scsi/scsi_bus: fix races in REPORT LUNS

Currently scsi_target_emulate_report_luns iterates over the child device list
twice, and there is no guarantee that this list is the same in both iterations.

The reason for iterating twice is that the first iteration calculates
how much memory to allocate.  However if we use a dynamic array we can
avoid iterating twice, and therefore we avoid this race.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1866707
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200913160259.32145-10-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201006123904.610658-14-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index eda8cb7e70f5d0d4e807cbff0551caf4a7a0af95..b901e701f081ffc5637fac0cdedbb285d4b4eeaa 100644 (file)
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -438,19 +438,23 @@ struct SCSITargetReq {
 static void store_lun(uint8_t *outbuf, int lun)
 {
     if (lun < 256) {
+        /* Simple logical unit addressing method*/
+        outbuf[0] = 0;
         outbuf[1] = lun;
-        return;
+    } else {
+        /* Flat space addressing method */
+        outbuf[0] = 0x40 | (lun >> 8);
+        outbuf[1] = (lun & 255);
     }
-    outbuf[1] = (lun & 255);
-    outbuf[0] = (lun >> 8) | 0x40;
 }
 
 static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
 {
     BusChild *kid;
-    int i, len, n;
     int channel, id;
-    bool found_lun0;
+    uint8_t tmp[8] = {0};
+    int len = 0;
+    GByteArray *buf;
 
     if (r->req.cmd.xfer < 16) {
         return false;
@@ -458,46 +462,40 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
     if (r->req.cmd.buf[2] > 2) {
         return false;
     }
+
+    /* reserve space for 63 LUNs*/
+    buf = g_byte_array_sized_new(512);
+
     channel = r->req.dev->channel;
     id = r->req.dev->id;
-    found_lun0 = false;
-    n = 0;
 
-    RCU_READ_LOCK_GUARD();
+    /* add size (will be updated later to correct value */
+    g_byte_array_append(buf, tmp, 8);
+    len += 8;
 
-    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
-        DeviceState *qdev = kid->child;
-        SCSIDevice *dev = SCSI_DEVICE(qdev);
+    /* add LUN0 */
+    g_byte_array_append(buf, tmp, 8);
+    len += 8;
 
-        if (dev->channel == channel && dev->id == id) {
-            if (dev->lun == 0) {
-                found_lun0 = true;
+    WITH_RCU_READ_LOCK_GUARD() {
+        QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
+            DeviceState *qdev = kid->child;
+            SCSIDevice *dev = SCSI_DEVICE(qdev);
+
+            if (dev->channel == channel && dev->id == id && dev->lun != 0) {
+                store_lun(tmp, dev->lun);
+                g_byte_array_append(buf, tmp, 8);
+                len += 8;
             }
-            n += 8;
         }
     }
-    if (!found_lun0) {
-        n += 8;
-    }
-
-    scsi_target_alloc_buf(&r->req, n + 8);
-
-    len = MIN(n + 8, r->req.cmd.xfer & ~7);
-    memset(r->buf, 0, len);
-    stl_be_p(&r->buf[0], n);
-    i = found_lun0 ? 8 : 16;
-    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
-        DeviceState *qdev = kid->child;
-        SCSIDevice *dev = SCSI_DEVICE(qdev);
 
-        if (dev->channel == channel && dev->id == id) {
-            store_lun(&r->buf[i], dev->lun);
-            i += 8;
-        }
-    }
+    r->buf_len = len;
+    r->buf = g_byte_array_free(buf, FALSE);
+    r->len = MIN(len, r->req.cmd.xfer & ~7);
 
-    assert(i == n + 8);
-    r->len = len;
+    /* store the LUN list length */
+    stl_be_p(&r->buf[0], len - 8);
     return true;
 }