vtls: fix ssl session cache race condition

Sessionid cache management is inseparable from managing individual
session lifetimes. E.g. for reference-counted sessions (like those in
SChannel and OpenSSL engines) every session addition and removal
should be accompanied with refcount increment and decrement
respectively. Failing to do so synchronously leads to a race condition
that causes symptoms like use-after-free and memory corruption.
This commit:
 - makes existing session cache locking explicit, thus allowing
   individual engines to manage lock's scope.
 - fixes OpenSSL and SChannel engines by putting refcount management
   inside this lock's scope in relevant places.
 - adds these explicit locking calls to other engines that use
   sessionid cache to accommodate for this change. Note, however,
   that it is unknown whether any of these engines could also have
   this race.

Bug: https://github.com/curl/curl/issues/815
Fixes #815
Closes #847

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/vtls/axtls.c b/lib/vtls/axtls.c
index 0afcfaa58..df6ae9562 100644
--- a/lib/vtls/axtls.c
+++ b/lib/vtls/axtls.c
@@ -259,14 +259,18 @@ static CURLcode connect_prep(struct connectdata *conn, int sockindex)
    */
 
   /* In axTLS, handshaking happens inside ssl_client_new. */
+  Curl_ssl_sessionid_lock(conn);
   if(!Curl_ssl_getsessionid(conn, (void **) &ssl_sessionid, &ssl_idsize)) {
     /* we got a session id, use it! */
     infof (data, "SSL re-using session ID\n");
     ssl = ssl_client_new(ssl_ctx, conn->sock[sockindex],
                          ssl_sessionid, (uint8_t)ssl_idsize);
+    Curl_ssl_sessionid_unlock();
   }
-  else
+  else {
+    Curl_ssl_sessionid_unlock();
     ssl = ssl_client_new(ssl_ctx, conn->sock[sockindex], NULL, 0);
+  }
 
   conn->ssl[sockindex].ssl = ssl;
   return CURLE_OK;
@@ -381,9 +385,11 @@ static CURLcode connect_finish(struct connectdata *conn, int sockindex)
   /* Put our freshly minted SSL session in cache */
   ssl_idsize = ssl_get_session_id_size(ssl);
   ssl_sessionid = ssl_get_session_id(ssl);
+  Curl_ssl_sessionid_lock(conn);
   if(Curl_ssl_addsessionid(conn, (void *) ssl_sessionid, ssl_idsize)
      != CURLE_OK)
     infof (data, "failed to add session to cache\n");
+  Curl_ssl_sessionid_unlock(conn);
 
   return CURLE_OK;
 }