[openssl-users] Confirmation of what I believe to be true from docs and observation

Wed Jan 10 20:37:51 UTC 2018

On 1/10/2018 14:07, Benjamin Kaduk wrote:
> On 01/10/2018 08:41 AM, Karl Denninger wrote:
>> We start with a context that I load a dhparam file to (so I can take a
>> DH connection) along with an edh curve, then set an acceptable cipher
>> list for it to use.
>>
> Why not just use AUTO_DH (the only option for 1.1.0, IIRC)?
That's a reasonable change (and I'll go ahead and make it); the dhparam
was only there in the first place for those browsers and such that can't
negotiate EC (which my cipher selection set prefers.)
>> Assume I next manually load both the CA store (using
>> X509_STORE_add_cert as many times as necessary to load the
>> intermediate components and the root of trust) and then load the
>> cert/key pair (using SSL_CTX_use_certificate/SSL_CTX_use_PrivateKey)
>>
>> I then create some number of SSLs from that context to perform
>> communication with and all is well.
>>
>> Now I want to rekey that context for some reason.  It appears that
>> while I can add things to
>>
> Why do you need to rekey the context as opposed to making a new one?
I could make a new one (or more-specifically, destroy the existing one
and re-initialize it), but that is more-complicated as the application
in question is multi-threaded -- and it's not at all clear from the
documentation if I destroy a context that has SSLs that have been
generated from it will cause "bad things" to happen (like a deference on
a freed object!)

The reason I may want to rekey is that the cert/key pair used for that
context may have changed.  The user's certificate may have expired for
example (or been revoked) and I wish to reload it (and the matching key)
without having to shut down the software and restart it.

> In general, making configuration changes to an SSL_CTX after it has been
> used to generate SSL objects is a risky proposition -- the locking model
> does not really account for doing synchronization properly, and there
> might be some latent race conditions.  In practice, some operations are
> currently safe, but there is no authoritative list of which ones, and at
> least my personal recommendation is to not try to rely on it.
Assuming that there are SSL objects that are in use and I destroy and
re-generate the CTX from which it was generated, is *THAT* safe?  Or do
I need to flag all the in-use descendants and wait for them to be closed
(or force them closed and release them) before I can safely destroy the
context?
>> the CA chain trying to load the same component that is already in
>> there returns a failure (somewhat-expected; that is, it does not
>> overwrite but rather adds, and if you try to add what's already there
>> you get an error back) and there's no call to CLEAR the certificate
>> validation chain -- if I want to *replace* the validation chain I have
>> to destroy the context and initialize a new one from scratch.
>>
> N.B. that the X509_STORE_add_cert behavior when presented with a
> duplicate certificate changed in commit
> c0452248ea1a59a41023a4765ef7d9825e80a62b (from returning an error to
> doing nothing and returning success); this will be in 1.1.1.
>
> As to the desired behavior, there does not seem to be an API to remove
> an entry from an X509_STORE.  With the above caveat about thread-safety
> in mind, couldn't you just make a call to SSL_CTX_set{1}_cert_store() to
> replace the X509_STORE without tearing down the whole SSL_CTX?
Yeah, I didn't see one either.  I'm not particularly concerned about the
verification chain being able to be modified while "in-use"; that ought
not happen except in an extreme circumstance (e.g. the intermediate
cert's key is compromised and thus both it and the cert have to be
regenerated and re-distributed.)  If it DOES happen when the end-entity
cert and key are reloaded (as they've been signed from the new
intermediate) they'll fail to validate against the old chain and the
user will get plenty of notice that there's trouble.
>> It appears, however, that I *can* load over the top of a certificate
>> and private key of the same type and that's acceptable.  In other
>> words, if I have an RSA key/cert pair in the context and I load
>> another one, the first one is replaced.  This *looks* to be working ok
>> as far as I can tell and it doesn't appear to leak memory doing that
>> but it's not explicitly stated that this is considered acceptable
>> (rather than destroying and re-creating the context.)
>>
> The leaf certificate and private key are stored as arrays (for different
> types of certificates) of pointers in the associated CERT structure, and
> the "set" routines do not currently check for and error out if there is
> already one set.
Yes, but do they replace the pointer and, if they do, do they decrement
the reference counter on the existing one before replacing it so it will
get freed?   If yes then all is ok but if not then I need to destroy the
context otherwise I'm going to be leaking memory.
>> Is my understanding correct?
>>
>>
> Mostly ... but I am not sure that your desired workflow is wise.
>
> -Ben
If it's safe to destroy a context with potential SSL options from it
still in existence then it's pretty easy to do it the other way
otherwise I have some work to do in order to make sure all the potential
objects created from the parent have gone away before the old context is
destroyed.

Thanks in advance.

-- 
Karl Denninger
karl at denninger.net <mailto:karl at denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/
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