[PATCH] qcow2: Fix removal of list members from BDRVQcow2State.cluster_allocs

[PATCH] qcow2: Fix removal of list members from BDRVQcow2State.cluster_allocs

[Alberto Garcia]

When a write request needs to allocate new clusters (or change the L2
bitmap of existing ones) a QCowL2Meta structure is created so the L2
metadata can be later updated and any copy-on-write can be performed
if necessary.

A write request can span a region consisting of an arbitrary
combination of previously unallocated and allocated clusters, and if
the unallocated ones can be put contiguous to the existing ones then
QEMU will do so in order to minimize the number of write operations.

In practice this means that a write request has not just one but a
number of QCowL2Meta structures. All of them are added to the
cluster_allocs list that is stored in BDRVQcow2State and is used to
detect overlapping requests. After the write request finishes all its
associated QCowL2Meta are removed from that list. calculate_l2_meta()
takes care of creating and putting those structures in the list, and
qcow2_handle_l2meta() takes care of removing them.

The problem is that the error path in handle_alloc() also tries to
remove an item in that list, a remnant from the time when this was
handled there (that code would not even be correct anymore because
it only removes one struct and not all the ones from the same write
request).

This can trigger a double removal of the same item from the list,
causing a crash. This is not easy to reproduce in practice because
it requires that do_alloc_cluster_offset() fails after a successful
previous allocation during the same write request, but it can be
reproduced with the included test case.

As a last thing, this patch also removes the condition that
l2meta->nb_clusters is not 0 in qcow2_handle_l2meta(). This was only
necessary when that structure was allocated in the stack in order
to detect if it had been initialized or not. This changed in commit
f50f88b9fe and nowadays nb_clusters is guaranteed to be > 0.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2-cluster.c      |  3 --
 block/qcow2.c              |  4 +-
 tests/qemu-iotests/305     | 75 ++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/305.out | 16 ++++++++
 tests/qemu-iotests/group   |  1 +
 5 files changed, 93 insertions(+), 6 deletions(-)
 create mode 100755 tests/qemu-iotests/305
 create mode 100644 tests/qemu-iotests/305.out

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 996b3314f4..25e38daa78 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1709,9 +1709,6 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
 
 out:
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
-    if (ret < 0 && *m && (*m)->nb_clusters > 0) {
-        QLIST_REMOVE(*m, next_in_flight);
-    }
     return ret;
 }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index da56b1a4df..54a7d2f475 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2112,9 +2112,7 @@ static coroutine_fn int qcow2_handle_l2meta(BlockDriverState *bs,
         }
 
         /* Take the request off the list of running requests */
-        if (l2meta->nb_clusters != 0) {
-            QLIST_REMOVE(l2meta, next_in_flight);
-        }
+        QLIST_REMOVE(l2meta, next_in_flight);
 
         qemu_co_queue_restart_all(&l2meta->dependent_requests);
 
diff --git a/tests/qemu-iotests/305 b/tests/qemu-iotests/305
new file mode 100755
index 0000000000..6de3180c17
--- /dev/null
+++ b/tests/qemu-iotests/305
@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+#
+# Test the handling of errors in write requests with multiple allocations
+#
+# 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 $seq"
+
+status=1    # failure is the default!
+
+_cleanup()
+{
+    _cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+_supported_fmt qcow2
+_supported_proto file
+_supported_os Linux
+_unsupported_imgopts cluster_size refcount_bits extended_l2
+
+refcount_table_offset=$((0x400))
+
+echo '### Create the image'
+_make_test_img -o refcount_bits=64,cluster_size=1k 1M
+
+# The reference counts of the clusters for the first 123k of this
+# write request are stored in the first refcount block. The last
+# cluster (guest offset 123k) is referenced in the second refcount
+# block.
+echo '### Fill the first refcount block and one data cluster from the second'
+$QEMU_IO -c 'write 0 124k' "$TEST_IMG" | _filter_qemu_io
+
+echo '### Discard two of the last data clusters, leave one in the middle'
+$QEMU_IO -c 'discard 121k 1k' "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c 'discard 123k 1k' "$TEST_IMG" | _filter_qemu_io
+
+echo '### Corrupt the offset of the second refcount block'
+poke_file "$TEST_IMG" $(($refcount_table_offset+14)) "\x06"
+
+# This tries to allocate the two clusters discarded earlier (guest
+# offsets 121k and 123k). Their reference counts are in the first and
+# second refcount blocks respectively, but only the first one can be
+# allocated correctly because the second entry of the refcount table
+# is corrupted.
+echo '### Try to allocate the discarded clusters again'
+$QEMU_IO -c 'write 121k 3k' "$TEST_IMG" | _filter_qemu_io
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/305.out b/tests/qemu-iotests/305.out
new file mode 100644
index 0000000000..538019e726
--- /dev/null
+++ b/tests/qemu-iotests/305.out
@@ -0,0 +1,16 @@
+QA output created by 305
+### Create the image
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576
+### Fill the first refcount block and one data cluster from the second
+wrote 126976/126976 bytes at offset 0
+124 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+### Discard two of the last data clusters, leave one in the middle
+discard 1024/1024 bytes at offset 123904
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+discard 1024/1024 bytes at offset 125952
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+### Corrupt the offset of the second refcount block
+### Try to allocate the discarded clusters again
+qcow2: Marking image as corrupt: Refblock offset 0x20600 unaligned (reftable index: 0x1); further corruption events will be suppressed
+write failed: Input/output error
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 5cad015231..ff59cfd2d4 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -313,3 +313,4 @@
 302 quick
 303 rw quick
 304 rw quick
+305 rw quick
-- 
2.20.1

Re: [PATCH] qcow2: Fix removal of list members from BDRVQcow2State.cluster_allocs

[Max Reitz]

[-- Attachment #1.1: Type: text/plain, Size: 3558 bytes --]

On 02.09.20 19:42, Alberto Garcia wrote:
> When a write request needs to allocate new clusters (or change the L2
> bitmap of existing ones) a QCowL2Meta structure is created so the L2
> metadata can be later updated and any copy-on-write can be performed
> if necessary.
> 
> A write request can span a region consisting of an arbitrary
> combination of previously unallocated and allocated clusters, and if
> the unallocated ones can be put contiguous to the existing ones then
> QEMU will do so in order to minimize the number of write operations.
> 
> In practice this means that a write request has not just one but a
> number of QCowL2Meta structures. All of them are added to the
> cluster_allocs list that is stored in BDRVQcow2State and is used to
> detect overlapping requests. After the write request finishes all its
> associated QCowL2Meta are removed from that list. calculate_l2_meta()
> takes care of creating and putting those structures in the list, and
> qcow2_handle_l2meta() takes care of removing them.
> 
> The problem is that the error path in handle_alloc() also tries to
> remove an item in that list, a remnant from the time when this was
> handled there (that code would not even be correct anymore because
> it only removes one struct and not all the ones from the same write
> request).
> 
> This can trigger a double removal of the same item from the list,
> causing a crash. This is not easy to reproduce in practice because
> it requires that do_alloc_cluster_offset() fails after a successful
> previous allocation during the same write request, but it can be
> reproduced with the included test case.
> 
> As a last thing, this patch also removes the condition that
> l2meta->nb_clusters is not 0 in qcow2_handle_l2meta(). This was only
> necessary when that structure was allocated in the stack in order
> to detect if it had been initialized or not. This changed in commit
> f50f88b9fe and nowadays nb_clusters is guaranteed to be > 0.

On my search for /(\.|->)nb_clusters/ to verify this, I noticed this
comment for qcow2_alloc_cluster_offset():

 * If the cluster was already allocated, m->nb_clusters is set to 0 and



 * other fields in m are meaningless.

Should that be dropped, too?

> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c      |  3 --
>  block/qcow2.c              |  4 +-
>  tests/qemu-iotests/305     | 75 ++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/305.out | 16 ++++++++
>  tests/qemu-iotests/group   |  1 +
>  5 files changed, 93 insertions(+), 6 deletions(-)
>  create mode 100755 tests/qemu-iotests/305
>  create mode 100644 tests/qemu-iotests/305.out

[...]

> diff --git a/tests/qemu-iotests/305 b/tests/qemu-iotests/305
> new file mode 100755
> index 0000000000..6de3180c17
> --- /dev/null
> +++ b/tests/qemu-iotests/305
> @@ -0,0 +1,75 @@

[...]

> +_supported_fmt qcow2
> +_supported_proto file
> +_supported_os Linux
> +_unsupported_imgopts cluster_size refcount_bits extended_l2

data_file, too, because the refcounts work differently there.  (As in:
Not at all for data clusters.)

Also, compat(=0.10), because that wouldn’t allow -o refcount_bits=64.

> +
> +refcount_table_offset=$((0x400))

I would like to suggest $(peek_file_be "$TEST_IMG" 48 8), to set an
example for future generations; but not strictly necessary, of course. O:)

Anyway, at least with the _unsupported_imgopts line completed:

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


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Re: [PATCH] qcow2: Fix removal of list members from BDRVQcow2State.cluster_allocs

[Alberto Garcia]

On Thu 03 Sep 2020 03:12:22 PM CEST, Max Reitz wrote:
> On my search for /(\.|->)nb_clusters/ to verify this, I noticed this
> comment for qcow2_alloc_cluster_offset():
>
>  * If the cluster was already allocated, m->nb_clusters is set to 0 and
>  * other fields in m are meaningless.
>
> Should that be dropped, too?

Actually the whole comment should be rewritten (the function does not
allocale just one cluster), I'll send v2 with that and the rest of your
proposed changes.

Berto