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Allow an ALPN callback to pretend to not exist

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RFC 7301 mandates that the server SHALL respond with a fatal
"no_application_protocol" alert when there is no overlap between
the client's supplied list and the server's list of supported protocols.
In commit 062178678f5374b09f00d70796f6e692e8775aca we changed from
ignoring non-success returns from the supplied alpn_select_cb() to
treating such non-success returns as indicative of non-overlap and
sending the fatal alert.

In effect, this is using the presence of an alpn_select_cb() as a proxy
to attempt to determine whether the application has configured a list
of supported protocols.  However, there may be cases in which an
application's architecture leads it to supply an alpn_select_cb() but
have that callback be configured to take no action on connections that
do not have ALPN configured; returning SSL_TLSEXT_ERR_NOACK from
the callback would be the natural way to do so.  Unfortunately, the
aforementioned behavior change also treated SSL_TLSEXT_ERR_NOACK as
indicative of no overlap and terminated the connection; this change
supplies special handling for SSL_TLSEXT_ERR_NOACK returns from the
callback.  In effect, it provides a way for a callback to obtain the
behavior that would have occurred if no callback was registered at
all, which was not possible prior to this change.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2570)
This commit is contained in:
Benjamin Kaduk 2017-02-07 16:23:16 -06:00 committed by Rich Salz
parent f120fa1efe
commit 8313a787d7
3 changed files with 11 additions and 2 deletions
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8
doc/man3/SSL_CTX_set_alpn_select_cb.pod
View File

@ -113,9 +113,15 @@ The ALPN select callback B<cb>, must return one of the following:
ALPN protocol selected.
=item SSL_TLSEXT_ERR_ALERT_FATAL
There was no overlap between the client's supplied list and the server
configuration.
=item SSL_TLSEXT_ERR_NOACK
ALPN protocol not selected.
ALPN protocol not selected, e.g., because no ALPN protocols are configured for
this connection.
=back

3
ssl/statem/extensions.c
View File

@ -919,6 +919,9 @@ static int final_alpn(SSL *s, unsigned int context, int sent, int *al)
/* ALPN takes precedence over NPN. */
s->s3->npn_seen = 0;
#endif
} else if (r == SSL_TLSEXT_ERR_NOACK) {
/* Behave as if no callback was present. */
return 1;
} else {
*al = SSL_AD_NO_APPLICATION_PROTOCOL;
return 0;

2
test/handshake_helper.c
View File

@ -413,7 +413,7 @@ static int server_alpn_cb(SSL *s, const unsigned char **out,
*out = tmp_out;
/* Unlike NPN, we don't tolerate a mismatch. */
return ret == OPENSSL_NPN_NEGOTIATED ? SSL_TLSEXT_ERR_OK
: SSL_TLSEXT_ERR_NOACK;
: SSL_TLSEXT_ERR_ALERT_FATAL;
}
#ifndef OPENSSL_NO_SRP