qemu: support unmanaged target tap dev for <interface type='ethernet'>

If managed='no', then the tap device must already exist, and setting
of MAC address and online status (IFF_UP) is skipped.

NB: we still set IFF_VNET_HDR and IFF_MULTI_QUEUE as appropriate,
because those bits must be properly set in the TUNSETIFF we use to set
the tap device name of the handle we've opened - if IFF_VNET_HDR has
not been set and we set it the request will be honored even when
running libvirtd unprivileged; if IFF_MULTI_QUEUE is requested to be
different than how it was created, that will result in an error from
the kernel. This means that you don't need to pay attention to
IFF_VNET_HDR when creating the tap devices, but you *do* need to set
IFF_MULTI_QUEUE if you're going to use multiple queues for your tap
device.

NB2: /dev/vhost-net normally has permissions 600, so it can't be
opened by an unprivileged process. This would normally cause a warning
message when using a virtio net device from an unprivileged
libvirtd. I've found that setting the permissions for /dev/vhost-net
permits unprivileged libvirtd to use vhost-net for virtio devices, but
have no idea what sort of security implications that has. I haven't
changed libvrit's code to avoid *attempting* to open /dev/vhost-net -
if you are concerned about the security of opening up permissions of
/dev/vhost-net (probably a good idea at least until we ask someone who
knows about the code) then add <driver name='qemu'/> to the interface
definition and you'll avoid the warning message.

Note that virNetDevTapCreate() is the correct function to call in the
case of an existing device, because the same ioctl() that creates a
new tap device will also open an existing tap device.

Resolves: https://bugzilla.redhat.com/1723367 (partially)
Signed-off-by: NLaine Stump <laine@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_interface.c

@@ -428,36 +428,53 @@ qemuInterfaceEthernetConnect(virDomainDefPtr def,
     if (virDomainNetIsVirtioModel(net))
         tap_create_flags |= VIR_NETDEV_TAP_CREATE_VNET_HDR;
 
-    if (!net->ifname ||
-        STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
-        strchr(net->ifname, '%')) {
-        VIR_FREE(net->ifname);
-        if (VIR_STRDUP(net->ifname, VIR_NET_GENERATED_TAP_PREFIX "%d") < 0)
+    if (net->managed_tap == VIR_TRISTATE_BOOL_NO) {
+        if (!net->ifname) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("target dev must be supplied when managed='no'"));
             goto cleanup;
-        /* avoid exposing vnet%d in getXMLDesc or error outputs */
-        template_ifname = true;
-    }
-    if (virNetDevTapCreate(&net->ifname, tunpath, tapfd, tapfdSize,
-                           tap_create_flags) < 0) {
-        goto cleanup;
-    }
+        }
+        if (virNetDevExists(net->ifname) != 1) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("target managed='no' but specified dev doesn't exist"));
+            goto cleanup;
+        }
+        if (virNetDevTapCreate(&net->ifname, tunpath, tapfd, tapfdSize,
+                               tap_create_flags) < 0) {
+            goto cleanup;
+        }
+    } else {
+        if (!net->ifname ||
+            STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+            strchr(net->ifname, '%')) {
+            VIR_FREE(net->ifname);
+            if (VIR_STRDUP(net->ifname, VIR_NET_GENERATED_TAP_PREFIX "%d") < 0)
+                goto cleanup;
+            /* avoid exposing vnet%d in getXMLDesc or error outputs */
+            template_ifname = true;
+        }
+        if (virNetDevTapCreate(&net->ifname, tunpath, tapfd, tapfdSize,
+                               tap_create_flags) < 0) {
+            goto cleanup;
+        }
 
-    /* The tap device's MAC address cannot match the MAC address
-     * used by the guest. This results in "received packet on
-     * vnetX with own address as source address" error logs from
-     * the kernel.
-     */
-    virMacAddrSet(&tapmac, &net->mac);
-    if (tapmac.addr[0] == 0xFE)
-        tapmac.addr[0] = 0xFA;
-    else
-        tapmac.addr[0] = 0xFE;
+        /* The tap device's MAC address cannot match the MAC address
+         * used by the guest. This results in "received packet on
+         * vnetX with own address as source address" error logs from
+         * the kernel.
+         */
+        virMacAddrSet(&tapmac, &net->mac);
+        if (tapmac.addr[0] == 0xFE)
+            tapmac.addr[0] = 0xFA;
+        else
+            tapmac.addr[0] = 0xFE;
 
-    if (virNetDevSetMAC(net->ifname, &tapmac) < 0)
-        goto cleanup;
+        if (virNetDevSetMAC(net->ifname, &tapmac) < 0)
+            goto cleanup;
 
-    if (virNetDevSetOnline(net->ifname, true) < 0)
-        goto cleanup;
+        if (virNetDevSetOnline(net->ifname, true) < 0)
+            goto cleanup;
+    }
 
     if (net->script &&
         virNetDevRunEthernetScript(net->ifname, net->script) < 0)

src/qemu/qemu_process.c

@@ -7559,7 +7559,7 @@ void qemuProcessStop(virQEMUDriverPtr driver,
                              cfg->stateDir));
             break;
         case VIR_DOMAIN_NET_TYPE_ETHERNET:
-            if (net->ifname) {
+            if (net->managed_tap != VIR_TRISTATE_BOOL_NO && net->ifname) {
                 ignore_value(virNetDevTapDelete(net->ifname, net->backend.tap));
                 VIR_FREE(net->ifname);
             }

src/util/virnetdev.h

@@ -150,7 +150,7 @@ int virNetDevSetupControl(const char *ifname,
     ATTRIBUTE_RETURN_CHECK;
 
 int virNetDevExists(const char *brname)
-    ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK;
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NOINLINE;
 
 int virNetDevSetOnline(const char *ifname,
                        bool online)

tests/qemuxml2argvdata/net-eth-unmanaged-tap.args

+LC_ALL=C \
+PATH=/bin \
+HOME=/tmp/lib/domain--1-QEMUGuest1 \
+USER=test \
+LOGNAME=test \
+XDG_DATA_HOME=/tmp/lib/domain--1-QEMUGuest1/.local/share \
+XDG_CACHE_HOME=/tmp/lib/domain--1-QEMUGuest1/.cache \
+XDG_CONFIG_HOME=/tmp/lib/domain--1-QEMUGuest1/.config \
+QEMU_AUDIO_DRV=none \
+/usr/bin/qemu-system-i686 \
+-name QEMUGuest1 \
+-S \
+-machine pc,accel=tcg,usb=off,dump-guest-core=off \
+-m 214 \
+-realtime mlock=off \
+-smp 1,sockets=1,cores=1,threads=1 \
+-uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \
+-display none \
+-no-user-config \
+-nodefaults \
+-chardev socket,id=charmonitor,path=/tmp/lib/domain--1-QEMUGuest1/monitor.sock,\
+server,nowait \
+-mon chardev=charmonitor,id=monitor,mode=control \
+-rtc base=utc \
+-no-shutdown \
+-no-acpi \
+-usb \
+-drive file=/dev/HostVG/QEMUGuest1,format=raw,if=none,id=drive-ide0-0-0 \
+-device ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 \
+-netdev tap,fd=3,id=hostnet0,vhost=on,vhostfd=44 \
+-device virtio-net-pci,netdev=hostnet0,id=net0,mac=fe:11:22:33:44:55,bus=pci.0,\
+addr=0x3

tests/qemuxml2argvmock.c

@@ -149,8 +149,12 @@ virNetDevTapCreate(char **ifname,
     for (i = 0; i < tapfdSize; i++)
         tapfd[i] = STDERR_FILENO + 1 + i;
 
-    VIR_FREE(*ifname);
-    return VIR_STRDUP(*ifname, "vnet0");
+    if (STREQ_NULLABLE(*ifname, "mytap0")) {
+        return 0;
+    } else {
+        VIR_FREE(*ifname);
+        return VIR_STRDUP(*ifname, "vnet0");
+    }
 }
 
 int
@@ -160,6 +164,14 @@ virNetDevSetMAC(const char *ifname ATTRIBUTE_UNUSED,
     return 0;
 }
 
+
+int
+virNetDevExists(const char *ifname)
+{
+    return STREQ_NULLABLE(ifname, "mytap0");
+}
+
+
 int virNetDevIPAddrAdd(const char *ifname ATTRIBUTE_UNUSED,
                        virSocketAddr *addr ATTRIBUTE_UNUSED,
                        virSocketAddr *peer ATTRIBUTE_UNUSED,

tests/qemuxml2argvtest.c

@@ -1318,6 +1318,7 @@ mymain(void)
     DO_TEST("net-eth-ifname", NONE);
     DO_TEST("net-eth-names", NONE);
     DO_TEST("net-eth-hostip", NONE);
+    DO_TEST("net-eth-unmanaged-tap", NONE);
     DO_TEST("net-client", NONE);
     DO_TEST("net-server", NONE);
     DO_TEST("net-many-models", NONE);