net/netfilter/xt_limit.c

0001 // SPDX-License-Identifier: GPL-2.0-only
0002 /* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
0003  * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>
0004  * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
0005  */
0006 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
0007
0008 #include <linux/slab.h>
0009 #include <linux/module.h>
0010 #include <linux/skbuff.h>
0011 #include <linux/interrupt.h>
0012
0013 #include <linux/netfilter/x_tables.h>
0014 #include <linux/netfilter/xt_limit.h>
0015
0016 struct xt_limit_priv {
0017     unsigned long prev;
0018     u32 credit;
0019 };
0020
0021 MODULE_LICENSE("GPL");
0022 MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
0023 MODULE_DESCRIPTION("Xtables: rate-limit match");
0024 MODULE_ALIAS("ipt_limit");
0025 MODULE_ALIAS("ip6t_limit");
0026
0027 /* The algorithm used is the Simple Token Bucket Filter (TBF)
0028  * see net/sched/sch_tbf.c in the linux source tree
0029  */
0030
0031 /* Rusty: This is my (non-mathematically-inclined) understanding of
0032    this algorithm.  The `average rate' in jiffies becomes your initial
0033    amount of credit `credit' and the most credit you can ever have
0034    `credit_cap'.  The `peak rate' becomes the cost of passing the
0035    test, `cost'.
0036
0037    `prev' tracks the last packet hit: you gain one credit per jiffy.
0038    If you get credit balance more than this, the extra credit is
0039    discarded.  Every time the match passes, you lose `cost' credits;
0040    if you don't have that many, the test fails.
0041
0042    See Alexey's formal explanation in net/sched/sch_tbf.c.
0043
0044    To get the maximum range, we multiply by this factor (ie. you get N
0045    credits per jiffy).  We want to allow a rate as low as 1 per day
0046    (slowest userspace tool allows), which means
0047    CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
0048 #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
0049
0050 /* Repeated shift and or gives us all 1s, final shift and add 1 gives
0051  * us the power of 2 below the theoretical max, so GCC simply does a
0052  * shift. */
0053 #define _POW2_BELOW2(x) ((x)|((x)>>1))
0054 #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
0055 #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
0056 #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
0057 #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
0058 #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
0059
0060 #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
0061
0062 static bool
0063 limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
0064 {
0065     const struct xt_rateinfo *r = par->matchinfo;
0066     struct xt_limit_priv *priv = r->master;
0067     unsigned long now;
0068     u32 old_credit, new_credit, credit_increase = 0;
0069     bool ret;
0070
0071     /* fastpath if there is nothing to update */
0072     if ((READ_ONCE(priv->credit) < r->cost) && (READ_ONCE(priv->prev) == jiffies))
0073         return false;
0074
0075     do {
0076         now = jiffies;
0077         credit_increase += (now - xchg(&priv->prev, now)) * CREDITS_PER_JIFFY;
0078         old_credit = READ_ONCE(priv->credit);
0079         new_credit = old_credit;
0080         new_credit += credit_increase;
0081         if (new_credit > r->credit_cap)
0082             new_credit = r->credit_cap;
0083         if (new_credit >= r->cost) {
0084             ret = true;
0085             new_credit -= r->cost;
0086         } else {
0087             ret = false;
0088         }
0089     } while (cmpxchg(&priv->credit, old_credit, new_credit) != old_credit);
0090
0091     return ret;
0092 }
0093
0094 /* Precision saver. */
0095 static u32 user2credits(u32 user)
0096 {
0097     /* If multiplying would overflow... */
0098     if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
0099         /* Divide first. */
0100         return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
0101
0102     return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
0103 }
0104
0105 static int limit_mt_check(const struct xt_mtchk_param *par)
0106 {
0107     struct xt_rateinfo *r = par->matchinfo;
0108     struct xt_limit_priv *priv;
0109
0110     /* Check for overflow. */
0111     if (r->burst == 0
0112         || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
0113         pr_info_ratelimited("Overflow, try lower: %u/%u\n",
0114                     r->avg, r->burst);
0115         return -ERANGE;
0116     }
0117
0118     priv = kmalloc(sizeof(*priv), GFP_KERNEL);
0119     if (priv == NULL)
0120         return -ENOMEM;
0121
0122     /* For SMP, we only want to use one set of state. */
0123     r->master = priv;
0124     /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
0125        128. */
0126     priv->prev = jiffies;
0127     priv->credit = user2credits(r->avg * r->burst); /* Credits full. */
0128     if (r->cost == 0) {
0129         r->credit_cap = priv->credit; /* Credits full. */
0130         r->cost = user2credits(r->avg);
0131     }
0132
0133     return 0;
0134 }
0135
0136 static void limit_mt_destroy(const struct xt_mtdtor_param *par)
0137 {
0138     const struct xt_rateinfo *info = par->matchinfo;
0139
0140     kfree(info->master);
0141 }
0142
0143 #ifdef CONFIG_NETFILTER_XTABLES_COMPAT
0144 struct compat_xt_rateinfo {
0145     u_int32_t avg;
0146     u_int32_t burst;
0147
0148     compat_ulong_t prev;
0149     u_int32_t credit;
0150     u_int32_t credit_cap, cost;
0151
0152     u_int32_t master;
0153 };
0154
0155 /* To keep the full "prev" timestamp, the upper 32 bits are stored in the
0156  * master pointer, which does not need to be preserved. */
0157 static void limit_mt_compat_from_user(void *dst, const void *src)
0158 {
0159     const struct compat_xt_rateinfo *cm = src;
0160     struct xt_rateinfo m = {
0161         .avg        = cm->avg,
0162         .burst      = cm->burst,
0163         .prev       = cm->prev | (unsigned long)cm->master << 32,
0164         .credit     = cm->credit,
0165         .credit_cap = cm->credit_cap,
0166         .cost       = cm->cost,
0167     };
0168     memcpy(dst, &m, sizeof(m));
0169 }
0170
0171 static int limit_mt_compat_to_user(void __user *dst, const void *src)
0172 {
0173     const struct xt_rateinfo *m = src;
0174     struct compat_xt_rateinfo cm = {
0175         .avg        = m->avg,
0176         .burst      = m->burst,
0177         .prev       = m->prev,
0178         .credit     = m->credit,
0179         .credit_cap = m->credit_cap,
0180         .cost       = m->cost,
0181         .master     = m->prev >> 32,
0182     };
0183     return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
0184 }
0185 #endif /* CONFIG_NETFILTER_XTABLES_COMPAT */
0186
0187 static struct xt_match limit_mt_reg __read_mostly = {
0188     .name             = "limit",
0189     .revision         = 0,
0190     .family           = NFPROTO_UNSPEC,
0191     .match            = limit_mt,
0192     .checkentry       = limit_mt_check,
0193     .destroy          = limit_mt_destroy,
0194     .matchsize        = sizeof(struct xt_rateinfo),
0195 #ifdef CONFIG_NETFILTER_XTABLES_COMPAT
0196     .compatsize       = sizeof(struct compat_xt_rateinfo),
0197     .compat_from_user = limit_mt_compat_from_user,
0198     .compat_to_user   = limit_mt_compat_to_user,
0199 #endif
0200     .usersize         = offsetof(struct xt_rateinfo, prev),
0201     .me               = THIS_MODULE,
0202 };
0203
0204 static int __init limit_mt_init(void)
0205 {
0206     return xt_register_match(&limit_mt_reg);
0207 }
0208
0209 static void __exit limit_mt_exit(void)
0210 {
0211     xt_unregister_match(&limit_mt_reg);
0212 }
0213
0214 module_init(limit_mt_init);
0215 module_exit(limit_mt_exit);