netfilter: xtables: make ip_tables reentrant

Currently, the table traverser stores return addresses in the ruleset
itself (struct ip6t_entry->comefrom). This has a well-known drawback:
the jumpstack is overwritten on reentry, making it necessary for
targets to return absolute verdicts. Also, the ruleset (which might
be heavy memory-wise) needs to be replicated for each CPU that can
possibly invoke ip6t_do_table.

This patch decouples the jumpstack from struct ip6t_entry and instead
puts it into xt_table_info. Not being restricted by 'comefrom'
anymore, we can set up a stack as needed. By default, there is room
allocated for two entries into the traverser.

arp_tables is not touched though, because there is just one/two
modules and further patches seek to collapse the table traverser
anyhow.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>

2 files changed, 39 insertions, 32 deletions

diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
index e8e363d90365..07a699059390 100644
--- a/net/ipv4/netfilter/arp_tables.c
+++ b/net/ipv4/netfilter/arp_tables.c
@@ -649,6 +649,9 @@ static int translate_table(struct xt_table_info *newinfo, void *entry0,
                 if (ret != 0)
                         break;
                 ++i;
+                if (strcmp(arpt_get_target(iter)->u.user.name,
+                    XT_ERROR_TARGET) == 0)
+                        ++newinfo->stacksize;
         }
         duprintf("translate_table: ARPT_ENTRY_ITERATE gives %d\n", ret);
         if (ret != 0)
@@ -1774,8 +1777,7 @@ struct xt_table *arpt_register_table(struct net *net,
 {
         int ret;
         struct xt_table_info *newinfo;
-        struct xt_table_info bootstrap
-                = { 0, 0, 0, { 0 }, { 0 }, { } };
+        struct xt_table_info bootstrap = {0};
         void *loc_cpu_entry;
         struct xt_table *new_table;
 
diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index 18c5b1573f3e..70900ecf88e2 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -321,8 +321,6 @@ ipt_do_table(struct sk_buff *skb,
              const struct net_device *out,
              struct xt_table *table)
 {
-#define tb_comefrom ((struct ipt_entry *)table_base)->comefrom
-
         static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long))));
         const struct iphdr *ip;
         bool hotdrop = false;
@@ -330,7 +328,8 @@ ipt_do_table(struct sk_buff *skb,
         unsigned int verdict = NF_DROP;
         const char *indev, *outdev;
         const void *table_base;
-        struct ipt_entry *e, *back;
+        struct ipt_entry *e, **jumpstack;
+        unsigned int *stackptr, origptr, cpu;
         const struct xt_table_info *private;
         struct xt_match_param mtpar;
         struct xt_target_param tgpar;
@@ -356,19 +355,23 @@ ipt_do_table(struct sk_buff *skb,
         IP_NF_ASSERT(table->valid_hooks & (1 << hook));
         xt_info_rdlock_bh();
         private = table->private;
-        table_base = private->entries[smp_processor_id()];
+        cpu        = smp_processor_id();
+        table_base = private->entries[cpu];
+        jumpstack  = (struct ipt_entry **)private->jumpstack[cpu];
+        stackptr   = &private->stackptr[cpu];
+        origptr    = *stackptr;
 
         e = get_entry(table_base, private->hook_entry[hook]);
 
-        /* For return from builtin chain */
-        back = get_entry(table_base, private->underflow[hook]);
+        pr_devel("Entering %s(hook %u); sp at %u (UF %p)\n",
+                 table->name, hook, origptr,
+                 get_entry(table_base, private->underflow[hook]));
 
         do {
                 const struct ipt_entry_target *t;
                 const struct xt_entry_match *ematch;
 
                 IP_NF_ASSERT(e);
-                IP_NF_ASSERT(back);
                 if (!ip_packet_match(ip, indev, outdev,
                     &e->ip, mtpar.fragoff)) {
  no_match:
@@ -403,17 +406,28 @@ ipt_do_table(struct sk_buff *skb,
                                         verdict = (unsigned)(-v) - 1;
                                         break;
                                 }
-                                e = back;
-                                back = get_entry(table_base, back->comefrom);
+                                if (*stackptr == 0) {
+                                        e = get_entry(table_base,
+                                            private->underflow[hook]);
+                                        pr_devel("Underflow (this is normal) "
+                                                 "to %p\n", e);
+                                } else {
+                                        e = jumpstack[--*stackptr];
+                                        pr_devel("Pulled %p out from pos %u\n",
+                                                 e, *stackptr);
+                                        e = ipt_next_entry(e);
+                                }
                                 continue;
                         }
                         if (table_base + v != ipt_next_entry(e) &&
                             !(e->ip.flags & IPT_F_GOTO)) {
-                                /* Save old back ptr in next entry */
-                                struct ipt_entry *next = ipt_next_entry(e);
-                                next->comefrom = (void *)back - table_base;
-                                /* set back pointer to next entry */
-                                back = next;
+                                if (*stackptr >= private->stacksize) {
+                                        verdict = NF_DROP;
+                                        break;
+                                }
+                                jumpstack[(*stackptr)++] = e;
+                                pr_devel("Pushed %p into pos %u\n",
+                                         e, *stackptr - 1);
                         }
 
                         e = get_entry(table_base, v);
@@ -426,18 +440,7 @@ ipt_do_table(struct sk_buff *skb,
                 tgpar.targinfo = t->data;
 
 
-#ifdef CONFIG_NETFILTER_DEBUG
-                tb_comefrom = 0xeeeeeeec;
-#endif
                 verdict = t->u.kernel.target->target(skb, &tgpar);
-#ifdef CONFIG_NETFILTER_DEBUG
-                if (tb_comefrom != 0xeeeeeeec && verdict == IPT_CONTINUE) {
-                        printk("Target %s reentered!\n",
-                               t->u.kernel.target->name);
-                        verdict = NF_DROP;
-                }
-                tb_comefrom = 0x57acc001;
-#endif
                 /* Target might have changed stuff. */
                 ip = ip_hdr(skb);
                 if (verdict == IPT_CONTINUE)
@@ -447,7 +450,9 @@ ipt_do_table(struct sk_buff *skb,
                         break;
         } while (!hotdrop);
         xt_info_rdunlock_bh();
-
+        pr_devel("Exiting %s; resetting sp from %u to %u\n",
+                 __func__, *stackptr, origptr);
+        *stackptr = origptr;
 #ifdef DEBUG_ALLOW_ALL
         return NF_ACCEPT;
 #else
@@ -455,8 +460,6 @@ ipt_do_table(struct sk_buff *skb,
                 return NF_DROP;
         else return verdict;
 #endif
-
-#undef tb_comefrom
 }
 
 /* Figures out from what hook each rule can be called: returns 0 if
@@ -838,6 +841,9 @@ translate_table(struct net *net, struct xt_table_info *newinfo, void *entry0,
                 if (ret != 0)
                         return ret;
                 ++i;
+                if (strcmp(ipt_get_target(iter)->u.user.name,
+                    XT_ERROR_TARGET) == 0)
+                        ++newinfo->stacksize;
         }
 
         if (i != repl->num_entries) {
@@ -2086,8 +2092,7 @@ struct xt_table *ipt_register_table(struct net *net,
 {
         int ret;
         struct xt_table_info *newinfo;
-        struct xt_table_info bootstrap
-                = { 0, 0, 0, { 0 }, { 0 }, { } };
+        struct xt_table_info bootstrap = {0};
         void *loc_cpu_entry;
         struct xt_table *new_table;