openssh/openssh-7.2p2-prevent_timing_user_enumeration.patch

# HG changeset patch
# Parent  4a254abf4ef391358257310ad2fe15c9e12dee34
Prevent user enumeration through password processing timing
CVE-2016-6210
bsc#989363

non-PAM part:
upstream commit: 9286875a73b2de7736b5e50692739d314cd8d9dc

PAM part:
upstream commit: 283b97ff33ea2c641161950849931bd578de6946

diff --git a/openssh-7.2p2/auth-pam.c b/openssh-7.2p2/auth-pam.c
--- a/openssh-7.2p2/auth-pam.c
+++ b/openssh-7.2p2/auth-pam.c
@@ -227,17 +227,16 @@ static pam_handle_t *sshpam_handle = NUL
 static int sshpam_err = 0;
 static int sshpam_authenticated = 0;
 static int sshpam_session_open = 0;
 static int sshpam_cred_established = 0;
 static int sshpam_account_status = -1;
 static char **sshpam_env = NULL;
 static Authctxt *sshpam_authctxt = NULL;
 static const char *sshpam_password = NULL;
-static char badpw[] = "\b\n\r\177INCORRECT";
 
 /* Some PAM implementations don't implement this */
 #ifndef HAVE_PAM_GETENVLIST
 static char **
 pam_getenvlist(pam_handle_t *pamh)
 {
 	/*
 	 * XXX - If necessary, we can still support envrionment passing
@@ -807,22 +806,45 @@ sshpam_query(void *ctx, char **name, cha
 			free(msg);
 			ctxt->pam_done = -1;
 			return (-1);
 		}
 	}
 	return (-1);
 }
 
+/*
+ * Returns a junk password of identical length to that the user supplied.
+ * Used to mitigate timing attacks against crypt(3)/PAM stacks that
+ * vary processing time in proportion to password length.
+ */
+static char *
+fake_password(const char *wire_password)
+{
+	const char junk[] = "\b\n\r\177INCORRECT";
+	char *ret = NULL;
+	size_t i, l = wire_password != NULL ? strlen(wire_password) : 0;
+
+	if (l >= INT_MAX)
+		fatal("%s: password length too long: %zu", __func__, l);
+
+	ret = xmalloc(l + 1);
+	for (i = 0; i < l; i++)
+		ret[i] = junk[i % (sizeof(junk) - 1)];
+	ret[i] = '\0';
+	return ret;
+}
+
 /* XXX - see also comment in auth-chall.c:verify_response */
 static int
 sshpam_respond(void *ctx, u_int num, char **resp)
 {
 	Buffer buffer;
 	struct pam_ctxt *ctxt = ctx;
+	char *fake;
 
 	debug2("PAM: %s entering, %u responses", __func__, num);
 	switch (ctxt->pam_done) {
 	case 1:
 		sshpam_authenticated = 1;
 		return (0);
 	case 0:
 		break;
@@ -833,18 +855,21 @@ sshpam_respond(void *ctx, u_int num, cha
 		error("PAM: expected one response, got %u", num);
 		return (-1);
 	}
 	buffer_init(&buffer);
 	if (sshpam_authctxt->valid &&
 	    (sshpam_authctxt->pw->pw_uid != 0 ||
 	    options.permit_root_login == PERMIT_YES))
 		buffer_put_cstring(&buffer, *resp);
-	else
-		buffer_put_cstring(&buffer, badpw);
+	else {
+		fake = fake_password(*resp);
+		buffer_put_cstring(&buffer, fake);
+		free(fake);
+	}
 	if (ssh_msg_send(ctxt->pam_psock, PAM_AUTHTOK, &buffer) == -1) {
 		buffer_free(&buffer);
 		return (-1);
 	}
 	buffer_free(&buffer);
 	return (1);
 }
 
@@ -1178,41 +1203,43 @@ static struct pam_conv passwd_conv = { s
 /*
  * Attempt password authentication via PAM
  */
 int
 sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 {
 	int flags = (options.permit_empty_passwd == 0 ?
 	    PAM_DISALLOW_NULL_AUTHTOK : 0);
+	char *fake = NULL;
 
 	if (!options.use_pam || sshpam_handle == NULL)
 		fatal("PAM: %s called when PAM disabled or failed to "
 		    "initialise.", __func__);
 
 	sshpam_password = password;
 	sshpam_authctxt = authctxt;
 
 	/*
 	 * If the user logging in is invalid, or is root but is not permitted
 	 * by PermitRootLogin, use an invalid password to prevent leaking
 	 * information via timing (eg if the PAM config has a delay on fail).
 	 */
 	if (!authctxt->valid || (authctxt->pw->pw_uid == 0 &&
 	    options.permit_root_login != PERMIT_YES))
-		sshpam_password = badpw;
+		sshpam_password = fake = fake_password(password);
 
 	sshpam_err = pam_set_item(sshpam_handle, PAM_CONV,
 	    (const void *)&passwd_conv);
 	if (sshpam_err != PAM_SUCCESS)
 		fatal("PAM: %s: failed to set PAM_CONV: %s", __func__,
 		    pam_strerror(sshpam_handle, sshpam_err));
 
 	sshpam_err = pam_authenticate(sshpam_handle, flags);
 	sshpam_password = NULL;
+	free(fake);
 	if (sshpam_err == PAM_SUCCESS && authctxt->valid) {
 		debug("PAM: password authentication accepted for %.100s",
 		    authctxt->user);
 		return 1;
 	} else {
 		debug("PAM: password authentication failed for %.100s: %s",
 		    authctxt->valid ? authctxt->user : "an illegal user",
 		    pam_strerror(sshpam_handle, sshpam_err));
diff --git a/openssh-7.2p2/auth-passwd.c b/openssh-7.2p2/auth-passwd.c
--- a/openssh-7.2p2/auth-passwd.c
+++ b/openssh-7.2p2/auth-passwd.c
@@ -188,28 +188,32 @@ sys_auth_passwd(Authctxt *authctxt, cons
 		return (auth_close(as));
 	}
 }
 #elif !defined(CUSTOM_SYS_AUTH_PASSWD)
 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;
 
 	/* Check for users with no password. */
 	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
 	 * are identical.
 	 */
 	return encrypted_password != NULL &&
 	    strcmp(encrypted_password, pw_password) == 0;
 }
diff --git a/openssh-7.2p2/openbsd-compat/xcrypt.c b/openssh-7.2p2/openbsd-compat/xcrypt.c
--- a/openssh-7.2p2/openbsd-compat/xcrypt.c
+++ b/openssh-7.2p2/openbsd-compat/xcrypt.c
@@ -20,16 +20,17 @@
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "includes.h"
 
 #include <sys/types.h>
+#include <string.h>
 #include <unistd.h>
 #include <pwd.h>
 
 # if defined(HAVE_CRYPT_H) && !defined(HAVE_SECUREWARE)
 #  include <crypt.h>
 # endif
 
 # ifdef __hpux
@@ -57,21 +58,54 @@
 #  include "md5crypt.h"
 # endif
 
 # if defined(WITH_OPENSSL) && !defined(HAVE_CRYPT) && defined(HAVE_DES_CRYPT)
 #  include <openssl/des.h>
 #  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);
         else
                 crypted = crypt(password, salt);
 # elif defined(__hpux) && !defined(HAVE_SECUREWARE)
 	if (iscomsec())
                 crypted = bigcrypt(password, salt);