krb5 commit [krb5-1.13]: Fix build_principal memory bug [CVE-2015-2697]

[Tom Yu]

https://github.com/krb5/krb5/commit/41d987867a29a8a8fa6f99de63d9e05b8792e91a
commit 41d987867a29a8a8fa6f99de63d9e05b8792e91a
Author: Greg Hudson <ghudson at mit.edu>
Date:   Fri Sep 25 12:51:47 2015 -0400

    Fix build_principal memory bug [CVE-2015-2697]
    
    In build_principal_va(), use k5memdup0() instead of strdup() to make a
    copy of the realm, to ensure that we allocate the correct number of
    bytes and do not read past the end of the input string.  This bug
    affects krb5_build_principal(), krb5_build_principal_va(), and
    krb5_build_principal_alloc_va().  krb5_build_principal_ext() is not
    affected.
    
    CVE-2015-2697:
    
    In MIT krb5 1.7 and later, an authenticated attacker may be able to
    cause a KDC to crash using a TGS request with a large realm field
    beginning with a null byte.  If the KDC attempts to find a referral to
    answer the request, it constructs a principal name for lookup using
    krb5_build_principal() with the requested realm.  Due to a bug in this
    function, the null byte causes only one byte be allocated for the
    realm field of the constructed principal, far less than its length.
    Subsequent operations on the lookup principal may cause a read beyond
    the end of the mapped memory region, causing the KDC process to crash.
    
    CVSSv2: AV:N/AC:L/Au:S/C:N/I:N/A:C/E:POC/RL:OF/RC:C
    
    (cherry picked from commit f0c094a1b745d91ef2f9a4eae2149aac026a5789)
    (cherry picked from commit 67bdf8189b24efca8a244316e7d51bd52d0dbda9)
    
    ticket: 8283 (new)
    version_fixed: 1.13.3
    status: resolved

 src/lib/krb5/krb/bld_princ.c |    6 ++----
 1 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/lib/krb5/krb/bld_princ.c b/src/lib/krb5/krb/bld_princ.c
index ab6fed8..8604268 100644
--- a/src/lib/krb5/krb/bld_princ.c
+++ b/src/lib/krb5/krb/bld_princ.c
@@ -40,10 +40,8 @@ build_principal_va(krb5_context context, krb5_principal princ,
     data = malloc(size * sizeof(krb5_data));
     if (!data) { retval = ENOMEM; }
 
-    if (!retval) {
-        r = strdup(realm);
-        if (!r) { retval = ENOMEM; }
-    }
+    if (!retval)
+        r = k5memdup0(realm, rlen, &retval);
 
     while (!retval && (component = va_arg(ap, char *))) {
         if (count == size) {