Patch "mnt: Account for MS_RDONLY in fs_fully_visible" has been added to the 4.6-stable tree

This is a note to let you know that I've just added the patch titled

mnt: Account for MS_RDONLY in fs_fully_visible

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mnt-account-for-ms_rdonly-in-fs_fully_visible.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Fri, 10 Jun 2016 12:21:40 -0500
Subject: mnt: Account for MS_RDONLY in fs_fully_visible

From: Eric W. Biederman <[email protected]>

commit 695e9df010e40f407f4830dc11d53dce957710ba upstream.

In rare cases it is possible for s_flags & MS_RDONLY to be set but MNT_READONLY to be clear. This starting combination can cause fs_fully_visible to fail to ensure that the new mount is readonly. Therefore force MNT_LOCK_READONLY in the new mount if MS_RDONLY is set on the source filesystem of the mount.

In general both MS_RDONLY and MNT_READONLY are set at the same for mounts so I don't expect any programs to care. Nor do I expect MS_RDONLY to be set on proc or sysfs in the initial user namespace, which further decreases the likelyhood of problems.

Which means this change should only affect system configurations by paranoid sysadmins who should welcome the additional protection as it keeps people from wriggling out of their policies.

---
 fs/namespace.c |    4 ++++
 1 file changed, 4 insertions(+)

@@ -3245,6 +3245,10 @@ static bool fs_fully_visible(struct file
 		if (mnt->mnt.mnt_sb->s_iflags & SB_I_NOEXEC)
 			mnt_flags &= ~(MNT_LOCK_NOSUID | MNT_LOCK_NOEXEC);
 
+		/* Don't miss readonly hidden in the superblock flags */
+		if (mnt->mnt.mnt_sb->s_flags & MS_RDONLY)
+			mnt_flags |= MNT_LOCK_READONLY;
+
 		/* Verify the mount flags are equal to or more permissive
 		 * than the proposed new mount.
 		 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/mnt-fs_fully_visible-test-the-proper-mount-for-mnt_locked.patch queue-4.6/mnt-if-fs_fully_visible-fails-call-put_filesystem.patch queue-4.6/mnt-account-for-ms_rdonly-in-fs_fully_visible.patch