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From: Mark Johnston <markj@FreeBSD.org>
Date: Wed, 1 Aug 2018 03:46:07 +0000 (UTC)
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Subject: svn commit: r337031 - head/sys/security/mac
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Author: markj
Date: Wed Aug  1 03:46:07 2018
New Revision: 337031
URL: https://svnweb.freebsd.org/changeset/base/337031

Log:
  Require that MAC label buffers be able to store a non-empty string.
  
  The buffer size may be used to initialize an sbuf in
  MAC_POLICY_EXTERNALIZE, and without this constraint it's possible to
  trigger an assertion failure in the sbuf code.  With INVARIANTS
  disabled, the first attempt to write to the sbuf will fail.
  
  Reported by:	pho
  Reviewed by:	delphij
  MFC after:	1 week
  Differential Revision:	https://reviews.freebsd.org/D16527

Modified:
  head/sys/security/mac/mac_framework.c

Modified: head/sys/security/mac/mac_framework.c
==============================================================================
--- head/sys/security/mac/mac_framework.c	Wed Aug  1 03:21:17 2018	(r337030)
+++ head/sys/security/mac/mac_framework.c	Wed Aug  1 03:46:07 2018	(r337031)
@@ -586,7 +586,9 @@ int
 mac_check_structmac_consistent(struct mac *mac)
 {
 
-	if (mac->m_buflen > MAC_MAX_LABEL_BUF_LEN)
+	/* Require that labels have a non-zero length. */
+	if (mac->m_buflen > MAC_MAX_LABEL_BUF_LEN ||
+	    mac->m_buflen <= sizeof(""))
 		return (EINVAL);
 
 	return (0);