Index: head/sys/fs/devfs/devfs_vnops.c
===================================================================
--- head/sys/fs/devfs/devfs_vnops.c
+++ head/sys/fs/devfs/devfs_vnops.c
@@ -1774,13 +1774,24 @@
 		return (EACCES);
 
 	/*
-	 * Character devices always share mappings, so
-	 * require a writable fd for writable mappings.
+	 * If we are sharing potential changes via MAP_SHARED and we
+	 * are trying to get write permission although we opened it
+	 * without asking for it, bail out.
+	 *
+	 * Note that most character devices always share mappings.
+	 * The one exception is that D_MMAP_ANON devices
+	 * (i.e. /dev/zero) permit private writable mappings.
+	 *
+	 * Rely on vm_mmap_cdev() to fail invalid MAP_PRIVATE requests
+	 * as well as updating maxprot to permit writing for
+	 * D_MMAP_ANON devices rather than doing that here.
 	 */
-	if ((fp->f_flag & FWRITE) != 0)
-		maxprot |= VM_PROT_WRITE;
-	else if ((prot & VM_PROT_WRITE) != 0)
-		return (EACCES);
+	if ((flags & MAP_SHARED) != 0) {
+		if ((fp->f_flag & FWRITE) != 0)
+			maxprot |= VM_PROT_WRITE;
+		else if ((prot & VM_PROT_WRITE) != 0)
+			return (EACCES);
+	}
 	maxprot &= cap_maxprot;
 
 	fpop = td->td_fpop;