Handle and detect #13709's unlock regression

module/zfs/dsl_crypt.c

@@ -2671,6 +2671,7 @@ spa_do_crypt_objset_mac_abd(boolean_t generate, spa_t *spa, uint64_t dsobj,
 	objset_phys_t *osp = buf;
 	uint8_t portable_mac[ZIO_OBJSET_MAC_LEN];
 	uint8_t local_mac[ZIO_OBJSET_MAC_LEN];
+	const uint8_t zeroed_mac[ZIO_OBJSET_MAC_LEN] = {0};
 
 	/* look up the key from the spa's keystore */
 	ret = spa_keystore_lookup_key(spa, dsobj, FTAG, &dck);
@@ -2696,8 +2697,21 @@ spa_do_crypt_objset_mac_abd(boolean_t generate, spa_t *spa, uint64_t dsobj,
 	if (memcmp(portable_mac, osp->os_portable_mac,
 	    ZIO_OBJSET_MAC_LEN) != 0 ||
 	    memcmp(local_mac, osp->os_local_mac, ZIO_OBJSET_MAC_LEN) != 0) {
-		abd_return_buf(abd, buf, datalen);
-		return (SET_ERROR(ECKSUM));
+		/*
+		 * If the MAC is zeroed out, we failed to decrypt it.
+		 * This should only arise, at least on Linux,
+		 * if we hit edge case handling for useraccounting, since we
+		 * shouldn't get here without bailing out on error earlier
+		 * otherwise.
+		 *
+		 * So if we're in that case, we can just fall through and
+		 * special-casing noticing that it's zero will handle it
+		 * elsewhere, since we can just regenerate it.
+		 */
+		if (memcmp(local_mac, zeroed_mac, ZIO_OBJSET_MAC_LEN) != 0) {
+			abd_return_buf(abd, buf, datalen);
+			return (SET_ERROR(ECKSUM));
+		}
 	}
 
 	abd_return_buf(abd, buf, datalen);

tests/runfiles/common.run

@@ -911,7 +911,7 @@ tests = [
     'userquota_007_pos', 'userquota_008_pos', 'userquota_009_pos',
     'userquota_010_pos', 'userquota_011_pos', 'userquota_012_neg',
     'userspace_001_pos', 'userspace_002_pos', 'userspace_encrypted',
-    'userspace_send_encrypted']
+    'userspace_send_encrypted', 'userspace_encrypted_13709']
 tags = ['functional', 'userquota']
 
 [tests/functional/vdev_zaps]

tests/zfs-tests/tests/Makefile.am

@@ -368,6 +368,7 @@ nobase_dist_datadir_zfs_tests_tests_DATA += \
 	functional/upgrade/upgrade_common.kshlib \
 	functional/user_namespace/user_namespace.cfg \
 	functional/user_namespace/user_namespace_common.kshlib \
+	functional/userquota/13709_reproducer.bz2 \
 	functional/userquota/userquota.cfg \
 	functional/userquota/userquota_common.kshlib \
 	functional/vdev_zaps/vdev_zaps.kshlib \
@@ -1935,6 +1936,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/userquota/userspace_003_pos.ksh \
 	functional/userquota/userspace_encrypted.ksh \
 	functional/userquota/userspace_send_encrypted.ksh \
+	functional/userquota/userspace_encrypted_13709.ksh \
 	functional/vdev_zaps/cleanup.ksh \
 	functional/vdev_zaps/setup.ksh \
 	functional/vdev_zaps/vdev_zaps_001_pos.ksh \

tests/zfs-tests/tests/functional/userquota/userspace_encrypted_13709.ksh

@@ -0,0 +1,45 @@
+#!/bin/ksh -p
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/userquota/userquota_common.kshlib
+
+#
+# DESCRIPTION:
+# Avoid allowing #11294/#13709 to recur a third time.
+#
+# So we hardcode a copy of a pool with this bug, try unlocking it,
+# and fail on error. Simple.
+
+function cleanup
+{
+	destroy_pool $POOLNAME
+	rm -f $FILEDEV
+}
+
+log_onexit cleanup
+
+FILEDEV="$TEST_BASE_DIR/userspace_13709"
+POOLNAME="testpool_13709"
+
+log_assert "ZFS should be able to unlock pools with #13709's failure mode"
+
+log_must bzcat $STF_SUITE/tests/functional/userquota/13709_reproducer.bz2 > $FILEDEV
+
+log_must zpool import -d $FILEDEV $POOLNAME
+
+echo -e 'password\npassword\n' | log_must zfs mount -al
+
+# Cleanup
+cleanup
+
+log_pass "#13709 not happening here"