Avoid passing null pointers to memcpy/memcmp

By a strict reading of the C standard, memcpy and memcmp have
undefined behavior if their pointer arguments aren't valid object
pointers, even if the length argument is 0.  Compilers are becoming
more aggressive about breaking code with undefined behavior, so we
should try to avoid it when possible.

In a krb5_data object, we frequently use NULL as the data value when
the length is 0.  Accordingly, we should avoid copying from or
comparing the data field of a length-0 krb5_data object.  Add checks
to our wrapper functions (like data_eq and k5_memdup) and to code
which works with possibly-empty krb5_data objects.  In a few places,
use wrapper functions to simplify the code rather than adding checks.

1 files changed, 8 insertions, 7 deletions

diff --git a/src/include/k5-int.h b/src/include/k5-int.h
index d80404759..a489ce383 100644
--- a/src/include/k5-int.h
+++ b/src/include/k5-int.h
@@ -2138,13 +2138,15 @@ krb5_error_code krb5_set_time_offsets(krb5_context, krb5_timestamp,
 static inline int
 data_eq(krb5_data d1, krb5_data d2)
 {
-    return (d1.length == d2.length && !memcmp(d1.data, d2.data, d1.length));
+    return (d1.length == d2.length && (d1.length == 0 ||
+                                       !memcmp(d1.data, d2.data, d1.length)));
 }
 
 static inline int
 data_eq_string (krb5_data d, const char *s)
 {
-    return (d.length == strlen(s) && !memcmp(d.data, s, d.length));
+    return (d.length == strlen(s) && (d.length == 0 ||
+                                      !memcmp(d.data, s, d.length)));
 }
 
 static inline krb5_data
@@ -2187,9 +2189,8 @@ alloc_data(krb5_data *data, unsigned int len)
 static inline int
 authdata_eq(krb5_authdata a1, krb5_authdata a2)
 {
-    return (a1.ad_type == a2.ad_type
-            && a1.length == a2.length
-            && !memcmp(a1.contents, a2.contents, a1.length));
+    return (a1.ad_type == a2.ad_type && a1.length == a2.length &&
+            (a1.length == 0 || !memcmp(a1.contents, a2.contents, a1.length)));
 }
 
 /* Allocate zeroed memory; set *code to 0 on success or ENOMEM on failure. */
@@ -2210,7 +2211,7 @@ k5memdup(const void *in, size_t len, krb5_error_code *code)
 {
     void *ptr = k5alloc(len, code);
 
-    if (ptr != NULL)
+    if (ptr != NULL && len > 0)
         memcpy(ptr, in, len);
     return ptr;
 }
@@ -2221,7 +2222,7 @@ k5memdup0(const void *in, size_t len, krb5_error_code *code)
 {
     void *ptr = k5alloc(len + 1, code);
 
-    if (ptr != NULL)
+    if (ptr != NULL && len > 0)
         memcpy(ptr, in, len);
     return ptr;
 }