[PATCH v5 9/9] scsi/scsi_bus: fix races in REPORT LUNS

[Maxim Levitsky <mlevitsk@redhat.com>]

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 twise 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>
---
 hw/scsi/scsi-bus.c | 65 ++++++++++++++++++++++------------------------
 1 file changed, 31 insertions(+), 34 deletions(-)

diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index dfd503f7ef..84c145c724 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -439,19 +439,23 @@ 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;
@@ -459,48 +463,41 @@ 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();
+    /* 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;
-            }
-            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;
+    rcu_read_lock();
     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;
+        if (dev->channel == channel && dev->id == id && dev->lun != 0) {
+            store_lun(tmp, dev->lun);
+            g_byte_array_append(buf, tmp, 8);
+            len += 8;
         }
     }
-
     rcu_read_unlock();
 
-    assert(i == n + 8);
-    r->len = len;
+    r->buf_len = len;
+    r->buf = g_byte_array_free(buf, FALSE);
+    r->len = MIN(len, r->req.cmd.xfer & ~7);
+
+    /* store the LUN list length */
+    stl_be_p(&r->buf[0], len - 8);
+
     return true;
 }
 
-- 
2.26.2