set sshpam_ctxt to NULL after free

Avoids use-after-free in monitor when privsep child is compromised.
Reported by Moritz Jodeit; ok dtucker@

1 files changed, 3 insertions, 1 deletions

diff --git a/monitor.c b/monitor.c
index f1b873dc4..a91420983 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1166,14 +1166,16 @@ mm_answer_pam_respond(int sock, Buffer *m)
 int
 mm_answer_pam_free_ctx(int sock, Buffer *m)
 {
+        int r = sshpam_authok != NULL && sshpam_authok == sshpam_ctxt;
 
         debug3("%s", __func__);
         (sshpam_device.free_ctx)(sshpam_ctxt);
+        sshpam_ctxt = sshpam_authok = NULL;
         buffer_clear(m);
         mm_request_send(sock, MONITOR_ANS_PAM_FREE_CTX, m);
         auth_method = "keyboard-interactive";
         auth_submethod = "pam";
-        return (sshpam_authok == sshpam_ctxt);
+        return r;
 }
 #endif