Fix race in finding available vnc port

The qemu driver contains a subtle race in the logic to find next
available vnc port.  Currently it iterates through all available ports
and returns the first for which bind(2) succeeds.  However it is possible
that a previously issued port has not yet been bound by qemu, resulting
in the same port used for a subsequent domain.

This patch addresses the race by using a simple bitmap to "reserve" the
ports allocated by libvirt.

V2:
  - Put port bitmap in struct qemud_driver
  - Initialize bitmap in qemudStartup

V3:
  - Check for failure of virBitmapGetBit
  - Additional check for port != -1 before calling virbitmapClearBit

V4:
  - Check for failure of virBitmap{Set,Clear}Bit

src/qemu/qemu_conf.h

@@ -39,6 +39,7 @@
 # include "pci.h"
 # include "cpu_conf.h"
 # include "driver.h"
+# include "bitmap.h"
 
 # define qemudDebug(fmt, ...) do {} while(0)
 
@@ -153,6 +154,8 @@ struct qemud_driver {
     char *saveImageFormat;
 
     pciDeviceList *activePciHostdevs;
+
+    virBitmapPtr reservedVNCPorts;
 };
 
 typedef struct _qemuDomainPCIAddressSet qemuDomainPCIAddressSet;

src/qemu/qemu_driver.c

@@ -1485,6 +1485,11 @@ qemudStartup(int privileged) {
          virEventAddTimeout(-1, qemuDomainEventFlush, qemu_driver, NULL)) < 0)
         goto error;
 
+    /* Allocate bitmap for vnc port reservation */
+    if ((qemu_driver->reservedVNCPorts =
+         virBitmapAlloc(QEMU_VNC_PORT_MAX - QEMU_VNC_PORT_MIN)) == NULL)
+        goto out_of_memory;
+
     if (privileged) {
         if (virAsprintf(&qemu_driver->logDir,
                         "%s/log/libvirt/qemu", LOCAL_STATE_DIR) == -1)
@@ -1781,6 +1786,7 @@ qemudShutdown(void) {
     virCapabilitiesFree(qemu_driver->caps);
 
     virDomainObjListDeinit(&qemu_driver->domains);
+    virBitmapFree(qemu_driver->reservedVNCPorts);
 
     VIR_FREE(qemu_driver->securityDriverName);
     VIR_FREE(qemu_driver->logDir);
@@ -2638,13 +2644,22 @@ qemuInitPCIAddresses(struct qemud_driver *driver,
     return ret;
 }
 
-static int qemudNextFreeVNCPort(struct qemud_driver *driver ATTRIBUTE_UNUSED) {
+static int qemudNextFreeVNCPort(struct qemud_driver *driver) {
     int i;
 
     for (i = QEMU_VNC_PORT_MIN; i < QEMU_VNC_PORT_MAX; i++) {
         int fd;
         int reuse = 1;
         struct sockaddr_in addr;
+        bool used = false;
+
+        if (virBitmapGetBit(driver->reservedVNCPorts,
+                            i - QEMU_VNC_PORT_MIN, &used) < 0)
+            VIR_DEBUG("virBitmapGetBit failed on bit %d", i - QEMU_VNC_PORT_MIN);
+
+        if (used)
+            continue;
+
         addr.sin_family = AF_INET;
         addr.sin_port = htons(i);
         addr.sin_addr.s_addr = htonl(INADDR_ANY);
@@ -2660,6 +2675,12 @@ static int qemudNextFreeVNCPort(struct qemud_driver *driver ATTRIBUTE_UNUSED) {
         if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) == 0) {
             /* Not in use, lets grab it */
             close(fd);
+            /* Add port to bitmap of reserved ports */
+            if (virBitmapSetBit(driver->reservedVNCPorts,
+                                i - QEMU_VNC_PORT_MIN) < 0) {
+                VIR_DEBUG("virBitmapSetBit failed on bit %d",
+                          i - QEMU_VNC_PORT_MIN);
+            }
             return i;
         }
         close(fd);
@@ -3700,6 +3721,21 @@ retry:
 
     qemudRemoveDomainStatus(driver, vm);
 
+    /* Remove VNC port from port reservation bitmap, but only if it was
+       reserved by the driver (autoport=yes)
+    */
+    if ((vm->def->ngraphics == 1) &&
+        vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
+        vm->def->graphics[0]->data.vnc.autoport &&
+        vm->def->graphics[0]->data.vnc.port != -1) {
+        if (virBitmapClearBit(driver->reservedVNCPorts,
+                              vm->def->graphics[0]->data.vnc.port - \
+                              QEMU_VNC_PORT_MIN) < 0) {
+            VIR_DEBUG("virBitmapClearBit failed on bit %d",
+                      vm->def->graphics[0]->data.vnc.port - QEMU_VNC_PORT_MIN);
+       }
+    }
+
     vm->pid = -1;
     vm->def->id = -1;
     vm->state = VIR_DOMAIN_SHUTOFF;