Add a workaround for the midi issue.

Reviewed by:	so

share/security/advisories/FreeBSD-SA-19:23.midi.asc

@@ -23,6 +23,11 @@ For general information regarding FreeBSD Security Advisories,
 including descriptions of the fields above, security branches, and the
 following sections, please visit <URL:https://security.FreeBSD.org/>.
 
+0.   Revision history
+
+v1.0   2019-08-20  Initial release.
+v1.1   2019-08-21  Updated workaround.
+
 I.   Background
 
 /dev/midistat is a device file which can be read to obtain a
@@ -47,8 +52,13 @@ fault in kernel mode, leading to a panic.
 
 IV.  Workaround
 
-No workaround is available.  Custom kernels without "device sound"
-are not vulnerable.
+Restrict permissions on /dev/midistat by adding an entry to
+/etc/devfs.conf and restarting the service:
+
+# echo "perm midistat 0600" >> /etc/devfs.conf
+# service devfs restart
+
+Custom kernels without "device sound" are not vulnerable.
 
 V.   Solution
 
@@ -120,19 +130,19 @@ The latest revision of this advisory is available at
 <URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:23.midi.asc>
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