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From:   David Howells <dhowells@redhat.com>
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To:     ebiederm@xmission.com (Eric W. Biederman)
Cc:     dhowells@redhat.com,
        Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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Date:   Thu, 11 Oct 2018 21:17:54 +0100
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Eric W. Biederman <ebiederm@xmission.com> wrote:

> It should just be a matter of replacing the test
> "if (p->mnt.mnt_sb->s_type == &nsfs)" with "if mnt_ns_loop(p->mnt.mnt_root)"

Okay, the attached seems to work.

Thanks,
David
---
diff --git a/fs/namespace.c b/fs/namespace.c
index e969ded7d54b..5548fb9b7de2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2388,6 +2388,27 @@ static inline int tree_contains_unbindable(struct mount *mnt)
 	return 0;
 }
 
+/*
+ * Object if there are any nsfs mounts in the specified subtree.  These can act
+ * as pins for mount namespaces that aren't checked by the mount-cycle checking
+ * code, thereby allowing cycles to be made.
+ */
+static bool check_for_nsfs_mounts(struct mount *subtree)
+{
+	struct mount *p;
+	bool ret = false;
+
+	lock_mount_hash();
+	for (p = subtree; p; p = next_mnt(p, subtree))
+		if (mnt_ns_loop(p->mnt.mnt_root))
+			goto out;
+
+	ret = true;
+out:
+	unlock_mount_hash();
+	return ret;
+}
+
 static int do_move_mount(struct path *old_path, struct path *new_path)
 {
 	struct path parent_path = {.mnt = NULL, .dentry = NULL};
@@ -2442,6 +2463,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
 		goto out1;
 	err = -ELOOP;
+	if (!check_for_nsfs_mounts(old))
+		goto out1;
 	for (; mnt_has_parent(p); p = p->mnt_parent)
 		if (p == old)
 			goto out1;