CVE-2018-17942: Out-of-bounds Write (CWE-787)
value: The convert_to_decimal function in vasnprintf.c in Gnulib before 2018-09-23 has a heap-based buffer overflow because memory is not allocated for a trailing '\0' character during %f processing.
CWE description: The product writes data past the end, or before the beginning, of the intended buffer.
CVSS v3 base score: 8.8
Published: 2018-10-03T08:29Z
Repository: https://github.com/coreutils/gnulib
Language: C
Files changed: ChangeLog, lib/vasnprintf.c, tests/test-vasnprintf.c
Commit date: 2018-09-23 14:15:03 +0200
Commit message: vasnprintf: Fix heap memory overrun bug.

Reported by Ben Pfaff <blp@cs.stanford.edu> in
<https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00107.html>.

* lib/vasnprintf.c (convert_to_decimal): Allocate one more byte of
memory.
* tests/test-vasnprintf.c (test_function): Add another test.

Diff: diff --git a/ChangeLog b/ChangeLog
index 4158be6da..904a5185e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,14 @@
+2018-09-23  Bruno Haible  <bruno@clisp.org>
+
+	vasnprintf: Fix heap memory overrun bug.
+	Reported by Ben Pfaff <blp@cs.stanford.edu> in
+	<https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00107.html>.
+	* lib/vasnprintf.c (convert_to_decimal): Allocate one more byte of
+	memory.
+	* tests/test-vasnprintf.c (test_function): Add another test.
+
 2018-09-19  Paul Eggert  <eggert@cs.ucla.edu>
 
 	maint: mktime.c now shared with glibc
 	* config/srclist.txt: intprops.h, timegm.c and mktime.c
 	are now the same in Gnulib and glibc.
diff --git a/lib/vasnprintf.c b/lib/vasnprintf.c
index 56ffbe308..30d021b28 100644
--- a/lib/vasnprintf.c
+++ b/lib/vasnprintf.c
@@ -858,11 +858,13 @@ convert_to_decimal (mpn_t a, size_t extra_zeroes)
 {
   mp_limb_t *a_ptr = a.limbs;
   size_t a_len = a.nlimbs;
   /* 0.03345 is slightly larger than log(2)/(9*log(10)).  */
   size_t c_len = 9 * ((size_t)(a_len * (GMP_LIMB_BITS * 0.03345f)) + 1);
-  char *c_ptr = (char *) malloc (xsum (c_len, extra_zeroes));
+  /* We need extra_zeroes bytes for zeroes, followed by c_len bytes for the
+     digits of a, followed by 1 byte for the terminating NUL.  */
+  char *c_ptr = (char *) malloc (xsum (xsum (extra_zeroes, c_len), 1));
   if (c_ptr != NULL)
     {
       char *d_ptr = c_ptr;
       for (; extra_zeroes > 0; extra_zeroes--)
         *d_ptr++ = '0';
diff --git a/tests/test-vasnprintf.c b/tests/test-vasnprintf.c
index 19731bc93..93d81d704 100644
--- a/tests/test-vasnprintf.c
+++ b/tests/test-vasnprintf.c
@@ -51,11 +51,30 @@ test_function (char * (*my_asnprintf) (char *, size_t *, const char *, ...))
       length = size;
       result = my_asnprintf (buf, &length, "%d", 12345);
       ASSERT (result != NULL);
       ASSERT (strcmp (result, "12345") == 0);
       ASSERT (length == 5);
-      if (size < 6)
+      if (size < 5 + 1)
+        ASSERT (result != buf);
+      ASSERT (memcmp (buf + size, &"DEADBEEF"[size], 8 - size) == 0);
+      if (result != buf)
+        free (result);
+    }
+
+  /* Note: This test assumes IEEE 754 representation of 'double' floats.  */
+  for (size = 0; size <= 8; size++)
+    {
+      size_t length;
+      char *result;
+
+      memcpy (buf, "DEADBEEF", 8);
+      length = size;
+      result = my_asnprintf (buf, &length, "%2.0f", 1.6314159265358979e+125);
+      ASSERT (result != NULL);
+      ASSERT (strcmp (result, "163141592653589790215729350939528493057529598899734151772468186268423257777068536614838678161083520756952076273094236944990208") == 0);
+      ASSERT (length == 126);
+      if (size < 126 + 1)
         ASSERT (result != buf);
       ASSERT (memcmp (buf + size, &"DEADBEEF"[size], 8 - size) == 0);
       if (result != buf)
         free (result);
     }

Vulnerable code:   char *c_ptr = (char *) malloc (xsum (c_len, extra_zeroes));
      if (size < 6)
Fixed code: 2018-09-23  Bruno Haible  <bruno@clisp.org>

	vasnprintf: Fix heap memory overrun bug.
	Reported by Ben Pfaff <blp@cs.stanford.edu> in
	<https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00107.html>.
	* lib/vasnprintf.c (convert_to_decimal): Allocate one more byte of
	memory.
	* tests/test-vasnprintf.c (test_function): Add another test.

  /* We need extra_zeroes bytes for zeroes, followed by c_len bytes for the
     digits of a, followed by 1 byte for the terminating NUL.  */
  char *c_ptr = (char *) malloc (xsum (xsum (extra_zeroes, c_len), 1));
      if (size < 5 + 1)
        ASSERT (result != buf);
      ASSERT (memcmp (buf + size, &"DEADBEEF"[size], 8 - size) == 0);
      if (result != buf)
        free (result);
    }

  /* Note: This test assumes IEEE 754 representation of 'double' floats.  */
  for (size = 0; size <= 8; size++)
    {
      size_t length;
      char *result;

      memcpy (buf, "DEADBEEF", 8);
      length = size;
      result = my_asnprintf (buf, &length, "%2.0f", 1.6314159265358979e+125);
      ASSERT (result != NULL);
      ASSERT (strcmp (result, "163141592653589790215729350939528493057529598899734151772468186268423257777068536614838678161083520756952076273094236944990208") == 0);
      ASSERT (length == 126);
      if (size < 126 + 1)