From: Eric W. Biederman <ebiederm@xmission.com>
Date: Fri, 22 Mar 2013 10:10:15 +0000 (-0700)
Subject: vfs: Add a mount flag to lock read only bind mounts
X-Git-Tag: v3.9-rc5~21^2~3
X-Git-Url: https://git.kernelconcepts.de/?p=karo-tx-linux.git;a=commitdiff_plain;h=90563b198e4c6674c63672fae1923da467215f45

vfs: Add a mount flag to lock read only bind mounts

When a read-only bind mount is copied from mount namespace in a higher
privileged user namespace to a mount namespace in a lesser privileged
user namespace, it should not be possible to remove the the read-only
restriction.

Add a MNT_LOCK_READONLY mount flag to indicate that a mount must
remain read-only.

CC: stable@vger.kernel.org
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---

diff --git a/fs/namespace.c b/fs/namespace.c
index a3035223d421..8505b5ece5de 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1713,6 +1713,9 @@ static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
 	if (readonly_request == __mnt_is_readonly(mnt))
 		return 0;
 
+	if (mnt->mnt_flags & MNT_LOCK_READONLY)
+		return -EPERM;
+
 	if (readonly_request)
 		error = mnt_make_readonly(real_mount(mnt));
 	else
diff --git a/include/linux/mount.h b/include/linux/mount.h
index d7029f4a191a..73005f9957ea 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -47,6 +47,8 @@ struct mnt_namespace;
 
 #define MNT_INTERNAL	0x4000
 
+#define MNT_LOCK_READONLY	0x400000
+
 struct vfsmount {
 	struct dentry *mnt_root;	/* root of the mounted tree */
 	struct super_block *mnt_sb;	/* pointer to superblock */