[PATCH] qemu: avoid denial of service reading from QEMU monitor (CVE-2018-xxxx)

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

We read from QEMU until seeing a \r\n pair to indicate a completed reply
or event. To avoid memory denial-of-service though, we must have a size
limit on amount of data we buffer. 10 MB is large enough that it ought
to cope with normal QEMU replies, and small enough that we're not
consuming unreasonable mem.

Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>
---
 src/qemu/qemu_monitor.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
index 046caf001c..85c7d68a13 100644
--- a/src/qemu/qemu_monitor.c
+++ b/src/qemu/qemu_monitor.c
@@ -55,6 +55,15 @@ VIR_LOG_INIT("qemu.qemu_monitor");
 #define DEBUG_IO 0
 #define DEBUG_RAW_IO 0
 
+/* We read from QEMU until seeing a \r\n pair to indicate a
+ * completed reply or event. To avoid memory denial-of-service
+ * though, we must have a size limit on amount of data we
+ * buffer. 10 MB is large enough that it ought to cope with
+ * normal QEMU replies, and small enough that we're not
+ * consuming unreasonable mem.
+ */
+#define QEMU_MONITOR_MAX_RESPONSE (10 * 1024 * 1024)
+
 struct _qemuMonitor {
     virObjectLockable parent;
 
@@ -575,6 +584,12 @@ qemuMonitorIORead(qemuMonitorPtr mon)
     int ret = 0;
 
     if (avail < 1024) {
+        if (mon->bufferLength >= QEMU_MONITOR_MAX_RESPONSE) {
+            virReportSystemError(ERANGE,
+                                 _("No complete monitor response found in %d bytes"),
+                                 QEMU_MONITOR_MAX_RESPONSE);
+            return -1;
+        }
         if (VIR_REALLOC_N(mon->buffer,
                           mon->bufferLength + 1024) < 0)
             return -1;
-- 
2.14.3

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