openssl-users Digest, Vol 77, Issue 4

Sat Apr 3 08:41:04 UTC 2021

Thank You Paul and Matthias for your help.

The reason I am trying to have separate RAND_METHOD for two threads is, the first thread which runs DNS bind code registers for RAND_METHOD through dnssec module in it. It registers via either ENGINE_set_default_RAND() or RAND_set_rand_method() based on OPENSSL_NO_ENGINE is defined or not. But problem is, under some circumstances the random number generator enters into blocking mode and starts to wait for some events on some FDs and it blocks in select() system call. dst__entropy_getdata()  from bind code is doing this. I am not sure under what cases it enters into blocking mode.

So If I use this RND_METHOD in second thread (basically this thread does different task of handling DoT, Dns Over TLS, connections, which is not related to first thread wrt SSL functionalities), then while creating SSL_CTX this thread gets stuck in select() system call randomly (happens very rarely as decided by dst__entropy_getdata()); this can happen at any time of SSL connection lifetime whenever it wants to get random data.

I agree with you that we should have done this as separate process instead of new thread; but I am trying figure out if I can somehow avoid this situation.

As you mentioned, I tried to look into implementation of RAND_bytes() and drbg_bytes().

When SSL_CTX_new() calls RAND_bytes(), it calls RAND_get_rand_method() which returns RAND_METHOD set by bind thread. So if I avoid configuring RAND_METHOD in bind thread, then RAND_get_rand_method() will return rand_meth which is OpenSSL default RAND_METHOD; but if I do this change bind thread will move away from its RAND_METHOD functions and start using OpenSSL default functions which may change its behaviour.

So I am still confused how can I do bind thread to use its own RAND_METHOD and DoT thread to use default OpenSSL RAND_METHOD. It would be really helpful if you can explain this with little more details (are there any APIs I can call from one thread to use its specific RAND_METHOD but other threads continue to use OpenSSL default RAND_METHOD?).

Thank You,
Vishwanath M

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Subject: openssl-users Digest, Vol 77, Issue 4

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Today's Topics:

   1. Re: Regarding RAND_set_rand_method (Dr Paul Dale)
   2. RE: Regarding RAND_set_rand_method (Dr. Matthias St. Pierre)

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Message: 1
Date: Fri, 2 Apr 2021 16:51:28 +1000
From: Dr Paul Dale <pauli at openssl.org>
To: openssl-users at openssl.org
Subject: Re: Regarding RAND_set_rand_method
Message-ID: <1781ab4c-2e2b-fa3b-8b3c-fb4fc5bd3371 at openssl.org>
Content-Type: text/plain; charset="windows-1252"; Format="flowed"

There isn't an easy a way to do what you want in 1.1.1.
RAND_set_rand_method replaces the RNG for all of OpenSSL.? In theory
your RAND_METHOD could detect which thread it is running in and do
different things for each.? I'm not sure this is a good idea however.

Why aren't the random number from your first thread good enough for the
second?? Good random numbers are just that - random.? It should be
impossible to distinguish the two streams.

In OpenSSL 3.0 there are ways to achieve what you're wanting.

Pauli

On 2/4/21 4:24 pm, Vishwanath Mahajanshetty wrote:
>
> Hi,
>
> I have some doubts/questions on how to use methods (for ex:
> RAND_set_rand_method) in multi threaded application which use OpenSSL.
> In my application (running on OpenSSL 1.1.1d) there are two threads
> which use OpenSSL, both threads perform very different operations. The
> issue I am facing is as below:
>
> Thread T1 calls RAND_set_rand_method() and sets RAND_METHOD structure.
> This is very specific to T1s use case. When thread T2 wants to create
> SSL_CTX it calls SSL_CTX_new() which then calls RAND_priv_bytes(). I
> am observing that the function RAND_priv_bytes() is calling the
> function set by T1 by RAND_METHOD in RAND_set_rand_method().
>
> Essentially RAND_METHOD function set by thread T1 are getting called
> by thread T2.
>
> *Q1: I want to know is there any way to avoid this problem? I want
> thread T2 to call default RAND methods and avoid calling methods set
> by thread T1. This is not only for RAND methods, but for any other
> methods.*
>
> **
>
> Q2: Also, is it possible to run OpenSSL as separate instance per
> thread (where each thread can do its own OpenSSL initialization) so
> that they can avoid above mentioned problem?
>
> Thank you,
>
> Vishwanath M
>

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Message: 2
Date: Fri, 2 Apr 2021 11:27:53 +0000
From: "Dr. Matthias St. Pierre" <Matthias.St.Pierre at ncp-e.com>
To: Dr Paul Dale <pauli at openssl.org>, "openssl-users at openssl.org"
        <openssl-users at openssl.org>
Subject: RE: Regarding RAND_set_rand_method
Message-ID: <7056523443ae4f94bca32240c4f24533 at ncp-e.com>
Content-Type: text/plain; charset="us-ascii"

Re Q1: I want to know is there any way to avoid this problem? I want thread T2 to call default RAND methods and avoid calling methods set by thread T1. This is not only for RAND methods, but for any other methods.

First of all, I agree with Pauli: your first question should be, why do you need different random generators for different threads in the same application? Is this necessary, or are you overengineering?

Let me clarify some details about the RNG implemention in OpenSSL 1.1.1.: The RAND_METHOD interface itself is not thread aware. It is only the new default RAND_METHOD implementation (added in 1.1.1.) of OpenSSL (RAND_OpenSSL()), which supports thread local random generators. The implementation is based on deterministic random bit generators (DRBG) as described in NIST.SP.800-90Ar1. Wenn a thread calls RAND_bytes() (resp. RAND_priv_bytes()), the call is forwarded to the thread-specific DRBG instance. All per-thread instances reseed from a single global DRBG instance, which in turn reseeds from  from random sources provided by the operating system.

In your case, by replacing the RAND_METHOD, you are changing the complete RAND implementation for all threads. Moreover, you are completely responsible yourself for reseeding your RNG properly.

You could however implement a smarter RAND_METHOD which calls your specific RNG for T1 and delegates to the thread local DRBG (RAND_DRBG_get0_public() resp. RAND_DRBG_get0_private()) for all other threads. To get an idea how it can be done, take a look at the default implementation of RAND_bytes(),  drbg_bytes() in drbg_lib.c:

https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/crypto/rand/drbg_lib.c#L958-L970

Re Q2: Also, is it possible to run OpenSSL as separate instance per thread (where each thread can do its own OpenSSL initialization) so that they can avoid above mentioned problem?

No. If you really need something like that, you might want to consider splitting your two threads into two processes.

HTH,
Matthias

From: openssl-users <openssl-users-bounces at openssl.org> On Behalf Of Dr Paul Dale
Sent: Friday, April 2, 2021 8:51 AM
To: openssl-users at openssl.org
Subject: Re: Regarding RAND_set_rand_method

There isn't an easy a way to do what you want in 1.1.1.  RAND_set_rand_method replaces the RNG for all of OpenSSL.  In theory your RAND_METHOD could detect which thread it is running in and do different things for each.  I'm not sure this is a good idea however.

Why aren't the random number from your first thread good enough for the second?  Good random numbers are just that - random.  It should be impossible to distinguish the two streams.

In OpenSSL 3.0 there are ways to achieve what you're wanting.

Pauli
On 2/4/21 4:24 pm, Vishwanath Mahajanshetty wrote:
Hi,

I have some doubts/questions on how to use methods (for ex: RAND_set_rand_method) in multi threaded application which use OpenSSL. In my application (running on OpenSSL 1.1.1d) there are two threads which use OpenSSL, both threads perform very different operations. The issue I am facing is as below:

Thread T1 calls RAND_set_rand_method() and sets RAND_METHOD structure. This is very specific to T1s use case. When thread T2 wants to create SSL_CTX it calls SSL_CTX_new() which then calls RAND_priv_bytes(). I am observing that the function RAND_priv_bytes() is calling the function set by T1 by RAND_METHOD in RAND_set_rand_method().

Essentially RAND_METHOD function set by thread T1 are getting called by thread T2.

Q1: I want to know is there any way to avoid this problem? I want thread T2 to call default RAND methods and avoid calling methods set by thread T1. This is not only for RAND methods, but for any other methods.

Q2: Also, is it possible to run OpenSSL as separate instance per thread (where each thread can do its own OpenSSL initialization) so that they can avoid above mentioned problem?

Thank you,
Vishwanath M

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