Hi,
I've a simple program (used as a stress tool) that creates a number of
threads, and uses a shared SSL_CTX to create an SSL object for each
thread. My understanding of the threading model is that this fits with
the intended thread-safety model of the library.
Sporadically, I get "SSL_R_WRONG_CIPHER_RETURNED" errors as the threads
attempt to establish their SSL layers. I think I've tracked this down to
a lack of thread safety in ssl3_get_server_hello: From
openssl-SNAP-20081117, ssl/s3_clnt.c, line 848:
> sk=ssl_get_ciphers_by_id(s);
> i=sk_SSL_CIPHER_find(sk,c);
The first line returns either the "cipher_list_by_id" field of the SSL
structure, or, if null, it's context's equivalent field. For my case,
the context's cipherlist is returned. So, sk_SSL_CIPHER_find() is
invoked on the stack that's shared by all threads.
The implementation of sk_SSL_CIPHER_find is a macro that ends up
invoking "sk_find", in turn invoking "internal_find". This ends up
calling sk_sort() before doing a binary search of the stack.
This means that sk_sort can be invoked from different threads
simultaneously without any lock protection. A quick analysis of the
SSL_CTX structure at the point that I get my "wrong cipher returned"
error shows that the "sorted" field of the stack is indeed set, but when
running through the list of ciphers, I see that the ID fields are
mostly, but not completely, sorted.
So, am I missing something or is this indeed a bug?
Assuming its a bug, I'm not familiar enough with OpenSSL to be sure what
the best fix is: I would imagine ensuring that the stack is sorted in
SSL_CTX_new and other places that change the SSL_CTXs cipher list would
be the minimal required change that wouldn't involve locking the SSL_CTX
for each operation. It might make sense to have ssl_create_cipher_list()
always sort the stack it intends to return after setting its comparison
function, if not doing so is just delaying the inevitable most of the
time anyhow.
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