CVE-2016-0798: avoid memory leak in SRP

The SRP user database lookup method SRP_VBASE_get_by_user had confusing
memory management semantics; the returned pointer was sometimes newly
allocated, and sometimes owned by the callee. The calling code has no
way of distinguishing these two cases.

Specifically, SRP servers that configure a secret seed to hide valid
login information are vulnerable to a memory leak: an attacker
connecting with an invalid username can cause a memory leak of around
300 bytes per connection.

Servers that do not configure SRP, or configure SRP but do not configure
a seed are not vulnerable.

In Apache, the seed directive is known as SSLSRPUnknownUserSeed.

To mitigate the memory leak, the seed handling in SRP_VBASE_get_by_user
is now disabled even if the user has configured a seed.

Applications are advised to migrate to SRP_VBASE_get1_by_user. However,
note that OpenSSL makes no strong guarantees about the
indistinguishability of valid and invalid logins. In particular,
computations are currently not carried out in constant time.

Reviewed-by: Rich Salz <rsalz@openssl.org>

1 files changed, 28 insertions, 16 deletions

diff --git a/apps/s_server.c b/apps/s_server.c
index 1380628..6645dc8 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -352,6 +352,8 @@ typedef struct srpsrvparm_st {
 static int ssl_srp_server_param_cb(SSL *s, int *ad, void *arg)
 {
     srpsrvparm *p = (srpsrvparm *) arg;
+    int ret = SSL3_AL_FATAL;
+
     if (p->login == NULL && p->user == NULL) {
         p->login = SSL_get_srp_username(s);
         BIO_printf(bio_err, "SRP username = \"%s\"\n", p->login);
@@ -360,21 +362,25 @@ static int ssl_srp_server_param_cb(SSL *s, int *ad, void *arg)
 
     if (p->user == NULL) {
         BIO_printf(bio_err, "User %s doesn't exist\n", p->login);
-        return SSL3_AL_FATAL;
+        goto err;
     }
+
     if (SSL_set_srp_server_param
         (s, p->user->N, p->user->g, p->user->s, p->user->v,
          p->user->info) < 0) {
         *ad = SSL_AD_INTERNAL_ERROR;
-        return SSL3_AL_FATAL;
+        goto err;
     }
     BIO_printf(bio_err,
                "SRP parameters set: username = \"%s\" info=\"%s\" \n",
                p->login, p->user->info);
-    /* need to check whether there are memory leaks */
+    ret = SSL_ERROR_NONE;
+
+err:
+    SRP_user_pwd_free(p->user);
     p->user = NULL;
     p->login = NULL;
-    return SSL_ERROR_NONE;
+    return ret;
 }
 
 #endif
@@ -2325,9 +2331,10 @@ static int sv_body(int s, int stype, unsigned char *context)
 #ifndef OPENSSL_NO_SRP
                 while (SSL_get_error(con, k) == SSL_ERROR_WANT_X509_LOOKUP) {
                     BIO_printf(bio_s_out, "LOOKUP renego during write\n");
+                    SRP_user_pwd_free(srp_callback_parm.user);
                     srp_callback_parm.user =
-                        SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                              srp_callback_parm.login);
+                        SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                               srp_callback_parm.login);
                     if (srp_callback_parm.user)
                         BIO_printf(bio_s_out, "LOOKUP done %s\n",
                                    srp_callback_parm.user->info);
@@ -2393,9 +2400,10 @@ static int sv_body(int s, int stype, unsigned char *context)
 #ifndef OPENSSL_NO_SRP
                 while (SSL_get_error(con, i) == SSL_ERROR_WANT_X509_LOOKUP) {
                     BIO_printf(bio_s_out, "LOOKUP renego during read\n");
+                    SRP_user_pwd_free(srp_callback_parm.user);
                     srp_callback_parm.user =
-                        SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                              srp_callback_parm.login);
+                        SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                               srp_callback_parm.login);
                     if (srp_callback_parm.user)
                         BIO_printf(bio_s_out, "LOOKUP done %s\n",
                                    srp_callback_parm.user->info);
@@ -2520,9 +2528,10 @@ static int init_ssl_connection(SSL *con)
         while (i <= 0 && SSL_get_error(con, i) == SSL_ERROR_WANT_X509_LOOKUP) {
             BIO_printf(bio_s_out, "LOOKUP during accept %s\n",
                        srp_callback_parm.login);
+            SRP_user_pwd_free(srp_callback_parm.user);
             srp_callback_parm.user =
-                SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                      srp_callback_parm.login);
+                SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                       srp_callback_parm.login);
             if (srp_callback_parm.user)
                 BIO_printf(bio_s_out, "LOOKUP done %s\n",
                            srp_callback_parm.user->info);
@@ -2732,9 +2741,10 @@ static int www_body(int s, int stype, unsigned char *context)
                 if (BIO_should_io_special(io)
                     && BIO_get_retry_reason(io) == BIO_RR_SSL_X509_LOOKUP) {
                     BIO_printf(bio_s_out, "LOOKUP renego during read\n");
+                    SRP_user_pwd_free(srp_callback_parm.user);
                     srp_callback_parm.user =
-                        SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                              srp_callback_parm.login);
+                        SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                               srp_callback_parm.login);
                     if (srp_callback_parm.user)
                         BIO_printf(bio_s_out, "LOOKUP done %s\n",
                                    srp_callback_parm.user->info);
@@ -3093,9 +3103,10 @@ static int rev_body(int s, int stype, unsigned char *context)
         if (BIO_should_io_special(io)
             && BIO_get_retry_reason(io) == BIO_RR_SSL_X509_LOOKUP) {
             BIO_printf(bio_s_out, "LOOKUP renego during accept\n");
+            SRP_user_pwd_free(srp_callback_parm.user);
             srp_callback_parm.user =
-                SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                      srp_callback_parm.login);
+                SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                       srp_callback_parm.login);
             if (srp_callback_parm.user)
                 BIO_printf(bio_s_out, "LOOKUP done %s\n",
                            srp_callback_parm.user->info);
@@ -3121,9 +3132,10 @@ static int rev_body(int s, int stype, unsigned char *context)
                 if (BIO_should_io_special(io)
                     && BIO_get_retry_reason(io) == BIO_RR_SSL_X509_LOOKUP) {
                     BIO_printf(bio_s_out, "LOOKUP renego during read\n");
+                    SRP_user_pwd_free(srp_callback_parm.user);
                     srp_callback_parm.user =
-                        SRP_VBASE_get_by_user(srp_callback_parm.vb,
-                                              srp_callback_parm.login);
+                        SRP_VBASE_get1_by_user(srp_callback_parm.vb,
+                                               srp_callback_parm.login);
                     if (srp_callback_parm.user)
                         BIO_printf(bio_s_out, "LOOKUP done %s\n",
                                    srp_callback_parm.user->info);