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
limit_mt(const struct sk_buff * skb,struct xt_action_param * par)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. */
user2credits(u32 user)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 
limit_mt_check(const struct xt_mtchk_param * par)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 
limit_mt_destroy(const struct xt_mtdtor_param * par)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. */
limit_mt_compat_from_user(void * dst,const void * src)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 
limit_mt_compat_to_user(void __user * dst,const void * src)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 
limit_mt_init(void)204 static int __init limit_mt_init(void)
205 {
206 	return xt_register_match(&limit_mt_reg);
207 }
208 
limit_mt_exit(void)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);
216