Add a data-independent variant of memcmp and a d-i memeq function.

The tor_memcmp code is by Robert Ransom, and the tor_memeq code is
by me.  Both incorporate some ideas from DJB's stuff.

changes/bug3122_memcmp

+  o Security fixes
+    - Replace all potentially sensitive memory comparison operations
+      with versions whose runtime does not depend on the data being
+      compared. This will help resist a class of attacks where an
+      adversary can use variations in timing information to learn
+      sensitive data.  Fix for one case of bug 3122.  (Safe memcmp
+      implementation by Robert Ransom based partially on code by DJB.)

configure.in

@@ -626,6 +626,24 @@ if test "$tor_cv_twos_complement" != no ; then
             [Define to 1 iff we represent negative integers with two's complement])
 fi
 
+# What does shifting a negative value do?
+AC_CACHE_CHECK([whether right-shift on negative values does sign-extension], tor_cv_sign_extend,
+[AC_RUN_IFELSE([AC_LANG_SOURCE(
+[[int main () { int okay = (-60 >> 8) == -1; return okay ? 0 : 1; }]])],
+       [tor_cv_sign_extend=yes],
+       [tor_cv_sign_extend=no],
+       [tor_cv_sign_extend=cross])])
+
+if test "$tor_cv_sign_extend" = cross ; then
+  # Cross-compiling; let's hope that the target isn't raving mad.
+  AC_MSG_NOTICE([Cross-compiling: we'll assume that right-shifting negative integers causes sign-extension])
+fi
+
+if test "$tor_cv_sign_extend" != no ; then
+  AC_DEFINE([RSHIFT_DOES_SIGN_EXTEND], 1,
+            [Define to 1 iff right-shifting a negative value performs sign-extension])
+fi
+
 # Whether we should use the dmalloc memory allocation debugging library.
 AC_MSG_CHECKING(whether to use dmalloc (debug memory allocation library))
 AC_ARG_WITH(dmalloc,

src/common/Makefile.am

@@ -10,7 +10,7 @@ libor_extra_source=
 endif
 
 libor_a_SOURCES = address.c log.c util.c compat.c container.c mempool.c \
-	memarea.c $(libor_extra_source)
+	memarea.c di_ops.c $(libor_extra_source)
 libor_crypto_a_SOURCES = crypto.c aes.c tortls.c torgzip.c
 
-noinst_HEADERS = address.h log.h crypto.h test.h util.h compat.h aes.h torint.h tortls.h strlcpy.c strlcat.c torgzip.h container.h ht.h mempool.h memarea.h ciphers.inc
+noinst_HEADERS = address.h log.h crypto.h test.h util.h compat.h aes.h torint.h tortls.h strlcpy.c strlcat.c torgzip.h container.h ht.h mempool.h memarea.h di_ops.h ciphers.inc

src/common/di_ops.c

+/* Copyright (c) 2011, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file di_ops.c
+ * \brief Functions for data-independent operations
+ **/
+
+#include "orconfig.h"
+#include "di_ops.h"
+
+/**
+ * Timing-safe version of memcmp.  As memcmp, compare the <b>sz</b> bytes
+ * at <b>a</b> with the <b>sz</b> bytes at <b>, and returns less than 0 if the
+ * bytes at <b>a</b> lexically precede those at <b>b</b>, 0 if the byte ranges
+ * are equal, and greater than zero if the bytes at <b>a</b> lexically follow
+ * those at <b>.
+ *
+ * This implementation differs from memcmp in that its timing behavior is not
+ * data-dependent: it should return in the same amount of time regardless of
+ * the contents of <b>a</b> and <b>b</b>.
+ */
+int
+tor_memcmp(const void *a, const void *b, size_t len)
+{
+  const uint8_t *x = a;
+  const uint8_t *y = b;
+  size_t i = len;
+  int retval = 0;
+
+  /* This loop goes from the end of the arrays to the start.  At the
+   * start of every iteration, before we decrement i, we have set
+   * "retval" equal to the result of memcmp(a+i,b+i,len-i).  During the
+   * loop, we update retval by leaving it unchanged if x[i]==y[i] and
+   * setting it to x[i]-y[i] if x[i]!= y[i].
+   *
+   * The following assumes we are on a system with two's-complement
+   * arithmetic.  We check for this at configure-time with the check
+   * that sets USING_TWOS_COMPLEMENT.  If we aren't two's complement, then
+   * torint.h will stop compilation with an error.
+   */
+  while (i--) {
+    int v1 = x[i];
+    int v2 = y[i];
+    int equal_p = v1 ^ v2;
+
+    /* The following sets bits 8 and above of equal_p to 'equal_p ==
+     * 0', and thus to v1 == v2.  (To see this, note that if v1 ==
+     * v2, then v1^v2 == equal_p == 0, so equal_p-1 == -1, which is the
+     * same as ~0 on a two's-complement machine.  Then note that if
+     * v1 != v2, then 0 < v1 ^ v2 < 256, so 0 <= equal_p - 1 < 255.)
+     */
+    --equal_p;
+
+    equal_p >>= 8;
+    /* Thanks to (sign-preserving) arithmetic shift, equal_p is now
+     * equal to -(v1 == v2), which is exactly what we need below.
+     * (Since we're assuming two's-complement arithmetic, -1 is the
+     * same as ~0 (all bits set).)
+     *
+     * (The result of an arithmetic shift on a negative value is
+     * actually implementation-defined in standard C.  So how do we
+     * get away with assuming it?  Easy.  We check.) */
+#if ((-60 >> 8) != -1)
+#error "According to cpp, right-shift doesn't perform sign-extension."
+#endif
+#ifndef RSHIFT_DOES_SIGN_EXTEND
+#error "According to configure, right-shift doesn't perform sign-extension."
+#endif
+
+    /* If v1 == v2, equal_p is ~0, so this will leave retval
+     * unchanged; otherwise, equal_p is 0, so this will zero it. */
+    retval &= equal_p;
+
+    /* If v1 == v2, then this adds 0, and leaves retval unchanged.
+     * Otherwise, we just zeroed retval, so this sets it to v1 - v2. */
+    retval += (v1 - v2);
+
+    /* There.  Now retval is equal to its previous value if v1 == v2, and
+     * equal to v1 - v2 if v1 != v2. */
+  }
+
+  return retval;
+}
+
+/**
+ * Timing-safe memory comparison.  Return true if the <b>sz</b> bytes at
+ * <b>a</b> are the same as the <b>sz</b> bytes at <b>, and 0 otherwise.
+ *
+ * This implementation differs from !memcmp(a,b,sz) in that its timing
+ * behavior is not data-dependent: it should return in the same amount of time
+ * regardless of the contents of <b>a</b> and <b>b</b>.  It differs from
+ * !tor_memcmp(a,b,sz) by being faster.
+ */
+int
+tor_memeq(const void *a, const void *b, size_t sz)
+{
+  /* Treat a and b as byte ranges. */
+  const uint8_t *ba = a, *bb = b;
+  uint32_t any_difference = 0;
+  while (sz--) {
+    /* Set byte_diff to all of those bits that are different in *ba and *bb,
+     * and advance both ba and bb. */
+    const uint8_t byte_diff = *ba++ ^ *bb++;
+
+    /* Set bits in any_difference if they are set in byte_diff. */
+    any_difference |= byte_diff;
+  }
+
+  /* Now any_difference is 0 if there are no bits different between
+   * a and b, and is nonzero if there are bits different between a
+   * and b.  Now for paranoia's sake, let's convert it to 0 or 1.
+   *
+   * (If we say "!any_difference", the compiler might get smart enough
+   * to optimize-out our data-independence stuff above.)
+   *
+   * To unpack:
+   *
+   * If any_difference == 0:
+   *            any_difference - 1 == ~0
+   *     (any_difference - 1) >> 8 == 0x00ffffff
+   *     1 & ((any_difference - 1) >> 8) == 1
+   *
+   * If any_difference != 0:
+   *            0 < any_difference < 256, so
+   *            0 < any_difference - 1 < 255
+   *            (any_difference - 1) >> 8 == 0
+   *            1 & ((any_difference - 1) >> 8) == 0
+   */
+
+  return 1 & ((any_difference - 1) >> 8);
+}
+

src/common/di_ops.h

+/* Copyright (c) 2003-2004, Roger Dingledine
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2011, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file di_ops.h
+ * \brief Headers for di_ops.c
+ **/
+
+#ifndef TOR_DI_OPS_H
+#define TOR_DI_OPS_H
+
+#include "orconfig.h"
+#include "torint.h"
+
+int tor_memcmp(const void *a, const void *b, size_t sz);
+int tor_memeq(const void *a, const void *b, size_t sz);
+#define tor_memneq(a,b,sz) (!tor_memeq((a),(b),(sz)))
+
+/** Alias for the platform's memcmp() function.  This function is
+ * <em>not</em> data-independent: we define this alias so that we can
+ * mark cases where we are deliberately using a data-dependent memcmp()
+ * implementation.
+ */
+#define fast_memcmp(a,b,c) (memcmp((a),(b),(c)))
+
+#endif

src/or/test.c

@@ -43,6 +43,7 @@ const char tor_svn_revision[] = "";
 #include "torgzip.h"
 #include "mempool.h"
 #include "memarea.h"
+#include "di_ops.h"
 
 #ifdef USE_DMALLOC
 #include <dmalloc.h>
@@ -1331,6 +1332,59 @@ test_util(void)
   ;
 }
 
+static void
+test_util_di_ops(void)
+{
+#define LT -1
+#define GT 1
+#define EQ 0
+  const struct {
+    const char *a; int want_sign; const char *b;
+  } examples[] = {
+    { "Foo", EQ, "Foo" },
+    { "foo", GT, "bar", },
+    { "foobar", EQ ,"foobar" },
+    { "foobar", LT, "foobaw" },
+    { "foobar", GT, "f00bar" },
+    { "foobar", GT, "boobar" },
+    { "", EQ, "" },
+    { NULL, 0, NULL },
+  };
+
+  int i;
+
+  for (i = 0; examples[i].a; ++i) {
+    size_t len = strlen(examples[i].a);
+    int eq1, eq2, neq1, neq2, cmp1, cmp2;
+    test_eq(len, strlen(examples[i].b));
+    /* We do all of the operations, with operands in both orders. */
+    eq1 = tor_memeq(examples[i].a, examples[i].b, len);
+    eq2 = tor_memeq(examples[i].b, examples[i].a, len);
+    neq1 = tor_memneq(examples[i].a, examples[i].b, len);
+    neq2 = tor_memneq(examples[i].b, examples[i].a, len);
+    cmp1 = tor_memcmp(examples[i].a, examples[i].b, len);
+    cmp2 = tor_memcmp(examples[i].b, examples[i].a, len);
+
+    /* Check for correctness of cmp1 */
+    if (cmp1 < 0 && examples[i].want_sign != LT)
+      test_fail();
+    else if (cmp1 > 0 && examples[i].want_sign != GT)
+      test_fail();
+    else if (cmp1 == 0 && examples[i].want_sign != EQ)
+      test_fail();
+
+    /* Check for consistency of everything else with cmp1 */
+    test_eq(eq1, eq2);
+    test_eq(neq1, neq2);
+    test_eq(cmp1, -cmp2);
+    test_eq(eq1, cmp1 == 0);
+    test_eq(neq1, !eq1);
+  }
+
+ done:
+  ;
+}
+
 /** Helper: assert that IPv6 addresses <b>a</b> and <b>b</b> are the same.  On
  * failure, reports an error, describing the addresses as <b>e1</b> and
  * <b>e2</b>, and reporting the line number as <b>line</b>. */
@@ -4753,6 +4807,7 @@ static struct {
   SUBENT(crypto, aes_iv),
   SUBENT(crypto, base32_decode),
   ENT(util),
+  SUBENT(util, di_ops),
   SUBENT(util, ip6_helpers),
   SUBENT(util, gzip),
   SUBENT(util, datadir),