Re: [PATCH] nwfilter: Avoid firewall hole during VM startup by checking rule presence

Upon VM bootstrapping (start,restore,incoming migration) iptablesCreateBaseChainsFW is called and unconditionally deletes and reinserts top-level firewall chain jumps (e.g. INPUT, FORWARD rules). This briefly opens a hole in the firewall, allowing packets through until the insertions complete. This commit ensures that the base chains are only created once per layer (IPV4/IPV6) and checks whether the expected rules already exist using `iptables -C`. If they do, no delete/insert operations are performed. This eliminates the short window where packets could bypass filters during VM lifecycle operations. Signed-off-by: Dion Bosschieter <dionbosschieter(a)gmail.com&gt; --- src/nwfilter/nwfilter_ebiptables_driver.c | 79 ++++++++++++++--------- 1 file changed, 47 insertions(+), 32 deletions(-) diff --git a/src/nwfilter/nwfilter_ebiptables_driver.c b/src/nwfilter/nwfilter_ebiptables_driver.c index 067df6e612..42a0133159 100644 --- a/src/nwfilter/nwfilter_ebiptables_driver.c +++ b/src/nwfilter/nwfilter_ebiptables_driver.c @@ -131,6 +131,14 @@ static char chainprefixes_host_temp[3] = { 0 }; +typedef struct { + const char *chain; + const char *position; + const char *targetChain; +} iptablesBaseChainFW; + +static bool baseChainFWDefined[VIR_FIREWALL_LAYER_LAST] = { false }; + static int printVar(virNWFilterVarCombIter *vars, char *buf, int bufsize, @@ -403,38 +411,45 @@ static void iptablesCreateBaseChainsFW(virFirewall *fw, virFirewallLayer layer) { - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-N", VIRT_IN_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-N", VIRT_OUT_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-N", VIRT_IN_POST_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-N", HOST_IN_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-D", "FORWARD", "-j", VIRT_IN_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-D", "FORWARD", "-j", VIRT_OUT_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-D", "FORWARD", "-j", VIRT_IN_POST_CHAIN, NULL); - virFirewallAddCmdFull(fw, layer, - true, NULL, NULL, - "-D", "INPUT", "-j", HOST_IN_CHAIN, NULL); - virFirewallAddCmd(fw, layer, - "-I", "FORWARD", "1", "-j", VIRT_IN_CHAIN, NULL); - virFirewallAddCmd(fw, layer, - "-I", "FORWARD", "2", "-j", VIRT_OUT_CHAIN, NULL); - virFirewallAddCmd(fw, layer, - "-I", "FORWARD", "3", "-j", VIRT_IN_POST_CHAIN, NULL); - virFirewallAddCmd(fw, layer, - "-I", "INPUT", "1", "-j", HOST_IN_CHAIN, NULL); + iptablesBaseChainFW fw_chains[] = { + {"FORWARD", "1", VIRT_IN_CHAIN}, + {"FORWARD", "2", VIRT_OUT_CHAIN}, + {"FORWARD", "3", VIRT_IN_POST_CHAIN}, + {"INPUT", "1", HOST_IN_CHAIN}, + }; + size_t i; + + // iptablesCreateBaseChainsFW already ran once for this layer, + // we don't have to recreate the base chains on every firewall update + if (baseChainFWDefined[layer]) + return; + + // set defined state so we skip the following logic next run + baseChainFWDefined[layer] = true; + + virFirewallStartTransaction(fw, 0); + + for (i = 0; i < G_N_ELEMENTS(fw_chains); i++) + virFirewallAddCmd(fw, layer, + "-C", fw_chains[i].chain, + "-j", fw_chains[i].targetChain, NULL); + + if (virFirewallApply(fw) == 0) + // rules already in place + return; + + for (i = 0; i < G_N_ELEMENTS(fw_chains); i++) { + virFirewallAddCmdFull(fw, layer, + true, NULL, NULL, + "-N", fw_chains[i].targetChain, NULL); + virFirewallAddCmdFull(fw, layer, + true, NULL, NULL, + "-D", fw_chains[i].chain, "-j", + fw_chains[i].targetChain, NULL); + virFirewallAddCmd(fw, layer, + "-I", fw_chains[i].chain, fw_chains[i].position, + "-j", fw_chains[i].targetChain, NULL); + } }