mnt = copy_tree(old_nd.mnt, old_nd.dentry);
-		else
-			mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
-	}
+	if (!check_mnt(nd->mnt))
+		goto out_path_release;
+
+	if (recurse && old_nd.mnt->mnt_namespace != ns1) {
+		ns2 = old_nd.mnt->mnt_namespace;
+		if (ns1 < ns2) {
+			down_write(&ns1->sem);
+			down_write(&ns2->sem);
+		} else {
+			down_write(&ns2->sem);
+			down_write(&ns1->sem);
+		}
+	} else
+		down_write(&ns1->sem);
+
+	err = -ENOMEM;
+	if (recurse)
+		mnt = copy_tree(old_nd.mnt, old_nd.dentry);
+	else
+		mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
+	
+	if (ns2)
+		up_write(&ns2->sem);
 
 	if (mnt) {
 		/* stop bind mounts from expiring */
@@ -657,7 +674,8 @@ static int do_loopback(struct nameidata 
 			mntput(mnt);
 	}
 
-	up_write(&current->namespace->sem);
+	up_write(&ns1->sem);
+out_path_release:
 	path_release(&old_nd);
 	return err;
 }
-
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