hw/cxl: Avoid accesses beyond the end of cel_log.

Add a check that the requested offset + length does not go beyond the end
of the cel_log.

Whilst the cci->cel_log is large enough to include all possible CEL
entries, the guest might still ask for entries beyond the end of it.
Move the comment to this new check rather than before the check on the
type of log requested.

Reported-by: Esifiel <esifiel@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20241101133917.27634-7-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/cxl/cxl-mailbox-utils.c

@@ -937,24 +937,28 @@ static CXLRetCode cmd_logs_get_log(const struct cxl_cmd *cmd,
 
     get_log = (void *)payload_in;
 
+    if (get_log->length > cci->payload_max) {
+        return CXL_MBOX_INVALID_INPUT;
+    }
+
+    if (!qemu_uuid_is_equal(&get_log->uuid, &cel_uuid)) {
+        return CXL_MBOX_INVALID_LOG;
+    }
+
     /*
      * CXL r3.1 Section 8.2.9.5.2: Get Log (Opcode 0401h)
      *   The device shall return Invalid Input if the Offset or Length
      *   fields attempt to access beyond the size of the log as reported by Get
-     *   Supported Logs.
+     *   Supported Log.
      *
-     * The CEL buffer is large enough to fit all commands in the emulation, so
-     * the only possible failure would be if the mailbox itself isn't big
-     * enough.
+     * Only valid for there to be one entry per opcode, but the length + offset
+     * may still be greater than that if the inputs are not valid and so access
+     * beyond the end of cci->cel_log.
      */
-    if (get_log->length > cci->payload_max) {
+    if ((uint64_t)get_log->offset + get_log->length >= sizeof(cci->cel_log)) {
         return CXL_MBOX_INVALID_INPUT;
     }
 
-    if (!qemu_uuid_is_equal(&get_log->uuid, &cel_uuid)) {
-        return CXL_MBOX_INVALID_LOG;
-    }
-
     /* Store off everything to local variables so we can wipe out the payload */
     *len_out = get_log->length;