References: bsc#944463

Subject: ui/vnc: limit client_cut_text msg payload size
From: Peter Lieven pl@kamp.de Mon Jun 30 10:07:54 2014 +0200
Date: Tue Jul 1 13:26:40 2014 +0200:
Git: f9a70e79391f6d7c2a912d785239ee8effc1922d

currently a malicious client could define a payload
size of 2^32 - 1 bytes and send up to that size of
data to the vnc server. The server would allocated
that amount of memory which could easily create an
out of memory condition.

This patch limits the payload size to 1MB max.

Please note that client_cut_text messages are currently
silently ignored.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Index: xen-4.5.1-testing/tools/qemu-xen-traditional-dir-remote/vnc.c
===================================================================
--- xen-4.5.1-testing.orig/tools/qemu-xen-traditional-dir-remote/vnc.c
+++ xen-4.5.1-testing/tools/qemu-xen-traditional-dir-remote/vnc.c
@@ -1779,14 +1779,21 @@ static int protocol_client_msg(VncState
 	pointer_event(vs, read_u8(data, 1), read_u16(data, 2), read_u16(data, 4));
 	break;
     case 6:
-	if (len == 1)
+	if (len == 1) {
 	    return 8;
-
+	}
 	if (len == 8) {
 	    uint32_t v;
 	    v = read_u32(data, 4);
-	    if (v)
+	    if (v > (1 << 20)) {
+		VNC_DEBUG("vnc: client_cut_text msg payload has %u bytes"
+			     " which exceeds our limit of 1MB.", v);
+		vnc_client_error(vs);
+		break;
+	    }
+	    if (v > 0) {
 		return 8 + v;
+	    }
 	}
 
 	client_cut_text(vs, read_u32(data, 4), (char *)(data + 8));