snapdir: add 'disabled' value to make .zfs inaccessible

in some environments, just making the .zfs control dir hidden from sight
might not be enough. in particular, the following scenarios might
warrant not allowing access at all:
- old snapshots with wrong permissions/ownership
- old snapshots with exploitable setuid/setgid binaries
- old snapshots with sensitive contents

introducing a new 'disabled' value that not only hides the control dir,
but prevents access to its contents by returning ENOENT solves all of
the above.

the new property value takes advantage of 'iuv' semantics ("ignore
unknown value") to automatically fall back to the old default value when
a pool is accessed by an older version of ZFS that doesn't yet know
about 'disabled' semantics.

I think that technically the zfs_dirlook change is enough to prevent
access, but preventing lookups and dir entries in an already opened .zfs
handle might also be a good idea to prevent races when modifying the
property at runtime.

Fixes: openzfs#3963

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

.zfs: don't return .zfs inode if disabled

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

include/os/linux/zfs/sys/zfs_ctldir.h

@@ -45,7 +45,7 @@
 	(ZTOZSB(zdp)->z_ctldir != NULL))
 #define	zfs_show_ctldir(zdp)	\
 	(zfs_has_ctldir(zdp) && \
-	(ZTOZSB(zdp)->z_show_ctldir))
+	(ZTOZSB(zdp)->z_show_ctldir == ZFS_SNAPDIR_VISIBLE))
 
 extern int zfs_expire_snapshot;
 

include/os/linux/zfs/sys/zfs_vfsops_os.h

@@ -110,7 +110,7 @@ struct zfsvfs {
 	kmutex_t	z_znodes_lock;	/* lock for z_all_znodes */
 	arc_prune_t	*z_arc_prune;	/* called by ARC to prune caches */
 	struct inode	*z_ctldir;	/* .zfs directory inode */
-	boolean_t	z_show_ctldir;	/* expose .zfs in the root dir */
+	uint_t		z_show_ctldir;	/* how to expose .zfs in the root dir */
 	boolean_t	z_issnap;	/* true if this is a snapshot */
 	boolean_t	z_use_fuids;	/* version allows fuids */
 	boolean_t	z_replay;	/* set during ZIL replay */

include/sys/zfs_ioctl.h

@@ -57,6 +57,7 @@ extern "C" {
  */
 #define	ZFS_SNAPDIR_HIDDEN		0
 #define	ZFS_SNAPDIR_VISIBLE		1
+#define	ZFS_SNAPDIR_DISABLED		2
 
 /*
  * Property values for snapdev

man/man7/zfsconcepts.7

@@ -71,7 +71,7 @@ File system snapshots can be accessed under the
 directory in the root of the file system.
 Snapshots are automatically mounted on demand and may be unmounted at regular
 intervals.
-The visibility of the
+The availability and visibility of the
 .Pa .zfs
 directory can be controlled by the
 .Sy snapdir

man/man7/zfsprops.7

@@ -1773,11 +1773,11 @@ Controls whether the volume snapshot devices under
 are hidden or visible.
 The default value is
 .Sy hidden .
-.It Sy snapdir Ns = Ns Sy hidden Ns | Ns Sy visible
+.It Sy snapdir Ns = Ns Sy disabled Ns | Ns Sy hidden Ns | Ns Sy visible
 Controls whether the
 .Pa .zfs
-directory is hidden or visible in the root of the file system as discussed in
-the
+directory is disabled, hidden or visible in the root of the file system as
+discussed in the
 .Sx Snapshots
 section of
 .Xr zfsconcepts 7 .

module/os/linux/zfs/zfs_ctldir.c

@@ -810,7 +810,9 @@ zfsctl_root_lookup(struct inode *dip, const char *name, struct inode **ipp,
 	if ((error = zfs_enter(zfsvfs, FTAG)) != 0)
 		return (error);
 
-	if (strcmp(name, "..") == 0) {
+	if (zfsvfs->z_show_ctldir == ZFS_SNAPDIR_DISABLED) {
+		error = SET_ERROR(ENOENT);
+	} else if (strcmp(name, "..") == 0) {
 		*ipp = dip->i_sb->s_root->d_inode;
 	} else if (strcmp(name, ZFS_SNAPDIR_NAME) == 0) {
 		*ipp = zfsctl_inode_lookup(zfsvfs, ZFSCTL_INO_SNAPDIR,

module/os/linux/zfs/zfs_dir.c

@@ -415,6 +415,9 @@ zfs_dirlook(znode_t *dzp, char *name, znode_t **zpp, int flags,
 			*zpp = zp;
 		rw_exit(&dzp->z_parent_lock);
 	} else if (zfs_has_ctldir(dzp) && strcmp(name, ZFS_CTLDIR_NAME) == 0) {
+		if (ZTOZSB(dzp)->z_show_ctldir == ZFS_SNAPDIR_DISABLED) {
+			return SET_ERROR(ENOENT);
+		}
 		ip = zfsctl_root(dzp);
 		*zpp = ITOZ(ip);
 	} else {

module/os/linux/zfs/zfs_vfsops.c

@@ -1764,6 +1764,11 @@ zfs_vget(struct super_block *sb, struct inode **ipp, fid_t *fidp)
 	    (object == ZFSCTL_INO_ROOT || object == ZFSCTL_INO_SNAPDIR)) {
 		*ipp = zfsvfs->z_ctldir;
 		ASSERT(*ipp != NULL);
+
+		if (zfsvfs->z_show_ctldir == ZFS_SNAPDIR_DISABLED) {
+			return (SET_ERROR(ENOENT));
+		}
+
 		if (object == ZFSCTL_INO_SNAPDIR) {
 			VERIFY(zfsctl_root_lookup(*ipp, "snapshot", ipp,
 			    0, kcred, NULL, NULL) == 0);

module/os/linux/zfs/zpl_ctldir.c

@@ -57,6 +57,10 @@ zpl_root_iterate(struct file *filp, zpl_dir_context_t *ctx)
 	zfsvfs_t *zfsvfs = ITOZSB(file_inode(filp));
 	int error = 0;
 
+	if (zfsvfs->z_show_ctldir == ZFS_SNAPDIR_DISABLED) {
+		return (SET_ERROR(ENOENT));
+	}
+
 	if ((error = zpl_enter(zfsvfs, FTAG)) != 0)
 		return (error);
 

module/zcommon/zfs_prop.c

@@ -237,6 +237,7 @@ zfs_prop_init(void)
 	static const zprop_index_t snapdir_table[] = {
 		{ "hidden",	ZFS_SNAPDIR_HIDDEN },
 		{ "visible",	ZFS_SNAPDIR_VISIBLE },
+		{ "disabled",	ZFS_SNAPDIR_DISABLED },
 		{ NULL }
 	};
 
@@ -428,7 +429,7 @@ zfs_prop_init(void)
 	    "COMPRESS", compress_table, sfeatures);
 	zprop_register_index(ZFS_PROP_SNAPDIR, "snapdir", ZFS_SNAPDIR_HIDDEN,
 	    PROP_INHERIT, ZFS_TYPE_FILESYSTEM,
-	    "hidden | visible", "SNAPDIR", snapdir_table, sfeatures);
+	    "disabled | hidden | visible", "SNAPDIR", snapdir_table, sfeatures);
 	zprop_register_index(ZFS_PROP_SNAPDEV, "snapdev", ZFS_SNAPDEV_HIDDEN,
 	    PROP_INHERIT, ZFS_TYPE_FILESYSTEM | ZFS_TYPE_VOLUME,
 	    "hidden | visible", "SNAPDEV", snapdev_table, sfeatures);

module/zfs/dsl_prop.c

@@ -698,6 +698,10 @@ dsl_prop_set_iuv(objset_t *mos, uint64_t zapobj, const char *propname,
 		    *(uint64_t *)value == ZFS_REDUNDANT_METADATA_NONE)
 			iuv = B_TRUE;
 		break;
+	case ZFS_PROP_SNAPDIR:
+		if (*(uint64_t *)value == ZFS_SNAPDIR_DISABLED)
+			iuv = B_TRUE;
+		break;
 	default:
 		break;
 	}

tests/zfs-tests/include/properties.shlib

@@ -30,7 +30,7 @@ typeset -a logbias_prop_vals=('latency' 'throughput')
 typeset -a primarycache_prop_vals=('all' 'none' 'metadata')
 typeset -a redundant_metadata_prop_vals=('all' 'most' 'some' 'none')
 typeset -a secondarycache_prop_vals=('all' 'none' 'metadata')
-typeset -a snapdir_prop_vals=('hidden' 'visible')
+typeset -a snapdir_prop_vals=('disabled' 'hidden' 'visible')
 typeset -a sync_prop_vals=('standard' 'always' 'disabled')
 
 typeset -a fs_props=('compress' 'checksum' 'recsize'