Index: lib/libc/sys/read.2
===================================================================
--- lib/libc/sys/read.2
+++ lib/libc/sys/read.2
@@ -28,7 +28,7 @@
 .\"     @(#)read.2	8.4 (Berkeley) 2/26/94
 .\" $FreeBSD$
 .\"
-.Dd March 30, 2020
+.Dd May 15, 2020
 .Dt READ 2
 .Os
 .Sh NAME
@@ -199,9 +199,14 @@
 The file was marked for non-blocking I/O,
 and no data were ready to be read.
 .It Bq Er EISDIR
-The file descriptor is associated with a directory residing
-on a file system that does not allow regular read operations on
-directories (e.g.\& NFS).
+The file descriptor is associated with a directory.
+Directories may only be read directly by root if the filesystem supports it and
+the
+.Dv security.bsd.allow_read_dir
+sysctl MIB is set to a non-zero value.
+For most scenarios, the
+.Xr readdir 3
+function should be used instead.
 .It Bq Er EOPNOTSUPP
 The file descriptor is associated with a file system and file type that
 do not allow regular read operations on it.
Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c
@@ -646,6 +646,12 @@
 	ZFS_ENTER(zfsvfs);
 	ZFS_VERIFY_ZP(zp);
 
+	/* We don't copy out anything useful for directories. */
+	if (vp->v_type == VDIR) {
+		ZFS_EXIT(zfsvfs);
+		return (SET_ERROR(EISDIR));
+	}
+
 	if (zp->z_pflags & ZFS_AV_QUARANTINED) {
 		ZFS_EXIT(zfsvfs);
 		return (SET_ERROR(EACCES));
Index: sys/kern/vfs_vnops.c
===================================================================
--- sys/kern/vfs_vnops.c
+++ sys/kern/vfs_vnops.c
@@ -135,6 +135,11 @@
 SYSCTL_ULONG(_debug, OID_AUTO, vn_io_faults, CTLFLAG_RD,
     &vn_io_faults_cnt, 0, "Count of vn_io_fault lock avoidance triggers");
 
+static int vfs_allow_read_dir = 0;
+SYSCTL_INT(_security_bsd, OID_AUTO, allow_read_dir, CTLFLAG_RW,
+    &vfs_allow_read_dir, 0,
+    "Enable read(2) of directory by root for filesystems that support it");
+
 /*
  * Returns true if vn_io_fault mode of handling the i/o request should
  * be used.
@@ -1160,6 +1165,24 @@
 
 	doio = uio->uio_rw == UIO_READ ? vn_read : vn_write;
 	vp = fp->f_vnode;
+
+	/*
+	 * The ability to read(2) on a directory has historically been
+	 * allowed for all users, but this can and has been the source of
+	 * at least one security issue in the past.  As such, it is now hidden
+	 * away behind a sysctl for those that actually need it to use it, and
+	 * restricted to root when it's turned on to make it relatively safe to
+	 * leave on for longer sessions of need.
+	 */
+	if (vp->v_type == VDIR) {
+		KASSERT(uio->uio_rw == UIO_READ,
+		    ("illegal write attempted on a directory"));
+		if (!vfs_allow_read_dir)
+			return (EISDIR);
+		if ((error = priv_check(td, PRIV_VFS_READ_DIR)) != 0)
+			return (EISDIR);
+	}
+
 	foffset_lock_uio(fp, uio, flags);
 	if (do_vn_io_fault(vp, uio)) {
 		args.kind = VN_IO_FAULT_FOP;
Index: sys/sys/priv.h
===================================================================
--- sys/sys/priv.h
+++ sys/sys/priv.h
@@ -283,6 +283,7 @@
 #define	PRIV_VFS_SYSFLAGS	342	/* Can modify system flags. */
 #define	PRIV_VFS_UNMOUNT	343	/* Can unmount(). */
 #define	PRIV_VFS_STAT		344	/* Override vnode MAC stat perm. */
+#define	PRIV_VFS_READ_DIR	345	/* Can read(2) a dirfd, needs sysctl. */
 
 /*
  * Virtual memory privileges.