[libvirt] [PATCH] util: allow tap-based guest interfaces to have MAC address prefix 0xFE

[Laine Stump]

Back in July 2010, commit 6ea90b84 (meant to resolve
https://bugzilla.redhat.com/571991 ) added code to set the MAC address
of any tap device to the associated guest interface's MAC, but with
the first byte replaced with 0xFE. This was done in order to assure
that

1) the tap MAC and guest interface MAC were different (otherwise L2
   forwarding through the tap would not work, and the kernel would
   repeatedly issue a warning stating as much).

2) any bridge device that had one of these taps attached would *not*
   take on the MAC of the tap (leading to network instability as
   guests started and stopped)

A couple years later, https://bugzilla.redhat.com/798467 was filed,
complaining that a user could configure a tap-based guest interface to
have a MAC address that itself had a first byte of 0xFE, silently
(other than the kernel warning messages) resulting in a non-working
configuration. This was fixed by commit 5d571045, which logged an
error and failed the guest start / interface attach if the MAC's first
byte was 0xFE.

Although this restriction only reduces the potential pool of MAC
addresses from 2^46 (last two bits of byte 1 must be set to 10) by
2^32 (still 4 orders of magnitude larger than the entire IPv4 address
space), it also means that management software that autogenerates MAC
addresses must have special code to avoid an 0xFE prefix. Now after 7
years, someone has noticed this restriction and requested that we
remove it.

So instead of failing when 0xFE is found as the first byte, this patch
removes the restriction by just replacing the first byte in the tap
device MAC with 0xFA if the first byte in the guest interface is
0xFE. 0xFA is the next-highest value that still has 10 as the lowest
two bits, and still

2) meets the requirement of "tap MAC must be different from guest
   interface MAC", and

3) is high enough that there should never be an issue of the attached
   bridge device taking on the MAC of the tap.

The result is that *any* MAC can be chosen by management software
(although it would still not work correctly if a multicast MAC (lowest
bit of first byte set to 1) was chosen), but that's a different
issue).

Signed-off-by: Laine Stump <laine at redhat.com>
---

Yes, I find it slightly problematic that the same setting is in two
different places in the code, but 1) that is pre-existing, and 2) if I
moved the MAC address setting down one level into virNetDevTapCreate(),
the code would *still* need to be duplicated, since there are two
different implementations of virNetDevTapCreate().  If anyone is
bothered by this, then I can resubmit with an extra patch to add a new
virNetDevTapCreate() that just calls a platform-specific
virNetDevTapCreateInternal(), then sets the tap mac address. Only by
request though :-).

 src/qemu/qemu_interface.c | 11 ++++++++++-
 src/util/virnetdevtap.c   | 26 ++++++++++++--------------
 2 files changed, 22 insertions(+), 15 deletions(-)

diff --git a/src/qemu/qemu_interface.c b/src/qemu/qemu_interface.c
index c8effa68f4..72ed51cb1f 100644
--- a/src/qemu/qemu_interface.c
+++ b/src/qemu/qemu_interface.c
@@ -444,8 +444,17 @@ qemuInterfaceEthernetConnect(virDomainDefPtr def,
     }
 
     virDomainAuditNetDevice(def, net, tunpath, true);
+
+    /* 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);
-    tapmac.addr[0] = 0xFE;
+    if (tapmac.addr[0] == 0xFE)
+        tapmac.addr[0] = 0xFA;
+    else
+        tapmac.addr[0] = 0xFE;
 
     if (virNetDevSetMAC(net->ifname, &tapmac) < 0)
         goto cleanup;
diff --git a/src/util/virnetdevtap.c b/src/util/virnetdevtap.c
index b65c26bee1..4548b51b5b 100644
--- a/src/util/virnetdevtap.c
+++ b/src/util/virnetdevtap.c
@@ -668,7 +668,6 @@ int virNetDevTapCreateInBridgePort(const char *brname,
                                    unsigned int flags)
 {
     virMacAddr tapmac;
-    char macaddrstr[VIR_MAC_STRING_BUFLEN];
     size_t i;
 
     if (virNetDevTapCreate(ifname, tunpath, tapfd, tapfdSize, flags) < 0)
@@ -682,19 +681,18 @@ int virNetDevTapCreateInBridgePort(const char *brname,
      */
     virMacAddrSet(&tapmac, macaddr);
     if (!(flags & VIR_NETDEV_TAP_CREATE_USE_MAC_FOR_BRIDGE)) {
-        if (macaddr->addr[0] == 0xFE) {
-            /* For normal use, 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.
-             */
-            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                           _("Unable to use MAC address starting with "
-                             "reserved value 0xFE - '%s' - "),
-                           virMacAddrFormat(macaddr, macaddrstr));
-            goto error;
-        }
-        tapmac.addr[0] = 0xFE; /* Discourage bridge from using TAP dev MAC */
+        /* 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. Making the tap address as high as possible
+         * discourages the bridge from using this tap's MAC as its own
+         * (a Linux host bridge will take on the lowest numbered MAC
+         * of all devices attached to it).
+         */
+        if (tapmac.addr[0] == 0xFE)
+            tapmac.addr[0] = 0xFA;
+        else
+            tapmac.addr[0] = 0xFE;
     }
 
     if (virNetDevSetMAC(*ifname, &tapmac) < 0)
-- 
2.21.0