pythongh-100372: Use BIO_eof to detect EOF for SSL_FILETYPE_ASN1 (pythonGH-100373)

In PEM, we need to parse until error and then suppress `PEM_R_NO_START_LINE`, because PEM allows arbitrary leading and trailing data. DER, however, does not. Parsing until error and suppressing `ASN1_R_HEADER_TOO_LONG` doesn't quite work because that error also covers some cases that should be rejected.

Instead, check `BIO_eof` early and stop the loop that way.

Automerge-Triggered-By: GH:Yhg1s

Author: davidben

Lib/test/test_ssl.py

@@ -1289,6 +1289,8 @@ def test_load_verify_cadata(self):
             "not enough data: cadata does not contain a certificate"
         ):
             ctx.load_verify_locations(cadata=b"broken")
+        with self.assertRaises(ssl.SSLError):
+            ctx.load_verify_locations(cadata=cacert_der + b"A")
 
     @unittest.skipIf(Py_DEBUG_WIN32, "Avoid mixing debug/release CRT on Windows")
     def test_load_dh_params(self):

Misc/NEWS.d/next/Library/2022-12-20-10-55-14.gh-issue-100372.utfP65.rst

@@ -0,0 +1,2 @@
+:meth:`ssl.SSLContext.load_verify_locations` no longer incorrectly accepts
+some cases of trailing data when parsing DER.

Modules/_ssl.c

@@ -3930,7 +3930,7 @@ _add_ca_certs(PySSLContext *self, const void *data, Py_ssize_t len,
 {
     BIO *biobuf = NULL;
     X509_STORE *store;
-    int retval = -1, err, loaded = 0;
+    int retval = -1, err, loaded = 0, was_bio_eof = 0;
 
     assert(filetype == SSL_FILETYPE_ASN1 || filetype == SSL_FILETYPE_PEM);
 
@@ -3958,6 +3958,10 @@ _add_ca_certs(PySSLContext *self, const void *data, Py_ssize_t len,
         int r;
 
         if (filetype == SSL_FILETYPE_ASN1) {
+            if (BIO_eof(biobuf)) {
+                was_bio_eof = 1;
+                break;
+            }
             cert = d2i_X509_bio(biobuf, NULL);
         } else {
             cert = PEM_read_bio_X509(biobuf, NULL,
@@ -3993,9 +3997,7 @@ _add_ca_certs(PySSLContext *self, const void *data, Py_ssize_t len,
         }
         _setSSLError(get_state_ctx(self), msg, 0, __FILE__, __LINE__);
         retval = -1;
-    } else if ((filetype == SSL_FILETYPE_ASN1) &&
-                    (ERR_GET_LIB(err) == ERR_LIB_ASN1) &&
-                    (ERR_GET_REASON(err) == ASN1_R_HEADER_TOO_LONG)) {
+    } else if ((filetype == SSL_FILETYPE_ASN1) && was_bio_eof) {
         /* EOF ASN1 file, not an error */
         ERR_clear_error();
         retval = 0;