Allocate regex in request's mempoll in @rx execute

If regex contains variable substitution, function
msre_op_rx_execute(), which handles execution of @rx operator,
will compile regex on each request (because variable, and this
regex, can be different each time). It does so by calling
msc_pregcomp_ex(), which accepts a mem pool because it does
allocations for regex data. Until this fix, msre_op_rx_execute()
passed rule->ruleset->mp to msc_pregcomp_ex(), which is a ruleset
mempool that persists as long as ruleset lives - which means it
is long-living and will persist until ModSecurity is
stopped/restarted. This created a memory leak because regex data
for non-constand regexes was allocated again to each request and
not freed after request was processed.
Fix that by passing msr->mp mempool to msc_pregcomp_ex() instead,
which is a mempool of record corresponding to a tx being processed
and will be freed as soon as processing is done.

Author: asterite3

apache2/re_operators.c

@@ -1018,7 +1018,7 @@ static int msre_op_rx_execute(modsec_rec *msr, msre_rule *rule, msre_var *var, c
                 msr_log(msr, 6, "Escaping pattern [%s]",pattern);
             }
 
-            regex = msc_pregcomp_ex(rule->ruleset->mp, pattern, PCRE_DOTALL | PCRE_DOLLAR_ENDONLY, &errptr, &erroffset, msc_pcre_match_limit, msc_pcre_match_limit_recursion);
+            regex = msc_pregcomp_ex(msr->mp, pattern, PCRE_DOTALL | PCRE_DOLLAR_ENDONLY, &errptr, &erroffset, msc_pcre_match_limit, msc_pcre_match_limit_recursion);
             if (regex == NULL) {
                 *error_msg = apr_psprintf(rule->ruleset->mp, "Error compiling pattern (offset %d): %s",
                         erroffset, errptr);