A discard request deallocates the selected clusters so they read back
as zeroes. This is done by clearing the cluster offset field and
setting QCOW_OFLAG_ZERO in the L2 entry.

This flag is however only supported when qcow_version >= 3. In older
images the cluster is simply deallocated, exposing any possible stale
data from the backing file.

Since discard is an advisory operation it's safer to simply forbid it
in this scenario.

Note that we are adding this check to qcow2_co_pdiscard() and not to
qcow2_cluster_discard() or discard_in_l2_slice() because the last
two are also used by qcow2_snapshot_create() to discard the clusters
used by the VM state. In this case there's no risk of exposing stale
data to the guest and we really want that the clusters are always
discarded.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
v2:

- Don't create the image with compat=0.10 in iotest 060 [Max]
- Use $TEST_IMG.base for the backing image name in iotest 289 [Max]
- Add list of unsupported options to iotest 289 [Max]

 block/qcow2.c              |  6 +++
 tests/qemu-iotests/060     | 10 ++---
 tests/qemu-iotests/060.out |  2 -
 tests/qemu-iotests/289     | 91 ++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/289.out | 52 ++++++++++++++++++++++
 tests/qemu-iotests/group   |  1 +
 6 files changed, 154 insertions(+), 8 deletions(-)
 create mode 100755 tests/qemu-iotests/289
 create mode 100644 tests/qemu-iotests/289.out

diff --git a/block/qcow2.c b/block/qcow2.c
index d44b45633d..7bb7e392e1 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3763,6 +3763,12 @@ static coroutine_fn int qcow2_co_pdiscard(BlockDriverState *bs,
     int ret;
     BDRVQcow2State *s = bs->opaque;
 
+    /* If the image does not support QCOW_OFLAG_ZERO then discarding
+     * clusters could expose stale data from the backing file. */
+    if (s->qcow_version < 3 && bs->backing) {
+        return -ENOTSUP;
+    }
+
     if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size)) {
         assert(bytes < s->cluster_size);
         /* Ignore partial clusters, except for the special case of the
diff --git a/tests/qemu-iotests/060 b/tests/qemu-iotests/060
index 043f12904a..8820290be6 100755
--- a/tests/qemu-iotests/060
+++ b/tests/qemu-iotests/060
@@ -160,16 +160,14 @@ TEST_IMG=$BACKING_IMG _make_test_img 1G
 
 $QEMU_IO -c 'write 0k 64k' "$BACKING_IMG" | _filter_qemu_io
 
-# compat=0.10 is required in order to make the following discard actually
-# unallocate the sector rather than make it a zero sector - we want COW, after
-# all.
-_make_test_img -o 'compat=0.10' -b "$BACKING_IMG" 1G
+_make_test_img -b "$BACKING_IMG" 1G
 # Write two clusters, the second one enforces creation of an L2 table after
 # the first data cluster.
 $QEMU_IO -c 'write 0k 64k' -c 'write 512M 64k' "$TEST_IMG" | _filter_qemu_io
-# Discard the first cluster. This cluster will soon enough be reallocated and
+# Free the first cluster. This cluster will soon enough be reallocated and
 # used for COW.
-$QEMU_IO -c 'discard 0k 64k' "$TEST_IMG" | _filter_qemu_io
+poke_file "$TEST_IMG" '262144' "\x00\x00\x00\x00\x00\x00\x00\x00" # 0x40000 - L2 entry
+poke_file "$TEST_IMG" '131082' "\x00\x00" # 0x2000a - Refcount entry
 # Now, corrupt the image by marking the second L2 table cluster as free.
 poke_file "$TEST_IMG" '131084' "\x00\x00" # 0x2000c
 # Start a write operation requiring COW on the image stopping it right before
diff --git a/tests/qemu-iotests/060.out b/tests/qemu-iotests/060.out
index d27692a33c..09caaea865 100644
--- a/tests/qemu-iotests/060.out
+++ b/tests/qemu-iotests/060.out
@@ -105,8 +105,6 @@ wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 536870912
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-discard 65536/65536 bytes at offset 0
-64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qcow2: Marking image as corrupt: Preventing invalid write on metadata (overlaps with active L2 table); further corruption events will be suppressed
 blkdebug: Suspended request '0'
 write failed: Input/output error
diff --git a/tests/qemu-iotests/289 b/tests/qemu-iotests/289
new file mode 100755
index 0000000000..629c992d79
--- /dev/null
+++ b/tests/qemu-iotests/289
@@ -0,0 +1,91 @@
+#!/usr/bin/env bash
+#
+# Test how 'qemu-io -c discard' behaves on v2 and v3 qcow2 images
+#
+# Copyright (C) 2020 Igalia, S.L.
+# Author: Alberto Garcia <berto@igalia.com>
+#
+# 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=berto@igalia.com
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+status=1    # failure is the default!
+
+_cleanup()
+{
+    _cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+_supported_fmt qcow2
+_supported_proto file
+_supported_os Linux
+_unsupported_imgopts 'compat=0.10' refcount_bits data_file
+
+echo
+echo "### Test 'qemu-io -c discard' on a QCOW2 image without a backing file"
+echo
+for qcow2_compat in 0.10 1.1; do
+    echo "# Create an image with compat=$qcow2_compat without a backing file"
+    _make_test_img -o "compat=$qcow2_compat" 128k
+
+    echo "# Fill all clusters with data and then discard them"
+    $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
+    $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io
+
+    echo "# Read the data from the discarded clusters"
+    $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io
+done
+
+echo
+echo "### Test 'qemu-io -c discard' on a QCOW2 image with a backing file"
+echo
+
+echo "# Create a backing image and fill it with data"
+BACKING_IMG="$TEST_IMG.base"
+TEST_IMG="$BACKING_IMG" _make_test_img 128k
+$QEMU_IO -c 'write -P 0xff 0 128k' "$BACKING_IMG" | _filter_qemu_io
+
+for qcow2_compat in 0.10 1.1; do
+    echo "# Create an image with compat=$qcow2_compat and a backing file"
+    _make_test_img -o "compat=$qcow2_compat" -b "$BACKING_IMG"
+
+    echo "# Fill all clusters with data and then discard them"
+    $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
+    $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io
+
+    echo "# Read the data from the discarded clusters"
+    if [ "$qcow2_compat" = "1.1" ]; then
+        # In qcow2 v3 clusters are zeroed (with QCOW_OFLAG_ZERO)
+        $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io
+    else
+        # In qcow2 v2 if there's a backing image we cannot zero the clusters
+        # without exposing the backing file data so discard does nothing
+        $QEMU_IO -c 'read -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
+    fi
+done
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/289.out b/tests/qemu-iotests/289.out
new file mode 100644
index 0000000000..9ded8ef552
--- /dev/null
+++ b/tests/qemu-iotests/289.out
@@ -0,0 +1,52 @@
+QA output created by 289
+
+### Test 'qemu-io -c discard' on a QCOW2 image without a backing file
+
+# Create an image with compat=0.10 without a backing file
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072
+# Fill all clusters with data and then discard them
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+discard 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Read the data from the discarded clusters
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Create an image with compat=1.1 without a backing file
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072
+# Fill all clusters with data and then discard them
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+discard 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Read the data from the discarded clusters
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+### Test 'qemu-io -c discard' on a QCOW2 image with a backing file
+
+# Create a backing image and fill it with data
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=131072
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Create an image with compat=0.10 and a backing file
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072 backing_file=TEST_DIR/t.IMGFMT.base
+# Fill all clusters with data and then discard them
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+discard 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Read the data from the discarded clusters
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Create an image with compat=1.1 and a backing file
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072 backing_file=TEST_DIR/t.IMGFMT.base
+# Fill all clusters with data and then discard them
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+discard 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+# Read the data from the discarded clusters
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index ec2b2302e5..891b3ce858 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -295,3 +295,4 @@
 284 rw
 286 rw quick
 288 quick
+289 rw auto quick
-- 
2.20.1

On 24.03.20 13:16, Alberto Garcia wrote:
> A discard request deallocates the selected clusters so they read back
> as zeroes. This is done by clearing the cluster offset field and
> setting QCOW_OFLAG_ZERO in the L2 entry.
> 
> This flag is however only supported when qcow_version >= 3. In older
> images the cluster is simply deallocated, exposing any possible stale
> data from the backing file.
> 
> Since discard is an advisory operation it's safer to simply forbid it
> in this scenario.
> 
> Note that we are adding this check to qcow2_co_pdiscard() and not to
> qcow2_cluster_discard() or discard_in_l2_slice() because the last
> two are also used by qcow2_snapshot_create() to discard the clusters
> used by the VM state. In this case there's no risk of exposing stale
> data to the guest and we really want that the clusters are always
> discarded.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
> v2:
> 
> - Don't create the image with compat=0.10 in iotest 060 [Max]
> - Use $TEST_IMG.base for the backing image name in iotest 289 [Max]
> - Add list of unsupported options to iotest 289 [Max]
> 
>  block/qcow2.c              |  6 +++
>  tests/qemu-iotests/060     | 10 ++---
>  tests/qemu-iotests/060.out |  2 -
>  tests/qemu-iotests/289     | 91 ++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/289.out | 52 ++++++++++++++++++++++
>  tests/qemu-iotests/group   |  1 +
>  6 files changed, 154 insertions(+), 8 deletions(-)
>  create mode 100755 tests/qemu-iotests/289
>  create mode 100644 tests/qemu-iotests/289.out

Reviewed-by: Max Reitz <mreitz@redhat.com>

(Maybe someone else wants to give a comment still, though)

Am 24.03.2020 um 13:16 hat Alberto Garcia geschrieben:
> A discard request deallocates the selected clusters so they read back
> as zeroes. This is done by clearing the cluster offset field and
> setting QCOW_OFLAG_ZERO in the L2 entry.
> 
> This flag is however only supported when qcow_version >= 3. In older
> images the cluster is simply deallocated, exposing any possible stale
> data from the backing file.
> 
> Since discard is an advisory operation it's safer to simply forbid it
> in this scenario.
> 
> Note that we are adding this check to qcow2_co_pdiscard() and not to
> qcow2_cluster_discard() or discard_in_l2_slice() because the last
> two are also used by qcow2_snapshot_create() to discard the clusters
> used by the VM state. In this case there's no risk of exposing stale
> data to the guest and we really want that the clusters are always
> discarded.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>

The test number 289 is already taken, so this needs a rebase.

> +echo
> +echo "### Test 'qemu-io -c discard' on a QCOW2 image without a backing file"
> +echo
> +for qcow2_compat in 0.10 1.1; do
> +    echo "# Create an image with compat=$qcow2_compat without a backing file"
> +    _make_test_img -o "compat=$qcow2_compat" 128k
> +
> +    echo "# Fill all clusters with data and then discard them"
> +    $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
> +    $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io
> +
> +    echo "# Read the data from the discarded clusters"
> +    $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io
> +done

How about adding a 'qemu-img map' call just to have some more direct
information about what happened to the allocation status?

> +
> +echo
> +echo "### Test 'qemu-io -c discard' on a QCOW2 image with a backing file"
> +echo
> +
> +echo "# Create a backing image and fill it with data"
> +BACKING_IMG="$TEST_IMG.base"
> +TEST_IMG="$BACKING_IMG" _make_test_img 128k
> +$QEMU_IO -c 'write -P 0xff 0 128k' "$BACKING_IMG" | _filter_qemu_io
> +
> +for qcow2_compat in 0.10 1.1; do
> +    echo "# Create an image with compat=$qcow2_compat and a backing file"
> +    _make_test_img -o "compat=$qcow2_compat" -b "$BACKING_IMG"

I would write some non-zero data to the backing file so that you can
later distinguish zero clusters from unallocated clusters.

> +    echo "# Fill all clusters with data and then discard them"
> +    $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
> +    $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io
> +
> +    echo "# Read the data from the discarded clusters"
> +    if [ "$qcow2_compat" = "1.1" ]; then
> +        # In qcow2 v3 clusters are zeroed (with QCOW_OFLAG_ZERO)
> +        $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io
> +    else
> +        # In qcow2 v2 if there's a backing image we cannot zero the clusters
> +        # without exposing the backing file data so discard does nothing
> +        $QEMU_IO -c 'read -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
> +    fi
> +done

Kevin

On Fri 27 Mar 2020 03:14:42 PM CET, Kevin Wolf wrote:
>> +echo "# Create a backing image and fill it with data"
>> +BACKING_IMG="$TEST_IMG.base"
>> +TEST_IMG="$BACKING_IMG" _make_test_img 128k
>> +$QEMU_IO -c 'write -P 0xff 0 128k' "$BACKING_IMG" | _filter_qemu_io
>> +
>> +for qcow2_compat in 0.10 1.1; do
>> +    echo "# Create an image with compat=$qcow2_compat and a backing file"
>> +    _make_test_img -o "compat=$qcow2_compat" -b "$BACKING_IMG"
>
> I would write some non-zero data to the backing file so that you can
> later distinguish zero clusters from unallocated clusters.

I'm doing that already (immediately before the 'for' loop).

I'll add the output of qemu-img map in v3.

Berto