krb5 commit: Allow gss_unwrap_iov() of unpadded RC4 tokens

[Greg Hudson]

https://github.com/krb5/krb5/commit/3f204ddd567715ef360b4bb0b32961b6a9877f9d
commit 3f204ddd567715ef360b4bb0b32961b6a9877f9d
Author: Greg Hudson <ghudson at mit.edu>
Date:   Sat Jul 11 21:57:30 2020 -0400

    Allow gss_unwrap_iov() of unpadded RC4 tokens
    
    Windows Remote Management, when used with an RC4 session key, appears
    to generate GSS wrap tokens with no padding instead of the expected
    one byte (RFC 4757 section 7.3).  These tokens cannot be decoded with
    gss_unwrap() or a STREAM buffer (even with Microsoft SSPI), but SSPI
    allows them to be decoded using explicit IOVs with either a
    zero-length padding buffer or no padding buffer.  Allow these cases to
    work in kg_fixup_padding_iov().  (It is already possible to make this
    work with HEADER | DATA | DATA, but only by
    accident--kg_fixup_padding_iov() doesn't find a data buffer because
    kg_locate_iov() only looks for singleton buffers, so it exits early.)
    
    ticket: 8926 (new)
    tags: pullup
    target_version: 1.18-next

 src/lib/gssapi/krb5/util_crypt.c |    9 +++------
 1 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/src/lib/gssapi/krb5/util_crypt.c b/src/lib/gssapi/krb5/util_crypt.c
index c9aaafd..84f1949 100644
--- a/src/lib/gssapi/krb5/util_crypt.c
+++ b/src/lib/gssapi/krb5/util_crypt.c
@@ -649,16 +649,13 @@ kg_fixup_padding_iov(OM_uint32 *minor_status, gss_iov_buffer_desc *iov,
     data = kg_locate_iov(iov, iov_count, GSS_IOV_BUFFER_TYPE_DATA);
     padding = kg_locate_iov(iov, iov_count, GSS_IOV_BUFFER_TYPE_PADDING);
 
-    if (data == NULL) {
+    /* Do nothing if padding is absent or empty, to allow unwrapping of WinRM
+     * unpadded RC4 tokens using an explicit IOV array. */
+    if (data == NULL || padding == NULL || padding->buffer.length == 0) {
         *minor_status = 0;
         return GSS_S_COMPLETE;
     }
 
-    if (padding == NULL || padding->buffer.length == 0) {
-        *minor_status = EINVAL;
-        return GSS_S_FAILURE;
-    }
-
     p = (unsigned char *)padding->buffer.value;
     padlength = p[padding->buffer.length - 1];