[libvirt] [PATCH v2] network: bridge: Don't start network if it collides with host routing

[Cole Robinson]

Fedora bug https://bugzilla.redhat.com/show_bug.cgi?id=235961

If using the default virtual network, an easy way to lose guest network
connectivity is to install libvirt inside the VM. The autostarted
default network inside the guest collides with host virtual network
routing. This is a long standing issue that has caused users quite a
bit of pain and confusion.

On network startup, parse /proc/net/route and compare the requested
IP+netmask against host routing destinations: if any matches are found,
refuse to start the network.

v2: Drop sscanf, fix a comment typo, comment that function could use
    libnl instead of /proc

Signed-off-by: Cole Robinson <crobinso at redhat.com>
---
 src/network/bridge_driver.c |  102 +++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 102 insertions(+), 0 deletions(-)

diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
index 5d7ef19..090bed7 100644
--- a/src/network/bridge_driver.c
+++ b/src/network/bridge_driver.c
@@ -42,6 +42,8 @@
 #include <stdio.h>
 #include <sys/wait.h>
 #include <sys/ioctl.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
 
 #include "virterror_internal.h"
 #include "datatypes.h"
@@ -908,6 +910,102 @@ cleanup:
     return ret;
 }
 
+#define PROC_NET_ROUTE "/proc/net/route"
+
+/* XXX: This function can be a lot more exhaustive, there are certainly
+ *      other scenarios where we can ruin host network connectivity.
+ * XXX: Using a proper library is preferred over parsing /proc
+ */
+static int networkCheckRouteCollision(virNetworkObjPtr network)
+{
+    int ret = -1, len;
+    char *cur, *buf = NULL;
+    enum {MAX_ROUTE_SIZE = 1024*64};
+    struct in_addr inaddress, innetmask;
+    char *netaddr = NULL;
+
+    if (!network->def->ipAddress || !network->def->netmask)
+        return 0;
+
+    if (inet_pton(AF_INET, network->def->ipAddress, &inaddress) <= 0) {
+        networkReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("cannot parse IP address '%s'"),
+                           network->def->ipAddress);
+        goto error;
+    }
+    if (inet_pton(AF_INET, network->def->netmask, &innetmask) <= 0) {
+        networkReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("cannot parse netmask '%s'"),
+                           network->def->netmask);
+        goto error;
+    }
+
+    inaddress.s_addr &= innetmask.s_addr;
+    netaddr = strdup(inet_ntoa(inaddress));
+    if (!netaddr) {
+        virReportOOMError();
+        goto error;
+    }
+
+    /* Read whole routing table into memory */
+    if ((len = virFileReadAll(PROC_NET_ROUTE, MAX_ROUTE_SIZE, &buf)) < 0)
+        goto error;
+
+    /* Dropping the last character shouldn't hurt */
+    buf[len-1] = '\0';
+
+    /* First line is just headings, skip it */
+    cur = strchr(buf, '\n');
+
+    while (cur) {
+        char *iface, *dest_raw;
+        char *dest_ip;
+        struct in_addr in;
+        unsigned int addr_val;
+
+        cur++;
+
+        /* Delimit interface field */
+        iface = cur;
+        while(*cur > ' ') {
+            cur++;
+        }
+        *cur++ = '\0';
+
+        /* Delimit destination field */
+        dest_raw = cur;
+        while(*cur > ' ') {
+            cur++;
+        }
+        *cur++ = '\0';
+
+        if (virStrToLong_ui(dest_raw, NULL, 16, &addr_val) < 0) {
+            networkReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Failed to convert network address %s"),
+                               dest_raw);
+            goto error;
+        }
+
+        in.s_addr = addr_val;
+        dest_ip = inet_ntoa(in);
+
+        if (STREQ(netaddr, dest_ip)) {
+            networkReportError(VIR_ERR_INTERNAL_ERROR,
+                              _("Network destination %s is already in use "
+                                "by interface %s"), netaddr, iface);
+            goto error;
+        }
+
+        cur = strchr(cur, '\n');
+    }
+
+    ret = 0;
+error:
+    VIR_FREE(buf);
+    VIR_FREE(netaddr);
+    return ret;
+}
+
 static int networkStartNetworkDaemon(struct network_driver *driver,
                                      virNetworkObjPtr network)
 {
@@ -919,6 +1017,10 @@ static int networkStartNetworkDaemon(struct network_driver *driver,
         return -1;
     }
 
+    /* Check to see if network collides with an existing route */
+    if (networkCheckRouteCollision(network) < 0)
+        return -1;
+
     if ((err = brAddBridge(driver->brctl, network->def->bridge))) {
         virReportSystemError(err,
                              _("cannot create bridge '%s'"),
-- 
1.6.6.1