[3.11.y.z extended stable] Patch "mnt: Correct permission checks in do_remount" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    mnt: Correct permission checks in do_remount

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 06d261659a95feba73d0f1db97cfbe3ba099ed52 Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm at xmission.com>
Date: Mon, 28 Jul 2014 17:26:07 -0700
Subject: mnt: Correct permission checks in do_remount

commit 9566d6742852c527bf5af38af5cbb878dad75705 upstream.

While invesgiating the issue where in "mount --bind -oremount,ro ..."
would result in later "mount --bind -oremount,rw" succeeding even if
the mount started off locked I realized that there are several
additional mount flags that should be locked and are not.

In particular MNT_NOSUID, MNT_NODEV, MNT_NOEXEC, and the atime
flags in addition to MNT_READONLY should all be locked.  These
flags are all per superblock, can all be changed with MS_BIND,
and should not be changable if set by a more privileged user.

The following additions to the current logic are added in this patch.
- nosuid may not be clearable by a less privileged user.
- nodev  may not be clearable by a less privielged user.
- noexec may not be clearable by a less privileged user.
- atime flags may not be changeable by a less privileged user.

The logic with atime is that always setting atime on access is a
global policy and backup software and auditing software could break if
atime bits are not updated (when they are configured to be updated),
and serious performance degradation could result (DOS attack) if atime
updates happen when they have been explicitly disabled.  Therefore an
unprivileged user should not be able to mess with the atime bits set
by a more privileged user.

The additional restrictions are implemented with the addition of
MNT_LOCK_NOSUID, MNT_LOCK_NODEV, MNT_LOCK_NOEXEC, and MNT_LOCK_ATIME
mnt flags.

Taken together these changes and the fixes for MNT_LOCK_READONLY
should make it safe for an unprivileged user to create a user
namespace and to call "mount --bind -o remount,... ..." without
the danger of mount flags being changed maliciously.

Acked-by: Serge E. Hallyn <serge.hallyn at ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/namespace.c        | 36 +++++++++++++++++++++++++++++++++---
 include/linux/mount.h |  5 +++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 343158cb021c..456c3e00cb04 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -828,8 +828,21 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,

 	mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;
 	/* Don't allow unprivileged users to change mount flags */
-	if ((flag & CL_UNPRIVILEGED) && (mnt->mnt.mnt_flags & MNT_READONLY))
-		mnt->mnt.mnt_flags |= MNT_LOCK_READONLY;
+	if (flag & CL_UNPRIVILEGED) {
+		mnt->mnt.mnt_flags |= MNT_LOCK_ATIME;
+
+		if (mnt->mnt.mnt_flags & MNT_READONLY)
+			mnt->mnt.mnt_flags |= MNT_LOCK_READONLY;
+
+		if (mnt->mnt.mnt_flags & MNT_NODEV)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NODEV;
+
+		if (mnt->mnt.mnt_flags & MNT_NOSUID)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NOSUID;
+
+		if (mnt->mnt.mnt_flags & MNT_NOEXEC)
+			mnt->mnt.mnt_flags |= MNT_LOCK_NOEXEC;
+	}

 	atomic_inc(&sb->s_active);
 	mnt->mnt.mnt_sb = sb;
@@ -1799,6 +1812,23 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 	    !(mnt_flags & MNT_READONLY)) {
 		return -EPERM;
 	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
+	    !(mnt_flags & MNT_NODEV)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NOSUID) &&
+	    !(mnt_flags & MNT_NOSUID)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_NOEXEC) &&
+	    !(mnt_flags & MNT_NOEXEC)) {
+		return -EPERM;
+	}
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_ATIME) &&
+	    ((mnt->mnt.mnt_flags & MNT_ATIME_MASK) != (mnt_flags & MNT_ATIME_MASK))) {
+		return -EPERM;
+	}
+
 	err = security_sb_remount(sb, data);
 	if (err)
 		return err;
@@ -1998,7 +2028,7 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
 		 */
 		if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
 			flags |= MS_NODEV;
-			mnt_flags |= MNT_NODEV;
+			mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
 		}
 	}

diff --git a/include/linux/mount.h b/include/linux/mount.h
index 16fc05d816d4..f058e131e6bb 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -45,10 +45,15 @@ struct mnt_namespace;
 #define MNT_USER_SETTABLE_MASK  (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
 				 | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
 				 | MNT_READONLY)
+#define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )


 #define MNT_INTERNAL	0x4000

+#define MNT_LOCK_ATIME		0x040000
+#define MNT_LOCK_NOEXEC		0x080000
+#define MNT_LOCK_NOSUID		0x100000
+#define MNT_LOCK_NODEV		0x200000
 #define MNT_LOCK_READONLY	0x400000

 struct vfsmount {
--
1.9.1