Use a write lock during ossl_provider_find()

A "find" operation on a stack can end up sorting the underlying stack. In
this case it is necessary to use a "write" lock to synchronise access to
the stack across multiple threads.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16980)

crypto/provider_core.c

diff --git a/crypto/provider_core.c b/crypto/provider_core.c
index c9dba6d..a46a96c 100644
--- a/crypto/provider_core.c
+++ b/crypto/provider_core.c
@@ -423,7 +423,11 @@
 #endif
 
         tmpl.name = (char *)name;
-        if (!CRYPTO_THREAD_read_lock(store->lock))
+        /*
+         * A "find" operation can sort the stack, and therefore a write lock is
+         * required.
+         */
+        if (!CRYPTO_THREAD_write_lock(store->lock))
             return NULL;
         if ((i = sk_OSSL_PROVIDER_find(store->providers, &tmpl)) != -1)
             prov = sk_OSSL_PROVIDER_value(store->providers, i);

doc/man3/DEFINE_STACK_OF.pod

diff --git a/doc/man3/DEFINE_STACK_OF.pod b/doc/man3/DEFINE_STACK_OF.pod
index d715246..ec9eda8 100644
--- a/doc/man3/DEFINE_STACK_OF.pod
+++ b/doc/man3/DEFINE_STACK_OF.pod
@@ -178,7 +178,10 @@
 B<sk_I<TYPE>_find>() returns the index of a matching element or B<-1> if there
 is no match. Note that, in this case the comparison function will usually
 compare the values pointed to rather than the pointers themselves and
-the order of elements in I<sk> can change.
+the order of elements in I<sk> can change. Note that because the stack may be
+sorted as the result of a B<sk_I<TYPE>_find>() call, if a lock is being used to
+synchronise access to the stack across multiple threads, then that lock must be
+a "write" lock.
 
 B<sk_I<TYPE>_find_ex>() operates like B<sk_I<TYPE>_find>() except when a
 comparison function has been specified and no matching element is found.