Index: share/man/man4/md.4
===================================================================
--- share/man/man4/md.4
+++ share/man/man4/md.4
@@ -7,7 +7,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 5, 2017
+.Dd December 7, 2017
 .Dt MD 4
 .Os
 .Sh NAME
@@ -79,6 +79,7 @@
 needs the following options:
 .Bd -literal -offset indent
 options 	MD_ROOT			# MD is a potential root device
+options 	MD_ROOT_READONLY	# disallow mounting root writeable
 options		MD_ROOT_SIZE=8192	# 8MB ram disk
 makeoptions	MFS_IMAGE=/h/foo/ARM-MD
 options 	ROOTDEVNAME=\\"ufs:md0\\"
Index: sys/conf/NOTES
===================================================================
--- sys/conf/NOTES
+++ sys/conf/NOTES
@@ -1116,6 +1116,9 @@
 # images of type mfs_root or md_root.
 options 	MD_ROOT
 
+# Write-protect the md root device so that it may not be mounted writeable.
+options 	MD_ROOT_READONLY
+
 # Disk quotas are supported when this option is enabled.
 options 	QUOTA			#enable disk quotas
 
Index: sys/conf/options
===================================================================
--- sys/conf/options
+++ sys/conf/options
@@ -165,6 +165,7 @@
 MAC_TEST	opt_dontuse.h
 MD_ROOT		opt_md.h
 MD_ROOT_FSTYPE	opt_md.h
+MD_ROOT_READONLY	opt_md.h
 MD_ROOT_SIZE	opt_md.h
 MFI_DEBUG	opt_mfi.h
 MFI_DECODE_LOG	opt_mfi.h
Index: sys/dev/md/md.c
===================================================================
--- sys/dev/md/md.c
+++ sys/dev/md/md.c
@@ -1791,10 +1791,16 @@
 	sc->start = mdstart_preload;
 	if (name != NULL)
 		strlcpy(sc->file, name, sizeof(sc->file));
-#if defined(MD_ROOT) && !defined(ROOTDEVNAME)
-	if (sc->unit == 0)
+#ifdef MD_ROOT
+	if (sc->unit == 0) {
+#ifndef ROOTDEVNAME
 		rootdevnames[0] = MD_ROOT_FSTYPE ":/dev/md0";
 #endif
+#ifdef MD_ROOT_READONLY
+		sc->flags |= MD_READONLY;
+#endif
+	}
+#endif
 	mdinit(sc);
 	if (name != NULL) {
 		printf("%s%d: Preloaded image <%s> %zd bytes at %p\n",