mirror of https://github.com/openssl/openssl.git
106 lines
4.4 KiB
Plaintext
106 lines
4.4 KiB
Plaintext
=pod
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=head1 NAME
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openssl-threads - Overview of thread safety in OpenSSL
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=head1 DESCRIPTION
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In this man page, we use the term B<thread-safe> to indicate that an
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object or function can be used by multiple threads at the same time.
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OpenSSL can be built with or without threads support. The most important
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use of this support is so that OpenSSL itself can use a single consistent
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API, as shown in L<CRYPTO_THREAD_run_once(3)/EXAMPLES>.
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Multi-platform applications can also use this API.
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In particular, being configured for threads support does not imply that
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all OpenSSL objects are thread-safe.
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To emphasize: I<most objects are not safe for simultaneous use>.
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Exceptions to this should be documented on the specific manual pages, and
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some general high-level guidance is given here.
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One major use of the OpenSSL thread API is to implement reference counting.
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Many objects within OpenSSL are reference-counted, so resources are not
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released, until the last reference is removed.
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References are often increased automatically (such as when an B<X509>
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certificate object is added into an B<X509_STORE> trust store).
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There is often an B<I<object>_up_ref>() function that can be used to increase
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the reference count.
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Failure to match B<I<object>_up_ref>() calls with the right number of
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B<I<object>_free>() calls is a common source of memory leaks when a program
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exits.
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Many objects have set and get API's to set attributes in the object.
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A C<set0> passes ownership from the caller to the object and a
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C<get0> returns a pointer but the attribute ownership
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remains with the object and a reference to it is returned.
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A C<set1> or C<get1> function does not change the ownership, but instead
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updates the attribute's reference count so that the object is shared
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between the caller and the object; the caller must free the returned
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attribute when finished.
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Functions that involve attributes that have reference counts themselves,
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but are named with just C<set> or C<get> are historical; and the documentation
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must state how the references are handled.
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Get methods are often thread-safe as long as the ownership requirements are
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met and shared objects are not modified.
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Set methods, or modifying shared objects, are generally not thread-safe
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as discussed below.
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Objects are thread-safe
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as long as the API's being invoked don't modify the object; in this
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case the parameter is usually marked in the API as C<const>.
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Not all parameters are marked this way.
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Note that a C<const> declaration does not mean immutable; for example
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L<X509_cmp(3)> takes pointers to C<const> objects, but the implementation
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uses a C cast to remove that so it can lock objects, generate and cache
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a DER encoding, and so on.
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Another instance of thread-safety is when updates to an object's
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internal state, such as cached values, are done with locks.
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One example of this is the reference counting API's described above.
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In all cases, however, it is generally not safe for one thread to
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mutate an object, such as setting elements of a private or public key,
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while another thread is using that object, such as verifying a signature.
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The same API's can usually be used simultaneously on different objects
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without interference.
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For example, two threads can calculate a signature using two different
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B<EVP_PKEY_CTX> objects.
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For implicit global state or singletons, thread-safety depends on the facility.
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The L<CRYPTO_secure_malloc(3)> and related API's have their own lock,
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while L<CRYPTO_malloc(3)> assumes the underlying platform allocation
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will do any necessary locking.
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Some API's, such as L<NCONF_load(3)> and related do no locking at all;
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this can be considered a bug.
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A separate, although related, issue is modifying "factory" objects
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when other objects have been created from that.
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For example, an B<SSL_CTX> object created by L<SSL_CTX_new(3)> is used
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to create per-connection B<SSL> objects by calling L<SSL_new(3)>.
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In this specific case, and probably for factory methods in general, it is
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not safe to modify the factory object after it has been used to create
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other objects.
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=head1 SEE ALSO
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CRYPTO_THREAD_run_once(3),
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local system threads documentation.
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=head1 BUGS
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This page is admittedly very incomplete.
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=head1 COPYRIGHT
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Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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