1 files changed, 31 insertions, 4 deletions

diff --git a/auth-pam.c b/auth-pam.c
index 8425af1ea..abd6a5e1b 100644
--- a/auth-pam.c
+++ b/auth-pam.c
@@ -232,7 +232,6 @@ 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
@@ -810,12 +809,35 @@ sshpam_query(void *ctx, char **name, char **info,
         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 = malloc(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) {
@@ -836,8 +858,11 @@ sshpam_respond(void *ctx, u_int num, char **resp)
             (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);
@@ -1181,6 +1206,7 @@ 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 "
@@ -1196,7 +1222,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
          */
         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);
@@ -1206,6 +1232,7 @@ sshpam_auth_passwd(Authctxt *authctxt, const char *password)
 
         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);