[PATCH v2 2/2] virnetdevtap.c: Disallow pre-existing TAP devices

[Michal Privoznik]

When starting a guest with <interface/> which has the target
device name set (i.e. not generated by us), it may happen that
the TAP device already exists. This then may lead to all sorts of
problems. For instance: for <interface type='network'/> the TAP
device is plugged into the network's bridge, but since the TAP
device is persistent it remains plugged there even after the
guest is shut off. We don't have a code that unplugs TAP devices
from the bridge because TAP devices we create are transient, i.e.
are removed automatically when QEMU closes their FD.

The only exception is <interface type='ethernet'/> with <target
managed='no'/> where we specifically want to let users use
pre-created TAP device and basically not touch it at all.

There's another reason for denying to use a pre-created TAP
devices: if we ever have bug in TAP name generation, we may
re-use a TAP device from another domain.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2144738
Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 src/qemu/qemu_interface.c |  2 ++
 src/util/virnetdevtap.c   | 23 +++++++++++++++++++++--
 src/util/virnetdevtap.h   |  2 ++
 3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_interface.c b/src/qemu/qemu_interface.c
index 4cc76e07a5..264d5e060c 100644
--- a/src/qemu/qemu_interface.c
+++ b/src/qemu/qemu_interface.c
@@ -461,6 +461,8 @@ qemuInterfaceEthernetConnect(virDomainDef *def,
         if (!net->ifname)
             template_ifname = true;
 
+        tap_create_flags |= VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING;
+
         if (virNetDevTapCreate(&net->ifname, tunpath, tapfd, tapfdSize,
                                tap_create_flags) < 0) {
             goto cleanup;
diff --git a/src/util/virnetdevtap.c b/src/util/virnetdevtap.c
index 112a1e8b99..a4ead0ae93 100644
--- a/src/util/virnetdevtap.c
+++ b/src/util/virnetdevtap.c
@@ -148,12 +148,15 @@ virNetDevTapGetRealDeviceName(char *ifname G_GNUC_UNUSED)
  * @tunpath: path to the tun device (if NULL, /dev/net/tun is used)
  * @tapfds: array of file descriptors return value for the new tap device
  * @tapfdSize: number of file descriptors in @tapfd
- * @flags: OR of virNetDevTapCreateFlags. Only one flag is recognized:
+ * @flags: OR of virNetDevTapCreateFlags. Only the following flags are
+ *         recognized:
  *
  *   VIR_NETDEV_TAP_CREATE_VNET_HDR
  *     - Enable IFF_VNET_HDR on the tap device
  *   VIR_NETDEV_TAP_CREATE_PERSIST
  *     - The device will persist after the file descriptor is closed
+ *   VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING
+ *     - The device creation fails if @ifname already exists
  *
  * Creates a tap interface. The caller must use virNetDevTapDelete to
  * remove a persistent TAP device when it is no longer needed. In case
@@ -170,6 +173,7 @@ int virNetDevTapCreate(char **ifname,
 {
     size_t i = 0;
     struct ifreq ifr;
+    int rc;
     int ret = -1;
     int fd = -1;
 
@@ -179,9 +183,24 @@ int virNetDevTapCreate(char **ifname,
      * can lead to race conditions).  if ifname is just a
      * user-provided name, virNetDevGenerateName leaves it
      * unchanged. */
-    if (virNetDevGenerateName(ifname, VIR_NET_DEV_GEN_NAME_VNET) < 0)
+    rc = virNetDevGenerateName(ifname, VIR_NET_DEV_GEN_NAME_VNET);
+    if (rc < 0)
         return -1;
 
+    if (rc > 0 &&
+        !(flags & VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING)) {
+        rc = virNetDevExists(*ifname);
+
+        if (rc < 0) {
+            return -1;
+        } else if (rc > 0) {
+            virReportError(VIR_ERR_OPERATION_INVALID,
+                           _("The %s interface already exists"),
+                           *ifname);
+            return -1;
+        }
+    }
+
     if (!tunpath)
         tunpath = "/dev/net/tun";
 
diff --git a/src/util/virnetdevtap.h b/src/util/virnetdevtap.h
index 197ea10f94..c9d29c0384 100644
--- a/src/util/virnetdevtap.h
+++ b/src/util/virnetdevtap.h
@@ -56,6 +56,8 @@ typedef enum {
    VIR_NETDEV_TAP_CREATE_USE_MAC_FOR_BRIDGE = 1 << 2,
    /* The device will persist after the file descriptor is closed */
    VIR_NETDEV_TAP_CREATE_PERSIST            = 1 << 3,
+   /* The device is allowed to exist before creation */
+   VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING     = 1 << 4,
 } virNetDevTapCreateFlags;
 
 int
-- 
2.37.4