qcow2: Don't crash qemu-img info with missing crypto header

qcow2_refresh_limits() assumes that s->crypto is non-NULL whenever
bs->encrypted is true. This is actually not the case: qcow2_do_open()
allows to open an image with a missing crypto header for BDRV_O_NO_IO,
and then bs->encrypted is true, but s->crypto is still NULL.

It doesn't make sense to open an invalid image, so remove the exception
for BDRV_O_NO_IO. This catches the problem early and any code that makes
the same assumption is safe now.

At the same time, in the name of defensive programming, we shouldn't
make the assumption in the first place. Let qcow2_refresh_limits() check
s->crypto rather than bs->encrypted. If s->crypto is NULL, it also can't
make any requirement on request alignment.

Finally, start a qcow2-encryption test case that only serves as a
regression test for this crash for now.

Reported-by: Leonid Reviakin <L.reviakin@fobos-nt.ru>
Reported-by: Denis Rastyogin <gerben@altlinux.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250318201143.70657-1-kwolf@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -1721,7 +1721,7 @@ qcow2_do_open(BlockDriverState *bs, QDict *options, int flags,
                 ret = -EINVAL;
                 goto fail;
             }
-        } else if (!(flags & BDRV_O_NO_IO)) {
+        } else {
             error_setg(errp, "Missing CRYPTO header for crypt method %d",
                        s->crypt_method_header);
             ret = -EINVAL;
@@ -1976,7 +1976,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
 
-    if (bs->encrypted) {
+    if (s->crypto) {
         /* Encryption works on a sector granularity */
         bs->bl.request_alignment = qcrypto_block_get_sector_size(s->crypto);
     }

tests/qemu-iotests/tests/qcow2-encryption

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# group: rw quick
+#
+# Test case for encryption support in qcow2
+#
+# Copyright (C) 2025 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# creator
+owner=kwolf@redhat.com
+
+seq="$(basename $0)"
+echo "QA output created by $seq"
+
+status=1	# failure is the default!
+
+_cleanup()
+{
+	_cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ../common.rc
+. ../common.filter
+
+# This tests qcow2-specific low-level functionality
+_supported_fmt qcow2
+_supported_proto file
+_require_working_luks
+
+IMG_SIZE=64M
+
+echo
+echo "=== Create an encrypted image ==="
+echo
+
+_make_test_img --object secret,id=sec0,data=123456 -o encrypt.format=luks,encrypt.key-secret=sec0 $IMG_SIZE
+$PYTHON ../qcow2.py "$TEST_IMG" dump-header-exts
+_img_info
+$QEMU_IMG check \
+    --object secret,id=sec0,data=123456 \
+    --image-opts file.filename="$TEST_IMG",encrypt.key-secret=sec0 \
+    | _filter_qemu_img_check
+
+echo
+echo "=== Remove the header extension ==="
+echo
+
+$PYTHON ../qcow2.py "$TEST_IMG" del-header-ext 0x0537be77
+$PYTHON ../qcow2.py "$TEST_IMG" dump-header-exts
+_img_info
+$QEMU_IMG check \
+    --object secret,id=sec0,data=123456 \
+    --image-opts file.filename="$TEST_IMG",encrypt.key-secret=sec0 2>&1 \
+    | _filter_qemu_img_check \
+    | _filter_testdir
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0

tests/qemu-iotests/tests/qcow2-encryption.out

@@ -0,0 +1,32 @@
+QA output created by qcow2-encryption
+
+=== Create an encrypted image ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+Header extension:
+magic                     0x537be77 (Crypto header)
+length                    16
+data                      <binary>
+
+Header extension:
+magic                     0x6803f857 (Feature table)
+length                    384
+data                      <binary>
+
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 64 MiB (67108864 bytes)
+encrypted: yes
+cluster_size: 65536
+No errors were found on the image.
+
+=== Remove the header extension ===
+
+Header extension:
+magic                     0x6803f857 (Feature table)
+length                    384
+data                      <binary>
+
+qemu-img: Could not open 'TEST_DIR/t.IMGFMT': Missing CRYPTO header for crypt method 2
+qemu-img: Could not open 'file.filename=TEST_DIR/t.qcow2,encrypt.key-secret=sec0': Missing CRYPTO header for crypt method 2
+*** done