Avoid overflows and underflows in sscanf and friends

(Patch from teor on 13104)

src/common/util.c

@@ -2804,10 +2804,14 @@ scan_unsigned(const char **bufp, unsigned long *out, int width, int base)
   while (**bufp && (hex?TOR_ISXDIGIT(**bufp):TOR_ISDIGIT(**bufp))
          && scanned_so_far < width) {
     int digit = hex?hex_decode_digit(*(*bufp)++):digit_to_num(*(*bufp)++);
-    unsigned long new_result = result * base + digit;
+    // Check for overflow beforehand, without actually causing any overflow
-    if (new_result < result)
+    // This preserves functionality on compilers that don't wrap overflow
-      return -1; /* over/underflow. */
+    // (i.e. that trap or optimise away overflow)
-    result = new_result;
+    // result * base + digit > ULONG_MAX
+    // result * base > ULONG_MAX - digit
+    if (result > (ULONG_MAX - digit)/base)
+      return -1; /* Processing this digit would overflow */
+    result = result * base + digit;
     ++scanned_so_far;
   }
 
@@ -2842,10 +2846,17 @@ scan_signed(const char **bufp, long *out, int width)
   if (scan_unsigned(bufp, &result, width, 10) < 0)
     return -1;
 
-  if (neg) {
+  if (neg && result > 0) {
     if (result > ((unsigned long)LONG_MAX) + 1)
       return -1; /* Underflow */
-    *out = -(long)result;
+    // Avoid overflow on the cast to signed long when result is LONG_MIN
+    // by subtracting 1 from the unsigned long positive value,
+    // then, after it has been cast to signed and negated,
+    // subtracting the original 1 (the double-subtraction is intentional).
+    // Otherwise, the cast to signed could cause a temporary long
+    // to equal LONG_MAX + 1, which is undefined.
+    // We avoid underflow on the subtraction by treating -0 as positive.
+    *out = (-(long)(result - 1)) - 1;
   } else {
     if (result > LONG_MAX)
       return -1; /* Overflow */

src/test/test_util.c

@@ -1672,6 +1672,7 @@ static void
 test_util_sscanf(void)
 {
   unsigned u1, u2, u3;
+  unsigned long ulng;
   char s1[20], s2[10], s3[10], ch;
   int r;
   long lng1,lng2;
@@ -1713,11 +1714,6 @@ test_util_sscanf(void)
   test_eq(0, tor_sscanf("", "%u", &u1)); /* absent number */
   test_eq(0, tor_sscanf("A", "%u", &u1)); /* bogus number */
   test_eq(0, tor_sscanf("-1", "%u", &u1)); /* negative number */
-  test_eq(1, tor_sscanf("4294967295", "%u", &u1)); /* UINT32_MAX should work */
-  test_eq(4294967295u, u1);
-  test_eq(0, tor_sscanf("4294967296", "%u", &u1)); /* But not at 32 bits */
-  test_eq(1, tor_sscanf("4294967296", "%9u", &u1)); /* but parsing only 9... */
-  test_eq(429496729u, u1);
 
   /* Numbers with size (eg. %2u) */
   test_eq(0, tor_sscanf("-1", "%2u", &u1));
@@ -1812,46 +1808,191 @@ test_util_sscanf(void)
   test_eq(int2, -1);
 
 #if SIZEOF_INT == 4
+  /* %u */
+  /* UINT32_MAX should work */
+  test_eq(1, tor_sscanf("4294967295", "%u", &u1));
+  test_eq(4294967295U, u1);
+
+  /* But UINT32_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("4294967296", "%u", &u1));
+  /* but parsing only 9... */
+  test_eq(1, tor_sscanf("4294967296", "%9u", &u1));
+  test_eq(429496729U, u1);
+
+  /* %x */
+  /* UINT32_MAX should work */
+  test_eq(1, tor_sscanf("FFFFFFFF", "%x", &u1));
+  test_eq(0xFFFFFFFF, u1);
+
+  /* But UINT32_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("100000000", "%x", &u1));
+
+  /* %d */
+  /* INT32_MIN and INT32_MAX should work */
   r = tor_sscanf("-2147483648. 2147483647.", "%d. %d.", &int1, &int2);
   test_eq(r,2);
-  test_eq(int1, -2147483647-1);
+  test_eq(int1, -2147483647 - 1);
   test_eq(int2, 2147483647);
 
-  r = tor_sscanf("-2147483679.", "%d.", &int1);
+  /* But INT32_MIN - 1 and INT32_MAX + 1 shouldn't work */
+  r = tor_sscanf("-2147483649.", "%d.", &int1);
   test_eq(r,0);
 
-  r = tor_sscanf("2147483678.", "%d.", &int1);
+  r = tor_sscanf("2147483648.", "%d.", &int1);
+  test_eq(r,0);
+
+  /* and the first failure stops further processing */
+  r = tor_sscanf("-2147483648. 2147483648.",
+                 "%d. %d.", &int1, &int2);
+  test_eq(r,1);
+
+  r = tor_sscanf("-2147483649. 2147483647.",
+                 "%d. %d.", &int1, &int2);
+  test_eq(r,0);
+
+  r = tor_sscanf("2147483648. -2147483649.",
+                 "%d. %d.", &int1, &int2);
   test_eq(r,0);
 #elif SIZEOF_INT == 8
+  /* %u */
+  /* UINT64_MAX should work */
+  test_eq(1, tor_sscanf("18446744073709551615", "%u", &u1));
+  test_eq(18446744073709551615U, u1);
+
+  /* But UINT64_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("18446744073709551616", "%u", &u1));
+  /* but parsing only 19... */
+  test_eq(1, tor_sscanf("18446744073709551616", "%19u", &u1));
+  test_eq(1844674407370955161U, u1);
+
+  /* %x */
+  /* UINT64_MAX should work */
+  test_eq(1, tor_sscanf("FFFFFFFFFFFFFFFF", "%x", &u1));
+  test_eq(0xFFFFFFFFFFFFFFFF, u1);
+
+  /* But UINT64_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("10000000000000000", "%x", &u1));
+
+  /* %d */
+  /* INT64_MIN and INT64_MAX should work */
   r = tor_sscanf("-9223372036854775808. 9223372036854775807.",
                  "%d. %d.", &int1, &int2);
   test_eq(r,2);
-  test_eq(int1, -9223372036854775807-1);
+  test_eq(int1, -9223372036854775807 - 1);
   test_eq(int2, 9223372036854775807);
 
+  /* But INT64_MIN - 1 and INT64_MAX + 1 shouldn't work */
   r = tor_sscanf("-9223372036854775809.", "%d.", &int1);
   test_eq(r,0);
 
   r = tor_sscanf("9223372036854775808.", "%d.", &int1);
   test_eq(r,0);
+
+  /* and the first failure stops further processing */
+  r = tor_sscanf("-9223372036854775808. 9223372036854775808.",
+                 "%d. %d.", &int1, &int2);
+  test_eq(r,1);
+
+  r = tor_sscanf("-9223372036854775809. 9223372036854775807.",
+                 "%d. %d.", &int1, &int2);
+  test_eq(r,0);
+
+  r = tor_sscanf("9223372036854775808. -9223372036854775809.",
+                 "%d. %d.", &int1, &int2);
+  test_eq(r,0);
 #endif
 
 #if SIZEOF_LONG == 4
+  /* %lu */
+  /* UINT32_MAX should work */
+  test_eq(1, tor_sscanf("4294967295", "%lu", &ulng));
+  test_eq(4294967295UL, ulng);
+
+  /* But UINT32_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("4294967296", "%lu", &ulng));
+  /* but parsing only 9... */
+  test_eq(1, tor_sscanf("4294967296", "%9lu", &ulng));
+  test_eq(429496729UL, ulng);
+
+  /* %lx */
+  /* UINT32_MAX should work */
+  test_eq(1, tor_sscanf("FFFFFFFF", "%lx", &ulng));
+  test_eq(0xFFFFFFFFUL, ulng);
+
+  /* But UINT32_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("100000000", "%lx", &ulng));
+
+  /* %ld */
+  /* INT32_MIN and INT32_MAX should work */
   r = tor_sscanf("-2147483648. 2147483647.", "%ld. %ld.", &lng1, &lng2);
   test_eq(r,2);
-  test_eq(lng1, -2147483647 - 1);
+  test_eq(lng1, -2147483647L - 1L);
-  test_eq(lng2, 2147483647);
+  test_eq(lng2, 2147483647L);
+
+  /* But INT32_MIN - 1 and INT32_MAX + 1 shouldn't work */
+  r = tor_sscanf("-2147483649.", "%ld.", &lng1);
+  test_eq(r,0);
+
+  r = tor_sscanf("2147483648.", "%ld.", &lng1);
+  test_eq(r,0);
+
+  /* and the first failure stops further processing */
+  r = tor_sscanf("-2147483648. 2147483648.",
+                 "%ld. %ld.", &lng1, &lng2);
+  test_eq(r,1);
+
+  r = tor_sscanf("-2147483649. 2147483647.",
+                 "%ld. %ld.", &lng1, &lng2);
+  test_eq(r,0);
+
+  r = tor_sscanf("2147483648. -2147483649.",
+                 "%ld. %ld.", &lng1, &lng2);
+  test_eq(r,0);
 #elif SIZEOF_LONG == 8
+  /* %lu */
+  /* UINT64_MAX should work */
+  test_eq(1, tor_sscanf("18446744073709551615", "%lu", &ulng));
+  test_eq(18446744073709551615UL, ulng);
+
+  /* But UINT64_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("18446744073709551616", "%lu", &ulng));
+  /* but parsing only 19... */
+  test_eq(1, tor_sscanf("18446744073709551616", "%19lu", &ulng));
+  test_eq(1844674407370955161UL, ulng);
+
+  /* %lx */
+  /* UINT64_MAX should work */
+  test_eq(1, tor_sscanf("FFFFFFFFFFFFFFFF", "%lx", &ulng));
+  test_eq(0xFFFFFFFFFFFFFFFFUL, ulng);
+
+  /* But UINT64_MAX + 1 shouldn't work */
+  test_eq(0, tor_sscanf("10000000000000000", "%lx", &ulng));
+
+  /* %ld */
+  /* INT64_MIN and INT64_MAX should work */
   r = tor_sscanf("-9223372036854775808. 9223372036854775807.",
                  "%ld. %ld.", &lng1, &lng2);
   test_eq(r,2);
-  test_eq(lng1, -9223372036854775807L - 1);
+  test_eq(lng1, -9223372036854775807L - 1L);
   test_eq(lng2, 9223372036854775807L);
 
+  /* But INT64_MIN - 1 and INT64_MAX + 1 shouldn't work */
+  r = tor_sscanf("-9223372036854775809.", "%ld.", &lng1);
+  test_eq(r,0);
+
+  r = tor_sscanf("9223372036854775808.", "%ld.", &lng1);
+  test_eq(r,0);
+
+  /* and the first failure stops further processing */
   r = tor_sscanf("-9223372036854775808. 9223372036854775808.",
                  "%ld. %ld.", &lng1, &lng2);
   test_eq(r,1);
-  r = tor_sscanf("-9223372036854775809. 9223372036854775808.",
+
+  r = tor_sscanf("-9223372036854775809. 9223372036854775807.",
+                 "%ld. %ld.", &lng1, &lng2);
+  test_eq(r,0);
+
+  r = tor_sscanf("9223372036854775808. -9223372036854775809.",
                  "%ld. %ld.", &lng1, &lng2);
   test_eq(r,0);
 #endif