Fix bug #6911 - Kerberos authentication from vista to samba fails when security blob size is greater than 16 kB

We were not correctly checking the output of asn1_start_tag().
asn1_start_tag() returns -1 and sets data->has_error if the
remaining blob size is too short to contain the tag length.
We were checking data->has_error and returning NT_STATUS_OK
(to allow the second asn.1 parse to fail in that case). We
should not be checking data->has_error in this case, but
falling through to the code that already checks the length.

Thanks to Jim for reproducing this for me. We don't get bitten
by this as we announce a max buffer size of 16k, greater than
Windows's 4k, which means that most krb5 spnego packets already
fit.

Jeremy.
(cherry picked from commit c718b7d43bc5616f7f5e110afdf5332a62e04cb8)

1 files changed, 25 insertions, 3 deletions

diff --git a/source3/smbd/sesssetup.c b/source3/smbd/sesssetup.c
index ab9d8551576..0de5a4bb653 100644
--- a/source3/smbd/sesssetup.c
+++ b/source3/smbd/sesssetup.c
@@ -1101,13 +1101,28 @@ static NTSTATUS check_spnego_blob_complete(struct smbd_server_connection *sconn,
 	}
 
 	asn1_load(data, *pblob);
-	asn1_start_tag(data, pblob->data[0]);
-	if (data->has_error || data->nesting == NULL) {
+	if (asn1_start_tag(data, pblob->data[0])) {
+		/* asn1_start_tag checks if the given
+		   length of the blob is enough to complete
+		   the tag. If it returns true we know
+		   there is nothing to do - the blob is
+		   complete. */
 		asn1_free(data);
-		/* Let caller catch. */
 		return NT_STATUS_OK;
 	}
 
+	if (data->nesting == NULL) {
+		/* Incorrect tag, allocation failed,
+		   or reading the tag length failed.
+		   Let the caller catch. */
+		asn1_free(data);
+		return NT_STATUS_OK;
+	}
+
+	/* Here we know asn1_start_tag() has set data->has_error to true.
+	   asn1_tag_remaining() will have failed due to the given blob
+	   being too short. We need to work out how short. */
+
 	/* Integer wrap paranoia.... */
 
 	if (data->nesting->taglen + data->nesting->start < data->nesting->taglen ||
@@ -1136,6 +1151,13 @@ static NTSTATUS check_spnego_blob_complete(struct smbd_server_connection *sconn,
 
 	if (needed_len <= pblob->length) {
 		/* Nothing to do - blob is complete. */
+		/* THIS SHOULD NOT HAPPEN - asn1_start_tag()
+		   above should have caught this !!! */
+		DEBUG(0,("check_spnego_blob_complete: logic "
+			"error (needed_len = %u, "
+			"pblob->length = %u).\n",
+			(unsigned int)needed_len,
+			(unsigned int)pblob->length ));
 		return NT_STATUS_OK;
 	}