enable SSL_MODE_RELEASE_BUFFERS

Patch by Cory Benfield.

Misc/NEWS

@@ -41,6 +41,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25672: In the ssl module, enable the SSL_MODE_RELEASE_BUFFERS mode
+  option if it is safe to do so.
+
 - Issue #22570: Add 'path' attribute to pathlib.Path objects,
   returning the same as str(), to make it more similar to DirEntry.
   Library code can now write getattr(p, 'path', p) to get the path as

Modules/_ssl.c

@@ -2219,6 +2219,7 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
     PySSLContext *self;
     long options;
     SSL_CTX *ctx = NULL;
+    unsigned long libver;
 
     PySSL_BEGIN_ALLOW_THREADS
     if (proto_version == PY_SSL_VERSION_TLS1)
@@ -2281,6 +2282,22 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         options |= SSL_OP_NO_SSLv3;
     SSL_CTX_set_options(self->ctx, options);
 
+#if defined(SSL_MODE_RELEASE_BUFFERS)
+    /* Set SSL_MODE_RELEASE_BUFFERS. This potentially greatly reduces memory
+       usage for no cost at all. However, don't do this for OpenSSL versions
+       between 1.0.1 and 1.0.1h or 1.0.0 and 1.0.0m, which are affected by CVE
+       2014-0198. I can't find exactly which beta fixed this CVE, so be
+       conservative and assume it wasn't fixed until release. We do this check
+       at runtime to avoid problems from the dynamic linker.
+       See #25672 for more on this. */
+    libver = SSLeay();
+    if (!(libver >= 0x10001000UL && libver < 0x1000108fUL) &&
+        !(libver >= 0x10000000UL && libver < 0x100000dfUL)) {
+        SSL_CTX_set_mode(self->ctx, SSL_MODE_RELEASE_BUFFERS);
+    }
+#endif
+
+
 #ifndef OPENSSL_NO_ECDH
     /* Allow automatic ECDH curve selection (on OpenSSL 1.0.2+), or use
        prime256v1 by default.  This is Apache mod_ssl's initialization