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Tsirkin" , qemu-devel@nongnu.org, qemu-stable@nongnu.org Subject: Re: [PATCH] qmp: Check vhost protocol features for NULL prior to dumping In-Reply-To: <20260319085050.1982608-1-kniv@yandex-team.ru> (Nikolay Kuratov's message of "Thu, 19 Mar 2026 11:50:50 +0300") References: <20260319085050.1982608-1-kniv@yandex-team.ru> Date: Thu, 19 Mar 2026 14:59:24 +0100 Message-ID: <871phfx6mb.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -3 X-Spam_score: -0.4 X-Spam_bar: / X-Spam_report: (-0.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.819, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.903, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Nikolay Kuratov writes: > vhost_dev->protocol_features field can be legitimately set to NULL > as apparently it's vhost-user only field. At least I was able to get > NULL deref with a vhost-net VM. > > Without that check querying vhost-net device: > info virtio-status /machine/peripheral-anon/device[0]/virtio-backend > will lead to SIGSEGV. > > Fixes: 8a8287981d1169f534894d983ecfd3b70b71918b ("hmp: add virtio commands") > Cc: qemu-stable@nongnu.org > Signed-off-by: Nikolay Kuratov > --- > hw/virtio/virtio-hmp-cmds.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/hw/virtio/virtio-hmp-cmds.c b/hw/virtio/virtio-hmp-cmds.c > index 1daae482d3..b9980197ef 100644 > --- a/hw/virtio/virtio-hmp-cmds.c > +++ b/hw/virtio/virtio-hmp-cmds.c > @@ -15,6 +15,9 @@ > static void hmp_virtio_dump_protocols(Monitor *mon, > VhostDeviceProtocols *pcol) > { > + if (pcol == NULL) { > + return; > + } > strList *pcol_list = pcol->protocols; > while (pcol_list) { > monitor_printf(mon, "\t%s", pcol_list->value); I fear this papers over the real bug. Here's the only caller: void hmp_virtio_status(Monitor *mon, const QDict *qdict) { Error *err = NULL; const char *path = qdict_get_try_str(qdict, "path"); VirtioStatus *s = qmp_x_query_virtio_status(path, &err); if (err != NULL) { Aside: I'd prefer if (!s) here. hmp_handle_error(mon, err); return; } [...] if (s->vhost_dev) { [...] monitor_printf(mon, " Protocol features:\n"); hmp_virtio_dump_protocols(mon, s->vhost_dev->protocol_features); We're passing VirtioStatus member @protocol-features. It's not supposed to be null, because ... } qapi_free_VirtioStatus(s); } In qapi/virtio.json: { 'struct': 'VhostStatus', 'data': { 'n-mem-sections': 'int', 'n-tmp-sections': 'int', 'nvqs': 'uint32', 'vq-index': 'int', 'features': 'VirtioDeviceFeatures', 'acked-features': 'VirtioDeviceFeatures', 'backend-features': 'VirtioDeviceFeatures', 'protocol-features': 'VhostDeviceProtocols', ... member @protocol-features is *not* optional. Since you obseved it to be null, whatever created s->vhost_dev must have screwed up. If s->vhost_dev->protocol-features needs to be null in certain situations, you must declare it optional in the QAPI schema. 'max-queues': 'uint64', 'backend-cap': 'uint64', 'log-enabled': 'bool', 'log-size': 'uint64' } }