gh-96931: Fix incorrect results in ssl.SSLSocket.shared_ciphers (#96932)

1 files changed, 28 insertions, 8 deletions

diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 321b6ec..36b66cd 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -1998,24 +1998,44 @@ static PyObject *
 _ssl__SSLSocket_shared_ciphers_impl(PySSLSocket *self)
 /*[clinic end generated code: output=3d174ead2e42c4fd input=0bfe149da8fe6306]*/
 {
-    STACK_OF(SSL_CIPHER) *ciphers;
-    int i;
+    STACK_OF(SSL_CIPHER) *server_ciphers;
+    STACK_OF(SSL_CIPHER) *client_ciphers;
+    int i, len;
     PyObject *res;
+    const SSL_CIPHER* cipher;
+
+    /* Rather than use SSL_get_shared_ciphers, we use an equivalent algorithm because:
+
+       1) It returns a colon seperated list of strings, in an undefined
+          order, that we would have to post process back into tuples.
+       2) It will return a truncated string with no indication that it has
+          done so, if the buffer is too small.
+     */
 
-    ciphers = SSL_get_ciphers(self->ssl);
-    if (!ciphers)
+    server_ciphers = SSL_get_ciphers(self->ssl);
+    if (!server_ciphers)
         Py_RETURN_NONE;
-    res = PyList_New(sk_SSL_CIPHER_num(ciphers));
+    client_ciphers = SSL_get_client_ciphers(self->ssl);
+    if (!client_ciphers)
+        Py_RETURN_NONE;
+
+    res = PyList_New(sk_SSL_CIPHER_num(server_ciphers));
     if (!res)
         return NULL;
-    for (i = 0; i < sk_SSL_CIPHER_num(ciphers); i++) {
-        PyObject *tup = cipher_to_tuple(sk_SSL_CIPHER_value(ciphers, i));
+    len = 0;
+    for (i = 0; i < sk_SSL_CIPHER_num(server_ciphers); i++) {
+        cipher = sk_SSL_CIPHER_value(server_ciphers, i);
+        if (sk_SSL_CIPHER_find(client_ciphers, cipher) < 0)
+            continue;
+
+        PyObject *tup = cipher_to_tuple(cipher);
         if (!tup) {
             Py_DECREF(res);
             return NULL;
         }
-        PyList_SET_ITEM(res, i, tup);
+        PyList_SET_ITEM(res, len++, tup);
     }
+    Py_SET_SIZE(res, len);
     return res;
 }