BUG/CRITICAL: using HTTP information in tcp-request content may crash the process

During normal HTTP request processing, request buffers are realigned if
there are less than global.maxrewrite bytes available after them, in
order to leave enough room for rewriting headers after the request. This
is done in http_wait_for_request().

However, if some HTTP inspection happens during a "tcp-request content"
rule, this realignment is not performed. In theory this is not a problem
because empty buffers are always aligned and TCP inspection happens at
the beginning of a connection. But with HTTP keep-alive, it also happens
at the beginning of each subsequent request. So if a second request was
pipelined by the client before the first one had a chance to be forwarded,
the second request will not be realigned. Then, http_wait_for_request()
will not perform such a realignment either because the request was
already parsed and marked as such. The consequence of this, is that the
rewrite of a sufficient number of such pipelined, unaligned requests may
leave less room past the request been processed than the configured
reserve, which can lead to a buffer overflow if request processing appends
some data past the end of the buffer.

A number of conditions are required for the bug to be triggered :
  - HTTP keep-alive must be enabled ;
  - HTTP inspection in TCP rules must be used ;
  - some request appending rules are needed (reqadd, x-forwarded-for)
  - since empty buffers are always realigned, the client must pipeline
    enough requests so that the buffer always contains something till
    the point where there is no more room for rewriting.

While such a configuration is quite unlikely to be met (which is
confirmed by the bug's lifetime), a few people do use these features
together for very specific usages. And more importantly, writing such
a configuration and the request to attack it is trivial.

A quick workaround consists in forcing keep-alive off by adding
"option httpclose" or "option forceclose" in the frontend. Alternatively,
disabling HTTP-based TCP inspection rules enough if the application
supports it.

At first glance, this bug does not look like it could lead to remote code
execution, as the overflowing part is controlled by the configuration and
not by the user. But some deeper analysis should be performed to confirm
this. And anyway, corrupting the process' memory and crashing it is quite
trivial.

Special thanks go to Yves Lafon from the W3C who reported this bug and
deployed significant efforts to collect the relevant data needed to
understand it in less than one week.

CVE-2013-1912 was assigned to this issue.

Note that 1.4 is also affected so the fix must be backported.

diff --git a/src/proto_http.c b/src/proto_http.c
index f4502bfe139ea70c4e80c262a701fb338faaad9d..4ab3f93b95c85dc1100483e090e145e1ed91afdb 100644 (file)
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -8360,6 +8360,13 @@ acl_prefetch_http(struct proxy *px, struct session *s, void *l7, unsigned int op
                if (unlikely(!s->req))
                        return 0;
 
+               /* If the buffer does not leave enough free space at the end,
+                * we must first realign it.
+                */
+               if (s->req->buf->p > s->req->buf->data &&
+                   s->req->buf->i + s->req->buf->p > s->req->buf->data + s->req->buf->size - global.tune.maxrewrite)
+                       buffer_slow_realign(s->req->buf);
+
                if (unlikely(txn->req.msg_state < HTTP_MSG_BODY)) {
                        if ((msg->msg_state == HTTP_MSG_ERROR) ||
                            buffer_full(s->req->buf, global.tune.maxrewrite)) {
@@ -8387,6 +8394,18 @@ acl_prefetch_http(struct proxy *px, struct session *s, void *l7, unsigned int op
                         * preparation to perform so that further checks can rely
                         * on HTTP tests.
                         */
+
+                       /* If the request was parsed but was too large, we must absolutely
+                        * return an error so that it is not processed. At the moment this
+                        * cannot happen, but if the parsers are to change in the future,
+                        * we want this check to be maintained.
+                        */
+                       if (unlikely(s->req->buf->i + s->req->buf->p >
+                                    s->req->buf->data + s->req->buf->size - global.tune.maxrewrite)) {
+                               msg->msg_state = HTTP_MSG_ERROR;
+                               return 1;
+                       }
+
                        txn->meth = find_http_meth(msg->chn->buf->p, msg->sl.rq.m_l);
                        if (txn->meth == HTTP_METH_GET || txn->meth == HTTP_METH_HEAD)
                                s->flags |= SN_REDIRECTABLE;