Re: [edk2-devel] [PATCH v1 0/4] Support HTTPS HostName validation feature(CVE-2019-14553)

["David Woodhouse" <dwmw2@infradead.org>]

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On Mon, 2019-10-14 at 18:15 +0200, Laszlo Ersek wrote:
> My understanding is that a fix purely in edk2 -- that is, without
> advancing our openssl submodule reference at once -- is possible, based
> on your comment
> 
>   https://bugzilla.tianocore.org/show_bug.cgi?id=960#c32
> 
> Namely, edk2 commit 9396cdfeaa7a ("CryptoPkg: Add new TlsLib library",
> 2016-12-22) added a SSL_set_verify() call (in function TlsSetVerify()).
> The last argument of that call is currently NULL.
> 
> We should change that, to a callback function that implements what
> ssl_app_verify_callback() and match_cert_hostname() do, in your source file
> 
> http://git.infradead.org/users/dwmw2/openconnect.git/blob/HEAD:/openssl.c


Hm, you are lost in a twisty maze of verify callbacks, all alike.

Actually the one you can set with SSL_set_verify() isn't the one you
want. That's a low-level one, called from within the generic
X509_verify_cert() function.

The "app callback" in my OpenConnect example is set on the SSL_CTX not
the SSL object, and is called from the top-level
ssl_verify_cert_chain() function *instead* of X509_verify_cert().

It is X509_verify_cert() which can do the hostname/IP checks for us, if
we can only tell it that we want it to. But the X509_VERIFY_PARAM
object is private to the SSL.

As discussed, we have the SSL_set1_host() accessor function which lets
us set the hostname. The implementation really is a simple one-liner,
calling X509_VERIFY_PARAM_set1_host(s->param, …). But there's no way
for use to set the IP address from the outside, without an equivalent
accessor function for that (and without SSL_set1_host() spotting that
the string it's given is an IP address, and doing so).

But what we can do is stash the target string in some ex_data hanging
off the SSL object, then have an app callback — which *can* reach the
underlying X509_VERIFY_PARAM — call X509_VERIFY_PARAM_set1_host() or
X509_VERIFY_PARAM_set1_ip_asc() accordingly, before just calling the
normal X509_verify_cert() function that it has overridden.

Something like this... and instead of calling SSL_set1_host(ssl, host)
your own code now has to call
SSL_set_ex_data(ssl, ssl_target_idx, strdup(host));

diff --git a/CryptoPkg/Library/TlsLib/TlsInit.c b/CryptoPkg/Library/TlsLib/TlsInit.c
index f9ad6f6b946c..add5810cc4bd 100644
--- a/CryptoPkg/Library/TlsLib/TlsInit.c
+++ b/CryptoPkg/Library/TlsLib/TlsInit.c
@@ -9,6 +9,49 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 
 #include "InternalTlsLib.h"
 
+/* You are lost in a twisty maze of SSL cert verify callbacks, all
+ * alike.  All we really wanted to do was call SSL_set1_host() and
+ * have it work for IP addresses too, which OpenSSL PR#9201 will do
+ * for us. But until we update OpenSSL, that doesn't work. And we
+ * can't get at the underlying X509_VERIFY_PARAM to set the IP address
+ * for ourselves.
+ *
+ * So we install an app_verify_callback in the SSL_CTX (which is
+ * different to the per-SSL callback wae can use, because it happens
+ * sooner. All our callback does it set the hostname or IP address in
+ * the X509_VERIFY_PARAM like we wanted to in the first place, and
+ * then call X509_verify_param() which is the default function.
+ *
+ * How does it find the hostname/IP string? It's attached to the SSL
+ * as ex_data, using this index:
+ */
+static int ssl_target_idx;
+
+void ssl_target_free(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
+		    int idx, long argl, void *argp)
+{
+	/* Free it */
+}
+
+int ssl_target_dup(CRYPTO_EX_DATA *to, const CRYPTO_EX_DATA *from,
+		   void *from_d, int idx, long argl, void *argp)
+{
+	/* strdup it */
+	return 0;
+}
+
+int app_verify_callback(X509_STORE_CTX *ctx, void *dummy)
+{
+	SSL *ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
+	char *hostname = SSL_get_ex_data(ssl, ssl_target_idx);
+	X509_VERIFY_PARAM *vpm = X509_STORE_CTX_get0_param(ctx);
+
+	if (hostname && !X509_VERIFY_PARAM_set1_ip_asc(vpm, hostname))
+		X509_VERIFY_PARAM_set1_host(vpm, hostname, 0);
+
+	return X509_verify_cert(ctx);
+}
+
 /**
   Initializes the OpenSSL library.
 
@@ -40,6 +83,9 @@ TlsInitialize (
     return FALSE;
   }
 
+  ssl_target_idx = SSL_get_ex_new_index(0, "TLS target hosthame/IP", NULL,
+					ssl_target_dup, ssl_target_free);
+
   //
   // Initialize the pseudorandom number generator.
   //
@@ -106,6 +152,10 @@ TlsCtxNew (
   //
   SSL_CTX_set_min_proto_version (TlsCtx, ProtoVersion);
 
+  /* SSL_CTX_set_cert_verify_callback. Not SSL_CTX_set_verify(), which
+   * we could have done as SSL_set_verify(). Twisty maze, remember? */
+  SSL_CTX_set_cert_verify_callback(TlsCtx, app_verify_callback, NULL);
+
   return (VOID *) TlsCtx;
 }
 

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