From 9286875a73b2de7736b5e50692739d314cd8d9dc Mon Sep 17 00:00:00 2001
From: Darren Tucker <dtucker@zip.com.au>
Date: Fri, 15 Jul 2016 13:32:45 +1000
Subject: [PATCH] Determine appropriate salt for invalid users.

When sshd is processing a non-PAM login for a non-existent user it uses
the string from the fakepw structure as the salt for crypt(3)ing the
password supplied by the client.  That string has a Blowfish prefix, so on
systems that don't understand that crypt will fail fast due to an invalid
salt, and even on those that do it may have significantly different timing
from the hash methods used for real accounts (eg sha512).  This allows
user enumeration by, eg, sending large password strings.  This was noted
by EddieEzra.Harari at verint.com (CVE-2016-6210).

To mitigate, use the same hash algorithm that root uses for hashing
passwords for users that do not exist on the system.  ok djm@

Upstream-Status: Backport
OpenSSH  < 7.3
CVE: CVE-2016-6210 patch1
Signed-off-by: Armin Kuster <akuster@mvista.com>

---
 auth-passwd.c           | 12 ++++++++----
 openbsd-compat/xcrypt.c | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 4 deletions(-)

Index: openssh-7.1p2/auth-passwd.c
===================================================================
--- openssh-7.1p2.orig/auth-passwd.c
+++ openssh-7.1p2/auth-passwd.c
@@ -198,7 +198,7 @@ int
 sys_auth_passwd(Authctxt *authctxt, const char *password)
 {
 	struct passwd *pw = authctxt->pw;
-	char *encrypted_password;
+	char *encrypted_password, *salt = NULL;
 
 	/* Just use the supplied fake password if authctxt is invalid */
 	char *pw_password = authctxt->valid ? shadow_pw(pw) : pw->pw_passwd;
@@ -207,9 +207,13 @@ sys_auth_passwd(Authctxt *authctxt, cons
 	if (strcmp(pw_password, "") == 0 && strcmp(password, "") == 0)
 		return (1);
 
-	/* Encrypt the candidate password using the proper salt. */
-	encrypted_password = xcrypt(password,
-	    (pw_password[0] && pw_password[1]) ? pw_password : "xx");
+	/*
+	 * Encrypt the candidate password using the proper salt, or pass a
+	 * NULL and let xcrypt pick one.
+	 */
+	if (authctxt->valid && pw_password[0] && pw_password[1])
+		salt = pw_password;
+	encrypted_password = xcrypt(password, salt);
 
 	/*
 	 * Authentication is accepted if the encrypted passwords
Index: openssh-7.1p2/openbsd-compat/xcrypt.c
===================================================================
--- openssh-7.1p2.orig/openbsd-compat/xcrypt.c
+++ openssh-7.1p2/openbsd-compat/xcrypt.c
@@ -25,6 +25,7 @@
 #include "includes.h"
 
 #include <sys/types.h>
+#include <string.h>
 #include <unistd.h>
 #include <pwd.h>
 
@@ -62,11 +63,44 @@
 #  define crypt DES_crypt
 # endif
 
+/*
+ * Pick an appropriate password encryption type and salt for the running
+ * system.
+ */
+static const char *
+pick_salt(void)
+{
+	struct passwd *pw;
+	char *passwd, *p;
+	size_t typelen;
+	static char salt[32];
+
+	if (salt[0] != '\0')
+		return salt;
+	strlcpy(salt, "xx", sizeof(salt));
+	if ((pw = getpwuid(0)) == NULL)
+		return salt;
+	passwd = shadow_pw(pw);
+	if (passwd[0] != '$' || (p = strrchr(passwd + 1, '$')) == NULL)
+		return salt;  /* no $, DES */
+	typelen = p - passwd + 1;
+	strlcpy(salt, passwd, MIN(typelen, sizeof(salt)));
+	explicit_bzero(passwd, strlen(passwd));
+	return salt;
+}
+
 char *
 xcrypt(const char *password, const char *salt)
 {
 	char *crypted;
 
+	/*
+	 * If we don't have a salt we are encrypting a fake password for
+	 * for timing purposes.  Pick an appropriate salt.
+	 */
+	if (salt == NULL)
+		salt = pick_salt();
+
 # ifdef HAVE_MD5_PASSWORDS
         if (is_md5_salt(salt))
                 crypted = md5_crypt(password, salt);