Patch "xfs: disallow rw remount on fs with unknown ro-compat features" has been added to the 4.4-stable tree

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

    xfs: disallow rw remount on fs with unknown ro-compat features

to the 4.4-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:
     xfs-disallow-rw-remount-on-fs-with-unknown-ro-compat-features.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let [off-list ref] know about it.

From d0a58e833931234c44e515b5b8bede32bd4e6eed Mon Sep 17 00:00:00 2001

From: Eric Sandeen <redacted>
Date: Wed, 6 Apr 2016 07:05:41 +1000
Subject: xfs: disallow rw remount on fs with unknown ro-compat features

From: Eric Sandeen <redacted>

commit d0a58e833931234c44e515b5b8bede32bd4e6eed upstream.

Today, a kernel which refuses to mount a filesystem read-write
due to unknown ro-compat features can still transition to read-write
via the remount path.  The old kernel is most likely none the wiser,
because it's unaware of the new feature, and isn't using it.  However,
writing to the filesystem may well corrupt metadata related to that
new feature, and moving to a newer kernel which understand the feature
will have problems.

Right now the only ro-compat feature we have is the free inode btree,
which showed up in v3.16.  It would be good to push this back to
all the active stable kernels, I think, so that if anyone is using
newer mkfs (which enables the finobt feature) with older kernel
releases, they'll be protected.

Signed-off-by: Eric Sandeen <redacted>
Reviewed-by: Bill O'Donnell <redacted>
Reviewed-by: Dave Chinner <redacted>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/xfs/xfs_super.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c

@@ -1233,6 +1233,16 @@ xfs_fs_remount(
 			return -EINVAL;
 		}
 
+		if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5 &&
+		    xfs_sb_has_ro_compat_feature(sbp,
+					XFS_SB_FEAT_RO_COMPAT_UNKNOWN)) {
+			xfs_warn(mp,
+"ro->rw transition prohibited on unknown (0x%x) ro-compat filesystem",
+				(sbp->sb_features_ro_compat &
+					XFS_SB_FEAT_RO_COMPAT_UNKNOWN));
+			return -EINVAL;
+		}
+
 		mp->m_flags &= ~XFS_MOUNT_RDONLY;
 
 		/*

Patches currently in stable-queue which might be from sandeen@redhat.com are

queue-4.4/xfs-print-name-of-verifier-if-it-fails.patch
queue-4.4/xfs-disallow-rw-remount-on-fs-with-unknown-ro-compat-features.patch