[stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections

Fri Sep 27 18:57:35 CEST 2019

I’ve been digging around the code and I am no longer sure whether stunnel is at fault or it is actually an issue in OpenSSL!

load_key_engine() in ctx.c shoves the key into the SSL context – SSL_CTX_use_PrivateKey(section->ctx, pkey).  So the key should be freed when that context is disposed of.

The SIGHUP ends up in case SIGNAL_RELOAD_CONFIG of signal_pipe_dispatch(), which calls options_free()  [stunnel.c line 830].

Eventually there is an SSL_CTX_free() call, but it looks like this does not actually free the private key.  Ephemeral keys are freed and loads of other stuff, but not the private key associated with the certificate.

I may try to build a hacked OpenSSL on Monday to see what happens if SSL_CTX_free also frees the private key…

Regards,
Andrew.

From: Brent Kimberley [mailto:brent_kimberley at rogers.com]
Sent: Friday, September 27, 2019 4:29 PM
To: stunnel-users at stunnel.org; Lynch, Andrew
Subject: Re: stunnel-users Digest, Vol 182, Issue 14

>> Over time, after numerous reloads of stunnel (kill -HUP) the HSM reports that its connection table is full.
>>  Logging from the engine shows that stunnel is never freeing the keys and therefore the engine is not closing the associated sessions with the HSM.  Each stunnel reload opens 70 new sessions until eventually the HSM's configured limit is exceeded.

Where does the signal dispatcher free keys on SIG_HUP?
https://github.com/mtrojnar/stunnel/blob/master/src/stunnel.c

Date: Fri, 27 Sep 2019 07:23:29 +0000
From: "Lynch, Andrew" <andrew.lynch at atos.net<mailto:andrew.lynch at atos.net>>
To: Eric Eberhard <flash at vicsmba.com<mailto:flash at vicsmba.com>>
Cc: "stunnel-users at stunnel.org<mailto:stunnel-users at stunnel.org>" <stunnel-users at stunnel.org<mailto:stunnel-users at stunnel.org>>
Subject: Re: [stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections

Hi Eric,

Thank you for your suggestion.  It is certainly worth looking into, although I suspect it may be impractical in our environment.

Now we have a single stunnel.conf with 70 services which would translate into 70 additional entries in inetd.conf.  Each of those entries then needs to have its own reduced stunnel.conf so that it will only load the cert and key for its endpoint.

The stunnel HOWTO recommends daemon mode over inetd mode due to the overhead for forking and SSL initialization.  We would have additional overhead for engine init and HSM connection.

Regards,
Andrew.

-----Original Message-----
From: Eric Eberhard [mailto:flash at vicsmba.com<mailto:flash at vicsmba.com>]
Sent: Thursday, September 26, 2019 7:20 PM
To: Lynch, Andrew; stunnel-users at stunnel.org<mailto:stunnel-users at stunnel.org>
Subject: RE: [stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections?

Try running in inetd mode -- even if you don't like this you will learn something.  Inetd will close connections as needed.  E

-----Original Message-----
From: stunnel-users [mailto:stunnel-users-bounces at stunnel.org<mailto:stunnel-users-bounces at stunnel.org>] On Behalf Of Lynch, Andrew
Sent: Thursday, September 26, 2019 5:56 AM
To: stunnel-users at stunnel.org<mailto:stunnel-users at stunnel.org>
Subject: [stunnel-users] stunnel with OpenSSL engine: reload leaks HSM connections?

Hi,

We are using stunnel as a server to terminate incoming TLS connections.  The config has around 70 services with certificates whose EC private keys are stored in an HSM and accessed using an OpenSSL engine.

Over time, after numerous reloads of stunnel (kill -HUP) the HSM reports that its connection table is full.  Logging from the engine shows that stunnel is never freeing the keys and therefore the engine is not closing the associated sessions with the HSM.  Each stunnel reload opens 70 new sessions until eventually the HSM's configured limit is exceeded.

This behaviour has been observed on Suse Enterprise Linux 12.3 with the system-provided stunnel-5.00-4.3.4, but I can reproduce it with my own build of the current version 5.55.

Is this a known issue?  It appears that other (ephemeral) keys are being freed, just not those associated with the service certificates.

Currently our workaround is to perform a full restart instead of a reload.
This closes all HSM sessions when the process terminates, but of course it also kills any open client connections so it can only be done during the scheduled maintenance windows.

Regards,
Andrew.

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