Make memwipe() do nothing when passed a NULL pointer or zero size

Check size argument to memwipe() for underflow.

Closes bug #18089. Reported by "gk", patch by "teor".
Bugfix on 0.2.3.25 and 0.2.4.6-alpha (#7352),
commit 49dd5ef3 on 7 Nov 2012.

changes/bug18089

+  o Minor fixes (security):
+    - Make memwipe() do nothing when passed a NULL pointer
+      or zero size. Check size argument to memwipe() for underflow.
+      Closes bug #18089. Reported by "gk", patch by "teor".
+      Bugfix on 0.2.3.25 and 0.2.4.6-alpha (#7352),
+      commit 49dd5ef3 on 7 Nov 2012.

src/common/crypto.c

@@ -3012,6 +3012,7 @@ base32_decode(char *dest, size_t destlen, const char *src, size_t srclen)
 /**
  * Destroy the <b>sz</b> bytes of data stored at <b>mem</b>, setting them to
  * the value <b>byte</b>.
+ * If <b>mem</b> is NULL or <b>sz</b> is zero, nothing happens.
  *
  * This function is preferable to memset, since many compilers will happily
  * optimize out memset() when they can convince themselves that the data being
@@ -3029,6 +3030,13 @@ base32_decode(char *dest, size_t destlen, const char *src, size_t srclen)
 void
 memwipe(void *mem, uint8_t byte, size_t sz)
 {
+  if (mem == NULL || sz == 0) {
+    return;
+  }
+
+  /* Data this large is likely to be an underflow. */
+  tor_assert(sz < SIZE_T_CEILING);
+
   /* Because whole-program-optimization exists, we may not be able to just
    * have this function call "memset".  A smart compiler could inline it, then
    * eliminate dead memsets, and declare itself to be clever. */