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

["Laszlo Ersek" <lersek@redhat.com>]

On 10/15/19 18:56, Laszlo Ersek wrote:
> On 10/15/19 15:54, Laszlo Ersek wrote:
>> On 10/15/19 13:03, David Woodhouse wrote:
> 
>>> 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;
>>>  }
> 
>> (4) What happens if we call SSL_set_ex_data(), but a non-NULL value has
>> already been stored for the same index?
>>
>> Do we have to first fetch it with SSL_get_ex_data() and free it, or will
>> it be automatically freed with "free_func"?
>>
>> (Note: I think that, if we used a "new_func" for allocating anything,
>> this question could be relevant the very first time SSL_set_ex_data()
>> were called.)
> 
> A similar question:
> 
> is it possible that app_verify_callback() is called more frequently than
> SSL_set_ex_data() (in TlsSetVerify())?
> 
> Because that means that the frequency of SSL_set1_host() calls changes.
> Previously we'd call SSL_set1_host() once per TlsSetVerify(), but now it
> could be called multiple times per TlsSetVerify(). Is that the case?
> 
> If it is, is it OK?

Ehh, I failed to ask the actual question.

Is it OK to call X509_VERIFY_PARAM_set1*() multiple times -- basically,
every time just before we call X509_verify_cert()?

My concern is not with the crypto functionality, but whether we could be
leaking memory allocations.

Thanks
Laszlo

> To me the ownership of these strings (i.e., what component is
> responsible for freeing the strings) is impenetrable. :(
> 
> Even the UEFI 2.8 spec doesn't explain whether EFI_TLS_SET_SESSION_DATA
> saves the array of characters pointed-to by
> "EFI_TLS_VERIFY_HOST.HostName", or just the pointer itself. :/
> 
> Laszlo
> 
> 
>