Public bug reported:
[ Impact ]
When running `block-stream` and `query-named-block-nodes` concurrently,
a null-pointer dereference causes QEMU to segfault.
This occurs in every version of QEMU shipped with Ubuntu, 22.04 thru
25.10. I have not yet reproduced the bug using an upstream build.
I will link the upstream bug report here as soon as I've written it.
[ Reproducer ]
In `query-named-block-nodes.sh`:
```sh
#!/bin/bash
while true; do
virsh qemu-monitor-command "$1" query-named-block-nodes > /dev/null
done
```
In `blockrebase-crash.sh`:
```sh
#!/bin/bash
set -ex
domain="$1"
if [ -z "${domain}" ]; then
echo "Missing domain name"
exit 1
fi
./query_named_block_nodes.sh "${domain}" &
query_pid=$!
while [ -n "$(virsh list --uuid)" ]; do
snap="snap0-$(uuidgen)"
virsh snapshot-create-as "${domain}" \
--name "${snap}" \
--disk-only file= \
--diskspec vda,snapshot=no \
--diskspec
"vdb,stype=file,file=/var/lib/libvirt/images/n0-blk0_${snap}.qcow2" \
--atomic \
--no-metadata
virsh blockpull "${domain}" vdb
while bjr=$(virsh blockjob "$domain" vdb); do
if [[ "$bjr" == *"No current block job for"* ]] ; then
break;
fi;
done;
done
kill "${query_pid}"
```
Provision (`Ctrl + ]` after boot):
```sh
wget
https://cloud-images.ubuntu.com/daily/server/noble/current/noble-server-cloudimg-amd64.img
sudo cp noble-server-cloudimg-amd64.img /var/lib/libvirt/images/n0-root.qcow2
sudo qemu-img create -f qcow2 /var/lib/libvirt/images/n0-blk0.qcow2 10G
touch network-config
touch meta-data
touch user-data
virt-install \
-n n0 \
--description "Test noble minimal" \
--os-variant=ubuntu24.04 \
--ram=1024 --vcpus=2 \
--import \
--disk
path=/var/lib/libvirt/images/n0-root.qcow2,bus=virtio,cache=writethrough,size=10
\
--disk
path=/var/lib/libvirt/images/n0-blk0.qcow2,bus=virtio,cache=writethrough,size=10
\
--graphics none \
--network network=default \
--cloud-init
user-data="user-data,meta-data=meta-data,network-config=network-config"
```
And run the script to cause the crash (you may need to manually kill
query-named-block-jobs.sh):
```sh
./blockrebase-crash n0
```
[ Details ]
Backtrace from the coredump (source at [1]):
```
#0 bdrv_refresh_filename (bs=0x5efed72f8350) at
/usr/src/qemu-1:10.1.0+ds-5ubuntu2/b/qemu/block.c:8082
#1 0x00005efea73cf9dc in bdrv_block_device_info (blk=0x0, bs=0x5efed72f8350,
flat=true, errp=0x7ffeb829ebd8)
at block/qapi.c:62
#2 0x00005efea7391ed3 in bdrv_named_nodes_list (flat=<optimized out>,
errp=0x7ffeb829ebd8)
at /usr/src/qemu-1:10.1.0+ds-5ubuntu2/b/qemu/block.c:6275
#3 0x00005efea7471993 in qmp_query_named_block_nodes (has_flat=<optimized
out>, flat=<optimized out>,
errp=0x7ffeb829ebd8) at
/usr/src/qemu-1:10.1.0+ds-5ubuntu2/b/qemu/blockdev.c:2834
#4 qmp_marshal_query_named_block_nodes (args=<optimized out>,
ret=0x7f2b753beec0, errp=0x7f2b753beec8)
at qapi/qapi-commands-block-core.c:553
#5 0x00005efea74f03a5 in do_qmp_dispatch_bh (opaque=0x7f2b753beed0) at
qapi/qmp-dispatch.c:128
#6 0x00005efea75108e6 in aio_bh_poll (ctx=0x5efed6f3f430) at util/async.c:219
#7 0x00005efea74ffdb2 in aio_dispatch (ctx=0x5efed6f3f430) at
util/aio-posix.c:436
#8 0x00005efea7512846 in aio_ctx_dispatch (source=<optimized out>,
callback=<optimized out>,
user_data=<optimized out>) at util/async.c:361
#9 0x00007f2b77809bfb in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#10 0x00007f2b77809e70 in g_main_context_dispatch () from
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#11 0x00005efea7517228 in glib_pollfds_poll () at util/main-loop.c:287
#12 os_host_main_loop_wait (timeout=0) at util/main-loop.c:310
#13 main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:589
#14 0x00005efea7140482 in qemu_main_loop () at system/runstate.c:905
#15 0x00005efea744e4e8 in qemu_default_main (opaque=opaque@entry=0x0) at
system/main.c:50
#16 0x00005efea6e76319 in main (argc=<optimized out>, argv=<optimized out>) at
system/main.c:93
```
The libvirt logs suggest that the crash occurs right at the end of the
blockjob, since it reaches "concluded" state before crashing. I assume that
this is one of:
- `stream_clean` is freeing/modifying the `cor_filter_bs` without holding a
lock that it needs to [2][3]
- `bdrv_refresh_filename` needs to handle the possibility that the QLIST of
children for a filter bs could be NULL [1]
[1]
https://git.launchpad.net/ubuntu/+source/qemu/tree/block.c?h=ubuntu/questing-devel#n8071
[2]
https://git.launchpad.net/ubuntu/+source/qemu/tree/block/stream.c?h=ubuntu/questing-devel#n131
[3]
https://git.launchpad.net/ubuntu/+source/qemu/tree/block/stream.c?h=ubuntu/questing-devel#n340
** Affects: qemu (Ubuntu)
Importance: Medium
Assignee: Wesley Hershberger (whershberger)
Status: Confirmed
** Tags: sts
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https://bugs.launchpad.net/bugs/2126951
Title:
`block-stream` segfault with concurrent `query-named-block-nodes`
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