Tor 0.4.9.0-alpha-dev
di_ops.c
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1/* Copyright (c) 2011-2021, The Tor Project, Inc. */
2/* See LICENSE for licensing information */
3
4/**
5 * \file di_ops.c
6 * \brief Functions for data-independent operations.
7 **/
8
9#include "orconfig.h"
10#include "lib/ctime/di_ops.h"
11#include "lib/err/torerr.h"
12#include "lib/malloc/malloc.h"
13
14#include <string.h>
15
16/**
17 * Timing-safe version of memcmp. As memcmp, compare the <b>sz</b> bytes at
18 * <b>a</b> with the <b>sz</b> bytes at <b>b</b>, and return less than 0 if
19 * the bytes at <b>a</b> lexically precede those at <b>b</b>, 0 if the byte
20 * ranges are equal, and greater than zero if the bytes at <b>a</b> lexically
21 * follow those at <b>b</b>.
22 *
23 * This implementation differs from memcmp in that its timing behavior is not
24 * data-dependent: it should return in the same amount of time regardless of
25 * the contents of <b>a</b> and <b>b</b>.
26 *
27 * Note that if all you care about is equality, this implementation is
28 * overkill: it would be better to use tor_memeq() or tor_memneq().
29 */
30int
31tor_memcmp(const void *a, const void *b, size_t len)
32{
33#ifdef HAVE_TIMINGSAFE_MEMCMP
34 return timingsafe_memcmp(a, b, len);
35#else
36 const uint8_t *x = a;
37 const uint8_t *y = b;
38 size_t i = len;
39 int retval = 0;
40
41 /* This loop goes from the end of the arrays to the start. At the
42 * start of every iteration, before we decrement i, we have set
43 * "retval" equal to the result of memcmp(a+i,b+i,len-i). During the
44 * loop, we update retval by leaving it unchanged if x[i]==y[i] and
45 * setting it to x[i]-y[i] if x[i]!= y[i].
46 *
47 * The following assumes we are on a system with two's-complement
48 * arithmetic. We check for this at configure-time with the check
49 * that sets USING_TWOS_COMPLEMENT. If we aren't two's complement, then
50 * torint.h will stop compilation with an error.
51 */
52 while (i--) {
53 int v1 = x[i];
54 int v2 = y[i];
55 int equal_p = v1 ^ v2;
56
57 /* The following sets bits 8 and above of equal_p to 'equal_p ==
58 * 0', and thus to v1 == v2. (To see this, note that if v1 ==
59 * v2, then v1^v2 == equal_p == 0, so equal_p-1 == -1, which is the
60 * same as ~0 on a two's-complement machine. Then note that if
61 * v1 != v2, then 0 < v1 ^ v2 < 256, so 0 <= equal_p - 1 < 255.)
62 */
63 --equal_p;
64
65 equal_p >>= 8;
66 /* Thanks to (sign-preserving) arithmetic shift, equal_p is now
67 * equal to -(v1 == v2), which is exactly what we need below.
68 * (Since we're assuming two's-complement arithmetic, -1 is the
69 * same as ~0 (all bits set).)
70 *
71 * (The result of an arithmetic shift on a negative value is
72 * actually implementation-defined in standard C. So how do we
73 * get away with assuming it? Easy. We check.) */
74#if ((-60 >> 8) != -1)
75#error "cpp says right-shift doesn't perform sign-extension."
76#endif
77#ifndef RSHIFT_DOES_SIGN_EXTEND
78#error "configure says right-shift doesn't perform sign-extension."
79#endif
80
81 /* If v1 == v2, equal_p is ~0, so this will leave retval
82 * unchanged; otherwise, equal_p is 0, so this will zero it. */
83 retval &= equal_p;
84
85 /* If v1 == v2, then this adds 0, and leaves retval unchanged.
86 * Otherwise, we just zeroed retval, so this sets it to v1 - v2. */
87 retval += (v1 - v2);
88
89 /* There. Now retval is equal to its previous value if v1 == v2, and
90 * equal to v1 - v2 if v1 != v2. */
91 }
92
93 return retval;
94#endif /* defined(HAVE_TIMINGSAFE_MEMCMP) */
95}
96
97/**
98 * Timing-safe memory comparison. Return true if the <b>sz</b> bytes at
99 * <b>a</b> are the same as the <b>sz</b> bytes at <b>b</b>, and 0 otherwise.
100 *
101 * This implementation differs from !memcmp(a,b,sz) in that its timing
102 * behavior is not data-dependent: it should return in the same amount of time
103 * regardless of the contents of <b>a</b> and <b>b</b>. It differs from
104 * !tor_memcmp(a,b,sz) by being faster.
105 */
106int
107tor_memeq(const void *a, const void *b, size_t sz)
108{
109 /* Treat a and b as byte ranges. */
110 const uint8_t *ba = a, *bb = b;
111 uint32_t any_difference = 0;
112 while (sz--) {
113 /* Set byte_diff to all of those bits that are different in *ba and *bb,
114 * and advance both ba and bb. */
115 const uint8_t byte_diff = *ba++ ^ *bb++;
116
117 /* Set bits in any_difference if they are set in byte_diff. */
118 any_difference |= byte_diff;
119 }
120
121 /* Now any_difference is 0 if there are no bits different between
122 * a and b, and is nonzero if there are bits different between a
123 * and b. Now for paranoia's sake, let's convert it to 0 or 1.
124 *
125 * (If we say "!any_difference", the compiler might get smart enough
126 * to optimize-out our data-independence stuff above.)
127 *
128 * To unpack:
129 *
130 * If any_difference == 0:
131 * any_difference - 1 == ~0
132 * (any_difference - 1) >> 8 == 0x00ffffff
133 * 1 & ((any_difference - 1) >> 8) == 1
134 *
135 * If any_difference != 0:
136 * 0 < any_difference < 256, so
137 * 0 <= any_difference - 1 < 255
138 * (any_difference - 1) >> 8 == 0
139 * 1 & ((any_difference - 1) >> 8) == 0
140 */
141
142 /*coverity[overflow]*/
143 return 1 & ((any_difference - 1) >> 8);
144}
145
146/* Implement di_digest256_map_t as a linked list of entries. */
148 /** Pointer to the next entry in the list. */
150 /** Key for this entry. */
151 uint8_t key[32];
152 /** Value for this entry. */
153 void *val;
154};
155
156/** Release all storage held in <b>map</b>, calling free_fn on each value
157 * as we go. */
158void
160{
161 while (map) {
162 di_digest256_map_t *victim = map;
163 map = map->next;
164 if (free_fn)
165 free_fn(victim->val);
166 tor_free(victim);
167 }
168}
169
170/** Adjust the map at *<b>map</b>, adding an entry for <b>key</b> ->
171 * <b>val</b>, where <b>key</b> is a DIGEST256_LEN-byte key.
172 *
173 * The caller MUST NOT add a key that already appears in the map.
174 */
175void
177 const uint8_t *key, void *val)
178{
179 di_digest256_map_t *new_ent;
180 {
181 void *old_val = dimap_search(*map, key, NULL);
182 raw_assert(! old_val);
183 raw_assert(val);
184 }
185 new_ent = tor_malloc_zero(sizeof(di_digest256_map_t));
186 new_ent->next = *map;
187 memcpy(new_ent->key, key, 32);
188 new_ent->val = val;
189 *map = new_ent;
190}
191
192/** Search the map at <b>map</b> for an entry whose key is <b>key</b> (a
193 * DIGEST256_LEN-byte key) returning the corresponding value if we found one,
194 * and returning <b>dflt_val</b> if the key wasn't found.
195 *
196 * This operation takes an amount of time dependent only on the length of
197 * <b>map</b>, not on the position or presence of <b>key</b> within <b>map</b>.
198 */
199void *
200dimap_search(const di_digest256_map_t *map, const uint8_t *key,
201 void *dflt_val)
202{
203 uintptr_t result = (uintptr_t)dflt_val;
204
205 while (map) {
206 uintptr_t r = (uintptr_t) tor_memeq(map->key, key, 32);
207 r -= 1; /* Now r is (uintptr_t)-1 if memeq returned false, and
208 * 0 if memeq returned true. */
209
210 result &= r;
211 result |= ((uintptr_t)(map->val)) & ~r;
212
213 map = map->next;
214 }
215
216 return (void *)result;
217}
218
219/**
220 * Return true iff the <b>sz</b> bytes at <b>mem</b> are all zero. Runs in
221 * time independent of the contents of <b>mem</b>.
222 */
223int
224safe_mem_is_zero(const void *mem, size_t sz)
225{
226 uint32_t total = 0;
227 const uint8_t *ptr = mem;
228
229 while (sz--) {
230 total |= *ptr++;
231 }
232
233 /*coverity[overflow]*/
234 return 1 & ((total - 1) >> 8);
235}
236
237/** Time-invariant 64-bit greater-than; works on two integers in the range
238 * (0,INT64_MAX). */
239#if SIZEOF_VOID_P == 8
240#define gt_i64_timei(a,b) ((a) > (b))
241#else
242static inline int
243gt_i64_timei(uint64_t a, uint64_t b)
244{
245 int64_t diff = (int64_t) (b - a);
246 int res = diff >> 63;
247 return res & 1;
248}
249#endif /* SIZEOF_VOID_P == 8 */
250
251/**
252 * Given an array of list of <b>n_entries</b> uint64_t values, whose sum is
253 * <b>total</b>, find the first i such that the total of all elements 0...i is
254 * greater than rand_val.
255 *
256 * Try to perform this operation in a constant-time way.
257 */
258int
259select_array_member_cumulative_timei(const uint64_t *entries, int n_entries,
260 uint64_t total, uint64_t rand_val)
261{
262 int i, i_chosen=-1, n_chosen=0;
263 uint64_t total_so_far = 0;
264
265 for (i = 0; i < n_entries; ++i) {
266 total_so_far += entries[i];
267 if (gt_i64_timei(total_so_far, rand_val)) {
268 i_chosen = i;
269 n_chosen++;
270 /* Set rand_val to INT64_MAX rather than stopping the loop. This way,
271 * the time we spend in the loop does not leak which element we chose. */
272 rand_val = INT64_MAX;
273 }
274 }
275 raw_assert(total_so_far == total);
276 raw_assert(n_chosen == 1);
277 raw_assert(i_chosen >= 0);
278 raw_assert(i_chosen < n_entries);
279
280 return i_chosen;
281}
282
283/**
284 * If <b>s</b> is true, then copy <b>n</b> bytes from <b>src</b> to
285 * <b>dest</b>. Otherwise leave <b>dest</b> alone.
286 *
287 * This function behaves the same as
288 *
289 * if (s)
290 * memcpy(dest, src, n);
291 *
292 * except that it tries to run in the same amount of time whether <b>s</b> is
293 * true or not.
294 **/
295void
296memcpy_if_true_timei(bool s, void *dest, const void *src, size_t n)
297{
298 // If s is true, mask will be ~0. If s is false, mask will be 0.
299 const char mask = (char) -(signed char)s;
300
301 char *destp = dest;
302 const char *srcp = src;
303 for (size_t i = 0; i < n; ++i) {
304 *destp = (*destp & ~mask) | (*srcp & mask);
305 ++destp;
306 ++srcp;
307 }
308}
void dimap_add_entry(di_digest256_map_t **map, const uint8_t *key, void *val)
Definition: di_ops.c:176
void memcpy_if_true_timei(bool s, void *dest, const void *src, size_t n)
Definition: di_ops.c:296
int tor_memeq(const void *a, const void *b, size_t sz)
Definition: di_ops.c:107
int tor_memcmp(const void *a, const void *b, size_t len)
Definition: di_ops.c:31
static int gt_i64_timei(uint64_t a, uint64_t b)
Definition: di_ops.c:243
int select_array_member_cumulative_timei(const uint64_t *entries, int n_entries, uint64_t total, uint64_t rand_val)
Definition: di_ops.c:259
int safe_mem_is_zero(const void *mem, size_t sz)
Definition: di_ops.c:224
void dimap_free_(di_digest256_map_t *map, dimap_free_fn free_fn)
Definition: di_ops.c:159
void * dimap_search(const di_digest256_map_t *map, const uint8_t *key, void *dflt_val)
Definition: di_ops.c:200
Headers for di_ops.c.
void(* dimap_free_fn)(void *)
Definition: di_ops.h:55
Headers for util_malloc.c.
#define tor_free(p)
Definition: malloc.h:56
uint8_t key[32]
Definition: di_ops.c:151
struct di_digest256_map_t * next
Definition: di_ops.c:149
Headers for torerr.c.