net/netfilter/xt_limit.c

   1 /* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
   2  * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  */
   8
   9 #include <linux/module.h>
  10 #include <linux/skbuff.h>
  11 #include <linux/spinlock.h>
  12 #include <linux/interrupt.h>
  13
  14 #include <linux/netfilter/x_tables.h>
  15 #include <linux/netfilter/xt_limit.h>
  16
  17 MODULE_LICENSE("GPL");
  18 MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
  19 MODULE_DESCRIPTION("Xtables: rate-limit match");
  20 MODULE_ALIAS("ipt_limit");
  21 MODULE_ALIAS("ip6t_limit");
  22
  23 /* The algorithm used is the Simple Token Bucket Filter (TBF)
  24  * see net/sched/sch_tbf.c in the linux source tree
  25  */
  26
  27 static DEFINE_SPINLOCK(limit_lock);
  28
  29 /* Rusty: This is my (non-mathematically-inclined) understanding of
  30    this algorithm.  The `average rate' in jiffies becomes your initial
  31    amount of credit `credit' and the most credit you can ever have
  32    `credit_cap'.  The `peak rate' becomes the cost of passing the
  33    test, `cost'.
  34
  35    `prev' tracks the last packet hit: you gain one credit per jiffy.
  36    If you get credit balance more than this, the extra credit is
  37    discarded.  Every time the match passes, you lose `cost' credits;
  38    if you don't have that many, the test fails.
  39
  40    See Alexey's formal explanation in net/sched/sch_tbf.c.
  41
  42    To get the maxmum range, we multiply by this factor (ie. you get N
  43    credits per jiffy).  We want to allow a rate as low as 1 per day
  44    (slowest userspace tool allows), which means
  45    CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  46 #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  47
  48 /* Repeated shift and or gives us all 1s, final shift and add 1 gives
  49  * us the power of 2 below the theoretical max, so GCC simply does a
  50  * shift. */
  51 #define _POW2_BELOW2(x) ((x)|((x)>>1))
  52 #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  53 #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  54 #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  55 #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  56 #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  57
  58 #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
  59
  60 static bool
  61 limit_mt(const struct sk_buff *skb, const struct xt_match_param *par)
  62 {
  63         struct xt_rateinfo *r =
  64                 ((const struct xt_rateinfo *)par->matchinfo)->master;
  65         unsigned long now = jiffies;
  66
  67         spin_lock_bh(&limit_lock);
  68         r->credit += (now - xchg(&r->prev, now)) * CREDITS_PER_JIFFY;
  69         if (r->credit > r->credit_cap)
  70                 r->credit = r->credit_cap;
  71
  72         if (r->credit >= r->cost) {
  73                 /* We're not limited. */
  74                 r->credit -= r->cost;
  75                 spin_unlock_bh(&limit_lock);
  76                 return true;
  77         }
  78
  79         spin_unlock_bh(&limit_lock);
  80         return false;
  81 }
  82
  83 /* Precision saver. */
  84 static u_int32_t
  85 user2credits(u_int32_t user)
  86 {
  87         /* If multiplying would overflow... */
  88         if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
  89                 /* Divide first. */
  90                 return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
  91
  92         return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
  93 }
  94
  95 static bool
  96 limit_mt_check(const char *tablename, const void *inf,
  97                const struct xt_match *match, void *matchinfo,
  98                unsigned int hook_mask)
  99 {
 100         struct xt_rateinfo *r = matchinfo;
 101
 102         /* Check for overflow. */
 103         if (r->burst == 0
 104             || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
 105                 printk("Overflow in xt_limit, try lower: %u/%u\n",
 106                        r->avg, r->burst);
 107                 return false;
 108         }
 109
 110         /* For SMP, we only want to use one set of counters. */
 111         r->master = r;
 112         if (r->cost == 0) {
 113                 /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
 114                    128. */
 115                 r->prev = jiffies;
 116                 r->credit = user2credits(r->avg * r->burst);     /* Credits full. */
 117                 r->credit_cap = user2credits(r->avg * r->burst); /* Credits full. */
 118                 r->cost = user2credits(r->avg);
 119         }
 120         return true;
 121 }
 122
 123 #ifdef CONFIG_COMPAT
 124 struct compat_xt_rateinfo {
 125         u_int32_t avg;
 126         u_int32_t burst;
 127
 128         compat_ulong_t prev;
 129         u_int32_t credit;
 130         u_int32_t credit_cap, cost;
 131
 132         u_int32_t master;
 133 };
 134
 135 /* To keep the full "prev" timestamp, the upper 32 bits are stored in the
 136  * master pointer, which does not need to be preserved. */
 137 static void limit_mt_compat_from_user(void *dst, void *src)
 138 {
 139         const struct compat_xt_rateinfo *cm = src;
 140         struct xt_rateinfo m = {
 141                 .avg            = cm->avg,
 142                 .burst          = cm->burst,
 143                 .prev           = cm->prev | (unsigned long)cm->master << 32,
 144                 .credit         = cm->credit,
 145                 .credit_cap     = cm->credit_cap,
 146                 .cost           = cm->cost,
 147         };
 148         memcpy(dst, &m, sizeof(m));
 149 }
 150
 151 static int limit_mt_compat_to_user(void __user *dst, void *src)
 152 {
 153         const struct xt_rateinfo *m = src;
 154         struct compat_xt_rateinfo cm = {
 155                 .avg            = m->avg,
 156                 .burst          = m->burst,
 157                 .prev           = m->prev,
 158                 .credit         = m->credit,
 159                 .credit_cap     = m->credit_cap,
 160                 .cost           = m->cost,
 161                 .master         = m->prev >> 32,
 162         };
 163         return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
 164 }
 165 #endif /* CONFIG_COMPAT */
 166
 167 static struct xt_match limit_mt_reg __read_mostly = {
 168         .name             = "limit",
 169         .revision         = 0,
 170         .family           = NFPROTO_UNSPEC,
 171         .match            = limit_mt,
 172         .checkentry       = limit_mt_check,
 173         .matchsize        = sizeof(struct xt_rateinfo),
 174 #ifdef CONFIG_COMPAT
 175         .compatsize       = sizeof(struct compat_xt_rateinfo),
 176         .compat_from_user = limit_mt_compat_from_user,
 177         .compat_to_user   = limit_mt_compat_to_user,
 178 #endif
 179         .me               = THIS_MODULE,
 180 };
 181
 182 static int __init limit_mt_init(void)
 183 {
 184         return xt_register_match(&limit_mt_reg);
 185 }
 186
 187 static void __exit limit_mt_exit(void)
 188 {
 189         xt_unregister_match(&limit_mt_reg);
 190 }
 191
 192 module_init(limit_mt_init);
 193 module_exit(limit_mt_exit);