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Re: [PATCH v2] network: Only check kernel added routes in virNetDevIPCheckIPv6Forwarding



On 9/11/20 7:34 AM, Ian Wienand wrote:
The original motivation for adding virNetDevIPCheckIPv6Forwarding
(00d28a78b5d1f6eaf79f06ac59e31c568af9da37) was that networking routes
would disappear when ipv6 forwarding was enabled for an interface.

This is a fairly undocumented side-effect of the "accept_ra" sysctl
for an interface.  1 means the interface will accept_ra's if not
forwarding, 2 means always accept_RAs; but it is not explained that
enabling forwarding when accept_ra==1 will also clear any kernel RA
assigned routes, very likely breaking your networking.

The check to warn about this currently uses netlink to go through all
the routes and then look at the accept_ra status of the interfaces.

However, it has been noticed that this problem does not affect systems
where IPv6 RA configuration is handled in userspace, e.g. via tools
such as NetworkManager.  In this case, the error message from libvirt
is spurious, and modifying the forwarding state will not affect the RA
state or disable your networking.

If you refer to the function rt6_purge_dflt_routers() in the kernel,
we can see that the routes being purged are only those with the
kernel's RTF_ADDRCONF flag set; that is, routes added by the kernel's
RA handling.  Why does it do this?  I think this is a Linux
implementation decision; it has always been like that and there are
some comments suggesting that it is because a router should be
statically configured, rather than accepting external configurations.

The solution implemented here is to convert the existing check into a
walk of /proc/net/ipv6_route and look for routes with this flag set.
We then check the accept_ra status for the interface, and if enabling
forwarding would break things raise an error.

This should hopefully avoid "interactive" users, who are likely to be
using NetworkManager and the like, having false warnings when enabling
IPv6, but retain the error check for users relying on kernel-based
IPv6 interface auto-configuration.

Signed-off-by: Ian Wienand <iwienand redhat com>
---


Cedric - I saw you've given your ACK; have you been able to try this code on a system that actually handles RA in the kernel, and gotten the error message? Every system I have readily available to test on does RA in userspace, so I can't reproduce the error condition. If you've actually seen it take effect, I'll push this with both our ACKs, otherwise I guess I'll need to take some time to disable NetworkManager on some machine and try it out.


My comments: Normally I would be against using /proc rather than netlink to get this information, but according to an earlier email exchange we had, apparently the RTF_ADDRCONF flag isn't exposed in netlink. And we also have a precedent in the check for conflicting IPv4 routes in the network driver (networkCheckRouteCollision(), which reads /proc/net/route). (I was actually hoping to get rid of that function in favor of reading the routing table via netlink as Cedric's code does, and still I still might, but in this case it sounds like we'll have to continue to use /proc.


So,


Reviewed-by: Laine Stump <laine redhat com>


(pending successful completion of CI (see below) and verification that the error is triggered when appropriate. Once I've verified those two, I'll push it).


Thanks for taking the time to dig down on this problem (which has been nagging at me for awhile now) and figuring out a workable solution!



  src/util/virnetdevip.c | 323 ++++++++++++++++-------------------------
  1 file changed, 124 insertions(+), 199 deletions(-)

diff --git a/src/util/virnetdevip.c b/src/util/virnetdevip.c
index 7bd5a75f85..e208089bd6 100644
--- a/src/util/virnetdevip.c
+++ b/src/util/virnetdevip.c
@@ -43,8 +43,11 @@
  #ifdef __linux__
  # include <linux/sockios.h>
  # include <linux/if_vlan.h>
+# include <linux/ipv6_route.h>
  #endif
+#define PROC_NET_IPV6_ROUTE "/proc/net/ipv6_route"
+
  #define VIR_FROM_THIS VIR_FROM_NONE
VIR_LOG_INIT("util.netdevip");
@@ -370,203 +373,6 @@ virNetDevIPRouteAdd(const char *ifname,
  }
-static int
-virNetDevIPGetAcceptRA(const char *ifname)
-{
-    g_autofree char *path = NULL;
-    g_autofree char *buf = NULL;
-    char *suffix;
-    int accept_ra = -1;
-
-    path = g_strdup_printf("/proc/sys/net/ipv6/conf/%s/accept_ra",
-                           ifname ? ifname : "all");
-
-    if ((virFileReadAll(path, 512, &buf) < 0) ||
-        (virStrToLong_i(buf, &suffix, 10, &accept_ra) < 0))
-        return -1;
-
-    return accept_ra;
-}
-
-struct virNetDevIPCheckIPv6ForwardingData {
-    bool hasRARoutes;
-
-    /* Devices with conflicting accept_ra */
-    char **devices;
-    size_t ndevices;
-};
-
-
-static int
-virNetDevIPCheckIPv6ForwardingAddIF(struct virNetDevIPCheckIPv6ForwardingData *data,
-                                    char **ifname)
-{
-    size_t i;
-
-    /* add ifname to the array if it's not already there
-     * (ifname is char** so VIR_APPEND_ELEMENT() will move the
-     * original pointer out of the way and avoid having it freed)
-     */
-    for (i = 0; i < data->ndevices; i++) {
-        if (STREQ(data->devices[i], *ifname))
-            return 0;
-    }
-    return VIR_APPEND_ELEMENT(data->devices, data->ndevices, *ifname);
-}
-
-
-static int
-virNetDevIPCheckIPv6ForwardingCallback(struct nlmsghdr *resp,
-                                       void *opaque)
-{
-    struct rtmsg *rtmsg = NLMSG_DATA(resp);
-    struct virNetDevIPCheckIPv6ForwardingData *data = opaque;
-    struct rtattr *rta_attr;
-    int accept_ra = -1;
-    int ifindex = -1;
-    g_autofree char *ifname = NULL;
-
-    /* Ignore messages other than route ones */
-    if (resp->nlmsg_type != RTM_NEWROUTE)
-        return 0;
-
-    /* No need to do anything else for non RA routes */
-    if (rtmsg->rtm_protocol != RTPROT_RA)
-        return 0;
-
-    rta_attr = (struct rtattr *)nlmsg_find_attr(resp, sizeof(struct rtmsg), RTA_OIF);
-    if (rta_attr) {
-        /* This is a single path route, with interface used to reach
-         * nexthop in the RTA_OIF attribute.
-         */
-        ifindex = *(int *)RTA_DATA(rta_attr);
-        ifname = virNetDevGetName(ifindex);
-
-        if (!ifname)
-           return -1;
-
-        accept_ra = virNetDevIPGetAcceptRA(ifname);
-
-        VIR_DEBUG("Checking route for device %s (%d), accept_ra: %d",
-                  ifname, ifindex, accept_ra);
-
-        if (accept_ra != 2 && virNetDevIPCheckIPv6ForwardingAddIF(data, &ifname) < 0)
-            return -1;
-
-        data->hasRARoutes = true;
-        return 0;
-    }
-
-    /* if no RTA_OIF was found, see if this is a multipath route (one
-     * which has an array of nexthops, each with its own interface)
-     */
-
-    rta_attr = (struct rtattr *)nlmsg_find_attr(resp, sizeof(struct rtmsg), RTA_MULTIPATH);
-    if (rta_attr) {
-        /* The data of the attribute is an array of rtnexthop */
-        struct rtnexthop *nh = RTA_DATA(rta_attr);
-        size_t len = RTA_PAYLOAD(rta_attr);
-
-        /* validate the attribute array length */
-        len = MIN(len, ((char *)resp + NLMSG_PAYLOAD(resp, 0) - (char *)rta_attr));
-
-        while (len >= sizeof(*nh) && len >= nh->rtnh_len) {
-            /* check accept_ra for the interface of each nexthop */
-
-            ifname = virNetDevGetName(nh->rtnh_ifindex);
-
-            if (!ifname)
-                return -1;
-
-            accept_ra = virNetDevIPGetAcceptRA(ifname);
-
-            VIR_DEBUG("Checking multipath route nexthop device %s (%d), accept_ra: %d",
-                      ifname, nh->rtnh_ifindex, accept_ra);
-
-            if (accept_ra != 2 && virNetDevIPCheckIPv6ForwardingAddIF(data, &ifname) < 0)
-                return -1;
-
-            VIR_FREE(ifname);
-            data->hasRARoutes = true;
-
-            len -= NLMSG_ALIGN(nh->rtnh_len);
-            VIR_WARNINGS_NO_CAST_ALIGN
-            nh = RTNH_NEXT(nh);
-            VIR_WARNINGS_RESET
-        }
-    }
-
-    return 0;
-}
-
-bool
-virNetDevIPCheckIPv6Forwarding(void)
-{
-    bool valid = false;
-    struct rtgenmsg genmsg;
-    size_t i;
-    struct virNetDevIPCheckIPv6ForwardingData data = {
-        .hasRARoutes = false,
-        .devices = NULL,
-        .ndevices = 0
-    };
-    g_autoptr(virNetlinkMsg) nlmsg = NULL;
-
-
-    /* Prepare the request message */
-    if (!(nlmsg = nlmsg_alloc_simple(RTM_GETROUTE,
-                                     NLM_F_REQUEST | NLM_F_DUMP))) {
-        virReportOOMError();
-        goto cleanup;
-    }
-
-    memset(&genmsg, 0, sizeof(genmsg));
-    genmsg.rtgen_family = AF_INET6;
-
-    if (nlmsg_append(nlmsg, &genmsg, sizeof(genmsg), NLMSG_ALIGNTO) < 0) {
-        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                       _("allocated netlink buffer is too small"));
-        goto cleanup;
-    }
-
-    /* Send the request and loop over the responses */
-    if (virNetlinkDumpCommand(nlmsg, virNetDevIPCheckIPv6ForwardingCallback,
-                              0, 0, NETLINK_ROUTE, 0, &data) < 0) {
-        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                       _("Failed to loop over IPv6 routes"));
-        goto cleanup;
-    }
-
-    valid = !data.hasRARoutes || data.ndevices == 0;
-
-    /* Check the global accept_ra if at least one isn't set on a
-       per-device basis */
-    if (!valid && data.hasRARoutes) {
-        int accept_ra = virNetDevIPGetAcceptRA(NULL);
-        valid = accept_ra == 2;
-        VIR_DEBUG("Checked global accept_ra: %d", accept_ra);
-    }
-
-    if (!valid) {
-        g_auto(virBuffer) buf = VIR_BUFFER_INITIALIZER;
-        for (i = 0; i < data.ndevices; i++) {
-            virBufferAdd(&buf, data.devices[i], -1);
-            if (i < data.ndevices - 1)
-                virBufferAddLit(&buf, ", ");
-        }
-
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("Check the host setup: enabling IPv6 forwarding with "
-                         "RA routes without accept_ra set to 2 is likely to cause "
-                         "routes loss. Interfaces to look at: %s"),
-                       virBufferCurrentContent(&buf));
-    }
-
- cleanup:
-    virStringListFreeCount(data.devices, data.ndevices);
-    return valid;
-}
-
  #else /* defined(__linux__) && defined(WITH_LIBNL) */
@@ -688,15 +494,134 @@ virNetDevIPRouteAdd(const char *ifname,
      return 0;
  }
+#endif /* defined(__linux__) && defined(HAVE_LIBNL) */
+
+
+#if defined(__linux__)
+
+static int
+virNetDevIPGetAcceptRA(const char *ifname)
+{
+    g_autofree char *path = NULL;
+    g_autofree char *buf = NULL;
+    char *suffix;
+    int accept_ra = -1;
+
+    path = g_strdup_printf("/proc/sys/net/ipv6/conf/%s/accept_ra",
+                           ifname ? ifname : "all");
+
+    if ((virFileReadAll(path, 512, &buf) < 0) ||
+        (virStrToLong_i(buf, &suffix, 10, &accept_ra) < 0))
+        return -1;
+
+    return accept_ra;
+}
+
+/**
+ * virNetDevIPCheckIPv6Forwarding
+ *
+ * This function checks if IPv6 routes have the RTF_ADDRCONF flag set,
+ * indicating they have been created by the kernel's RA configuration
+ * handling.  These routes are subject to being flushed when ipv6
+ * forwarding is enabled unless accept_ra is explicitly set to "2".
+ * This will most likely result in ipv6 networking being broken.
+ *
+ * Returns: true if it is safe to enable forwarding, or false if
+ *          breakable routes are found.
+ *
+ **/
+bool
+virNetDevIPCheckIPv6Forwarding(void)
+{
+    int len;
+    char *cur;
+    g_autofree char *buf = NULL;
+    /* lines are 150 chars */
+    enum {MAX_ROUTE_SIZE = 150*100000};
+
+    /* This is /proc/sys/net/ipv6/conf/all/accept_ra */
+    int all_accept_ra = virNetDevIPGetAcceptRA(NULL);
+
+    /* Read ipv6 routes */
+    if ((len = virFileReadAll(PROC_NET_IPV6_ROUTE,
+                              MAX_ROUTE_SIZE, &buf)) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Unable to read %s "
+                         "for ipv6 forwarding checks"), PROC_NET_IPV6_ROUTE);
+        return false;
+    }
+
+    /* VIR_DEBUG("%s output:\n%s", PROC_NET_IPV6_ROUTE, buf); */
+
+    /* Dropping the last character to stop the loop */
+    if (len > 0)
+        buf[len-1] = '\0';
+
+    cur = buf;
+    while (cur) {
+        char route[33], flags[9], iface[9];
+        unsigned int flags_val;
+        char *iface_val;
+        int num;
+        char *nl = strchr(cur, '\n');
+
+        if (nl)
+            *nl++ = '\0';
+
+        num = sscanf(cur, "%32s %*s %*s %*s %*s %*s %*s %*s %8s %8s",
+                     route, flags, iface);
+
+        cur = nl;
+        if (num != 3) {
+            VIR_DEBUG("Failed to parse route line: %s", cur);
+            continue;
+        }
+
+        if (virStrToLong_ui(flags, NULL, 16, &flags_val)) {
+            VIR_DEBUG("Failed to parse flags: %s", flags);
+            continue;
+        }
+
+        /* This is right justified, strip leading spaces */
+        iface_val = &iface[0];
+        while (*iface_val && g_ascii_isspace(*iface_val))
+            iface_val++;
+
+        VIR_DEBUG("%s iface %s flags %s : RTF_ADDRCONF %sset",
+                  route, iface_val, flags,
+                  (flags_val & RTF_ADDRCONF ? "" : "not "));
+
+        if (flags_val & RTF_ADDRCONF) {
+            int ret = virNetDevIPGetAcceptRA(iface_val);
+            VIR_DEBUG("%s reports accept_ra of %d",
+                      iface_val, ret);
+            /* If the interface for this autoconfigured route
+             * has accept_ra == 1, or it is default and the "all"
+             * value of accept_ra == 1, it will be subject to
+             * flushing if forwarding is enabled.
+             */
+            if (ret == 1 || (ret == 0 && all_accept_ra == 1)) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Check the host setup: interface %s has kernel "
+                                 "autoconfigured IPv6 routes and enabling forwarding "
+                                 " without accept_ra set to 2 will cause the kernel "
+                                 "to flush them, breaking networking."), iface_val);
+                return false;
+            }
+        }
+    }
+    return true;
+}
+#else
bool
  virNetDevIPCheckIPv6Forwarding(void)
  {
-    VIR_WARN("built without libnl: unable to check if IPv6 forwarding can be safely enabled");
+    VIR_DEBUG("No checks for IPv6 forwarding issues on non-Linux systems", flags);


You've got an extra arg there, which won't cause any problems until the CI builds run for a non-linux platform. I'll remove it before pushing (I've just now pushed it to a branch on a private libvirt fork on gitlab, which will cause all the normal upstream CI to run without needing to push it to upstream master first; I'll incorporate any other similar fixes I might find).


      return true;
  }
-#endif /* defined(__linux__) && defined(WITH_LIBNL) */
+#endif /* defined(__linux__) */
/**



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