Re: [krbdev.mit.edu #8914] Invalid negative record length in keytab file

<URL: https://krbdev.mit.edu/rt/Ticket/Display.html?id=8914 >

Here's a patch to fail fast with a format error. It's not much code but still protects against this unlikely edge case.

--- /krb5-1.18.2.orig/src/lib/krb5/keytab/kt_file.c	2020-05-22 00:21:40.000000000 +0000
+++ /krb5-1.18.2/src/lib/krb5/keytab/kt_file.c	2020-07-01 19:16:42.000000000 +0000
@@ -921,6 +921,9 @@
             size = ntohl(size);
 
         if (size < 0) {
+            if (size == INT32_MIN)
+                return KRB5_KT_FORMAT;
+
             if (fseek(KTFILEP(id), -size, SEEK_CUR)) {
                 return errno;
             }
@@ -1347,6 +1350,8 @@
                 return errno;
         } else if (size < 0) {
             /* Empty record; use if it's big enough, seek past otherwise. */
+            if (size == INT32_MIN)
+                return KRB5_KT_FORMAT;
             size = -size;
             if (size >= *size_needed) {
                 *size_needed = size;
--- /krb5-1.18.2.orig/src/tests/t_keytab.py	2020-05-22 00:21:40.000000000 +0000
+++ /krb5-1.18.2/src/tests/t_keytab.py	2020-07-03 00:58:00.000000000 +0000
@@ -185,5 +185,13 @@
 test_addent(realm, 'exp', '-f')
 test_addent(realm, 'pexp', '-f')
 
+# Test for proper INT32_MIN record length handling.
+mark('invalid record length')
+f = open(realm.keytab, 'wb')
+f.write(b'\x05\x02\x80\x00\x00\x00')
+f.close()
+msg = 'Bad format in keytab while scanning keytab'
+realm.run([klist, '-k'], expected_code=1, expected_msg=msg)
+
 success('Keytab-related tests')
 success('Keytab-related tests')