Hand-conversion and audit phase of memcmp transition

Here I looked at the results of the automated conversion and cleaned
them up as follows:

   If there was a tor_memcmp or tor_memeq that was in fact "safe"[*] I
   changed it to a fast_memcmp or fast_memeq.

   Otherwise if there was a tor_memcmp that could turn into a
   tor_memneq or tor_memeq, I converted it.

This wants close attention.

[*] I'm erring on the side of caution here, and leaving some things
as tor_memcmp that could in my opinion use the data-dependent
fast_memcmp variant.

7 files changed, 23 insertions, 14 deletions

diff --git a/src/common/compat.c b/src/common/compat.c
index ea46d43d8..39651084a 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -312,6 +312,8 @@ tor_vsnprintf(char *str, size_t size, const char *format, va_list args)
  * <b>needle</b>, return a pointer to the first occurrence of the needle
  * within the haystack, or NULL if there is no such occurrence.
  *
+ * This function is <em>not</em> timing-safe.
+ *
  * Requires that nlen be greater than zero.
  */
 const void *
@@ -336,7 +338,7 @@ tor_memmem(const void *_haystack, size_t hlen,
   while ((p = memchr(p, first, end-p))) {
     if (p+nlen > end)
       return NULL;
-    if (tor_memeq(p, needle, nlen))
+    if (fast_memeq(p, needle, nlen))
       return p;
     ++p;
   }
diff --git a/src/common/container.c b/src/common/container.c
index d1d5ce339..c741eb020 100644
--- a/src/common/container.c
+++ b/src/common/container.c
@@ -223,7 +223,7 @@ smartlist_digest_isin(const smartlist_t *sl, const char *element)
   int i;
   if (!sl) return 0;
   for (i=0; i < sl->num_used; i++)
-    if (tor_memcmp((const char*)sl->list[i],element,DIGEST_LEN)==0)
+    if (tor_memeq((const char*)sl->list[i],element,DIGEST_LEN))
       return 1;
   return 0;
 }
diff --git a/src/common/crypto.c b/src/common/crypto.c
index 926942897..f3268fe18 100644
--- a/src/common/crypto.c
+++ b/src/common/crypto.c
@@ -845,7 +845,7 @@ crypto_pk_public_checksig_digest(crypto_pk_env_t *env, const char *data,
     tor_free(buf);
     return -1;
   }
-  if (tor_memcmp(buf, digest, DIGEST_LEN)) {
+  if (tor_memneq(buf, digest, DIGEST_LEN)) {
     log_warn(LD_CRYPTO, "Signature mismatched with digest.");
     tor_free(buf);
     return -1;
diff --git a/src/common/di_ops.h b/src/common/di_ops.h
index 1b223d9ab..4a212b0ca 100644
--- a/src/common/di_ops.h
+++ b/src/common/di_ops.h
@@ -24,5 +24,7 @@ int tor_memeq(const void *a, const void *b, size_t sz);
  * implementation.
  */
 #define fast_memcmp(a,b,c) (memcmp((a),(b),(c)))
+#define fast_memeq(a,b,c)  (0==memcmp((a),(b),(c)))
+#define fast_memneq(a,b,c) (0!=memcmp((a),(b),(c)))
 
 #endif
diff --git a/src/common/torgzip.c b/src/common/torgzip.c
index 51b29baca..f5709aaf3 100644
--- a/src/common/torgzip.c
+++ b/src/common/torgzip.c
@@ -356,7 +356,7 @@ tor_gzip_uncompress(char **out, size_t *out_len,
 compress_method_t
 detect_compression_method(const char *in, size_t in_len)
 {
-  if (in_len > 2 && tor_memeq(in, "\x1f\x8b", 2)) {
+  if (in_len > 2 && fast_memeq(in, "\x1f\x8b", 2)) {
     return GZIP_METHOD;
   } else if (in_len > 2 && (in[0] & 0x0f) == 8 &&
              (ntohs(get_uint16(in)) % 31) == 0) {
diff --git a/src/common/util.c b/src/common/util.c
index cb2cfed64..879a0e4bd 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -459,7 +459,7 @@ strcmp_len(const char *s1, const char *s2, size_t s1_len)
     return -1;
   if (s1_len > s2_len)
     return 1;
-  return tor_memcmp(s1, s2, s2_len);
+  return fast_memcmp(s1, s2, s2_len);
 }
 
 /** Compares the first strlen(s2) characters of s1 with s2.  Returns as for
@@ -501,17 +501,17 @@ strcasecmpend(const char *s1, const char *s2)
 /** Compare the value of the string <b>prefix</b> with the start of the
  * <b>memlen</b>-byte memory chunk at <b>mem</b>.  Return as for strcmp.
  *
- * [As tor_memcmp(mem, prefix, strlen(prefix)) but returns -1 if memlen is less
- * than strlen(prefix).]
+ * [As fast_memcmp(mem, prefix, strlen(prefix)) but returns -1 if memlen is
+ * less than strlen(prefix).]
  */
 int
-memcmpstart(const void *mem, size_t memlen,
+fast_memcmpstart(const void *mem, size_t memlen,
                 const char *prefix)
 {
   size_t plen = strlen(prefix);
   if (memlen < plen)
     return -1;
-  return tor_memcmp(mem, prefix, plen);
+  return fast_memcmp(mem, prefix, plen);
 }
 
 /** Return a pointer to the first char of s that is not whitespace and
@@ -644,14 +644,16 @@ tor_mem_is_zero(const char *mem, size_t len)
     0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0,
   };
   while (len >= sizeof(ZERO)) {
-    if (tor_memcmp(mem, ZERO, sizeof(ZERO)))
+    /* It's safe to use fast_memcmp here, since the very worst thing an
+     * attacker could learn is how many initial bytes of a secret were zero */
+    if (fast_memcmp(mem, ZERO, sizeof(ZERO)))
       return 0;
     len -= sizeof(ZERO);
     mem += sizeof(ZERO);
   }
   /* Deal with leftover bytes. */
   if (len)
-    return tor_memeq(mem, ZERO, len);
+    return fast_memeq(mem, ZERO, len);
 
   return 1;
 }
@@ -660,7 +662,10 @@ tor_mem_is_zero(const char *mem, size_t len)
 int
 tor_digest_is_zero(const char *digest)
 {
-  return tor_mem_is_zero(digest, DIGEST_LEN);
+  static const uint8_t ZERO_DIGEST[] = {
+    0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0
+  };
+  return tor_memeq(digest, ZERO_DIGEST, DIGEST_LEN);
 }
 
 /* Helper: common code to check whether the result of a strtol or strtoul or
diff --git a/src/common/util.h b/src/common/util.h
index 7bc5286a8..1012a111d 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -172,8 +172,8 @@ int strcasecmpstart(const char *s1, const char *s2)
 int strcmpend(const char *s1, const char *s2) ATTR_PURE ATTR_NONNULL((1,2));
 int strcasecmpend(const char *s1, const char *s2)
   ATTR_PURE ATTR_NONNULL((1,2));
-int memcmpstart(const void *mem, size_t memlen,
-                const char *prefix) ATTR_PURE;
+int fast_memcmpstart(const void *mem, size_t memlen,
+                     const char *prefix) ATTR_PURE;
 
 void tor_strstrip(char *s, const char *strip) ATTR_NONNULL((1,2));
 long tor_parse_long(const char *s, int base, long min,