[Qemu-devel] [PATCH 2/2] CVE-2015-1779: limit size of HTTP headers from websockets clients

[Daniel P. Berrange]

The VNC server websockets decoder will read and buffer data from
websockets clients until it sees the end of the HTTP headers,
as indicated by \r\n\r\n. In theory this allows a malicious to
trick QEMU into consuming an arbitrary amount of RAM. In practice,
because QEMU runs g_strstr_len() across the buffered header data,
it will spend increasingly long burning CPU time searching for
the substring match and less & less time reading data. So while
this does cause arbitrary memory growth, the bigger problem is
that QEMU will be burning 100% of available CPU time.

A novnc websockets client typically sends headers of around
512 bytes in length. As such it is reasonable to place a 4096
byte limit on the amount of data buffered while searching for
the end of HTTP headers.

Signed-off-by: Daniel P. Berrange <address@hidden>
---
 ui/vnc-ws.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/ui/vnc-ws.c b/ui/vnc-ws.c
index e8146d0..525e2d7 100644
--- a/ui/vnc-ws.c
+++ b/ui/vnc-ws.c
@@ -81,8 +81,11 @@ void vncws_handshake_read(void *opaque)
     VncState *vs = opaque;
     uint8_t *handshake_end;
     long ret;
-    buffer_reserve(&vs->ws_input, 4096);
-    ret = vnc_client_read_buf(vs, buffer_end(&vs->ws_input), 4096);
+    /* Typical HTTP headers from novnc are 512 bytes, so limiting
+     * total header size to 4096 is easily enough. */
+    size_t want = 4096 - vs->ws_input.offset;
+    buffer_reserve(&vs->ws_input, want);
+    ret = vnc_client_read_buf(vs, buffer_end(&vs->ws_input), want);
 
     if (!ret) {
         if (vs->csock == -1) {
@@ -99,6 +102,9 @@ void vncws_handshake_read(void *opaque)
         vncws_process_handshake(vs, vs->ws_input.buffer, vs->ws_input.offset);
         buffer_advance(&vs->ws_input, handshake_end - vs->ws_input.buffer +
                 strlen(WS_HANDSHAKE_END));
+    } else if (vs->ws_input.offset >= 4096) {
+        VNC_DEBUG("End of headers not found in first 4096 bytes\n");
+        vnc_client_error(vs);
     }
 }
 
-- 
2.1.0