net/netfilter/xt_limit.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
   3  * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>
   4  * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
   5  */
   6 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   7
   8 #include <linux/slab.h>
   9 #include <linux/module.h>
  10 #include <linux/skbuff.h>
  11 #include <linux/interrupt.h>
  12
  13 #include <linux/netfilter/x_tables.h>
  14 #include <linux/netfilter/xt_limit.h>
  15
  16 struct xt_limit_priv {
  17         unsigned long prev;
  18         u32 credit;
  19 };
  20
  21 MODULE_LICENSE("GPL");
  22 MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
  23 MODULE_DESCRIPTION("Xtables: rate-limit match");
  24 MODULE_ALIAS("ipt_limit");
  25 MODULE_ALIAS("ip6t_limit");
  26
  27 /* The algorithm used is the Simple Token Bucket Filter (TBF)
  28  * see net/sched/sch_tbf.c in the linux source tree
  29  */
  30
  31 /* Rusty: This is my (non-mathematically-inclined) understanding of
  32    this algorithm.  The `average rate' in jiffies becomes your initial
  33    amount of credit `credit' and the most credit you can ever have
  34    `credit_cap'.  The `peak rate' becomes the cost of passing the
  35    test, `cost'.
  36
  37    `prev' tracks the last packet hit: you gain one credit per jiffy.
  38    If you get credit balance more than this, the extra credit is
  39    discarded.  Every time the match passes, you lose `cost' credits;
  40    if you don't have that many, the test fails.
  41
  42    See Alexey's formal explanation in net/sched/sch_tbf.c.
  43
  44    To get the maximum range, we multiply by this factor (ie. you get N
  45    credits per jiffy).  We want to allow a rate as low as 1 per day
  46    (slowest userspace tool allows), which means
  47    CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  48 #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  49
  50 /* Repeated shift and or gives us all 1s, final shift and add 1 gives
  51  * us the power of 2 below the theoretical max, so GCC simply does a
  52  * shift. */
  53 #define _POW2_BELOW2(x) ((x)|((x)>>1))
  54 #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  55 #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  56 #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  57 #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  58 #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  59
  60 #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
  61
  62 static bool
  63 limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
  64 {
  65         const struct xt_rateinfo *r = par->matchinfo;
  66         struct xt_limit_priv *priv = r->master;
  67         unsigned long now;
  68         u32 old_credit, new_credit, credit_increase = 0;
  69         bool ret;
  70
  71         /* fastpath if there is nothing to update */
  72         if ((READ_ONCE(priv->credit) < r->cost) && (READ_ONCE(priv->prev) == jiffies))
  73                 return false;
  74
  75         do {
  76                 now = jiffies;
  77                 credit_increase += (now - xchg(&priv->prev, now)) * CREDITS_PER_JIFFY;
  78                 old_credit = READ_ONCE(priv->credit);
  79                 new_credit = old_credit;
  80                 new_credit += credit_increase;
  81                 if (new_credit > r->credit_cap)
  82                         new_credit = r->credit_cap;
  83                 if (new_credit >= r->cost) {
  84                         ret = true;
  85                         new_credit -= r->cost;
  86                 } else {
  87                         ret = false;
  88                 }
  89         } while (cmpxchg(&priv->credit, old_credit, new_credit) != old_credit);
  90
  91         return ret;
  92 }
  93
  94 /* Precision saver. */
  95 static u32 user2credits(u32 user)
  96 {
  97         /* If multiplying would overflow... */
  98         if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
  99                 /* Divide first. */
 100                 return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
 101
 102         return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
 103 }
 104
 105 static int limit_mt_check(const struct xt_mtchk_param *par)
 106 {
 107         struct xt_rateinfo *r = par->matchinfo;
 108         struct xt_limit_priv *priv;
 109
 110         /* Check for overflow. */
 111         if (r->burst == 0
 112             || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
 113                 pr_info_ratelimited("Overflow, try lower: %u/%u\n",
 114                                     r->avg, r->burst);
 115                 return -ERANGE;
 116         }
 117
 118         priv = kmalloc(sizeof(*priv), GFP_KERNEL);
 119         if (priv == NULL)
 120                 return -ENOMEM;
 121
 122         /* For SMP, we only want to use one set of state. */
 123         r->master = priv;
 124         /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
 125            128. */
 126         priv->prev = jiffies;
 127         priv->credit = user2credits(r->avg * r->burst); /* Credits full. */
 128         if (r->cost == 0) {
 129                 r->credit_cap = priv->credit; /* Credits full. */
 130                 r->cost = user2credits(r->avg);
 131         }
 132
 133         return 0;
 134 }
 135
 136 static void limit_mt_destroy(const struct xt_mtdtor_param *par)
 137 {
 138         const struct xt_rateinfo *info = par->matchinfo;
 139
 140         kfree(info->master);
 141 }
 142
 143 #ifdef CONFIG_NETFILTER_XTABLES_COMPAT
 144 struct compat_xt_rateinfo {
 145         u_int32_t avg;
 146         u_int32_t burst;
 147
 148         compat_ulong_t prev;
 149         u_int32_t credit;
 150         u_int32_t credit_cap, cost;
 151
 152         u_int32_t master;
 153 };
 154
 155 /* To keep the full "prev" timestamp, the upper 32 bits are stored in the
 156  * master pointer, which does not need to be preserved. */
 157 static void limit_mt_compat_from_user(void *dst, const void *src)
 158 {
 159         const struct compat_xt_rateinfo *cm = src;
 160         struct xt_rateinfo m = {
 161                 .avg            = cm->avg,
 162                 .burst          = cm->burst,
 163                 .prev           = cm->prev | (unsigned long)cm->master << 32,
 164                 .credit         = cm->credit,
 165                 .credit_cap     = cm->credit_cap,
 166                 .cost           = cm->cost,
 167         };
 168         memcpy(dst, &m, sizeof(m));
 169 }
 170
 171 static int limit_mt_compat_to_user(void __user *dst, const void *src)
 172 {
 173         const struct xt_rateinfo *m = src;
 174         struct compat_xt_rateinfo cm = {
 175                 .avg            = m->avg,
 176                 .burst          = m->burst,
 177                 .prev           = m->prev,
 178                 .credit         = m->credit,
 179                 .credit_cap     = m->credit_cap,
 180                 .cost           = m->cost,
 181                 .master         = m->prev >> 32,
 182         };
 183         return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
 184 }
 185 #endif /* CONFIG_NETFILTER_XTABLES_COMPAT */
 186
 187 static struct xt_match limit_mt_reg __read_mostly = {
 188         .name             = "limit",
 189         .revision         = 0,
 190         .family           = NFPROTO_UNSPEC,
 191         .match            = limit_mt,
 192         .checkentry       = limit_mt_check,
 193         .destroy          = limit_mt_destroy,
 194         .matchsize        = sizeof(struct xt_rateinfo),
 195 #ifdef CONFIG_NETFILTER_XTABLES_COMPAT
 196         .compatsize       = sizeof(struct compat_xt_rateinfo),
 197         .compat_from_user = limit_mt_compat_from_user,
 198         .compat_to_user   = limit_mt_compat_to_user,
 199 #endif
 200         .usersize         = offsetof(struct xt_rateinfo, prev),
 201         .me               = THIS_MODULE,
 202 };
 203
 204 static int __init limit_mt_init(void)
 205 {
 206         return xt_register_match(&limit_mt_reg);
 207 }
 208
 209 static void __exit limit_mt_exit(void)
 210 {
 211         xt_unregister_match(&limit_mt_reg);
 212 }
 213
 214 module_init(limit_mt_init);
 215 module_exit(limit_mt_exit);