dump: add fallback KDBG using in Windows dump

KdDebuggerDataBlock may be encrypted in guest memory and dump will be
useless in this case. But guest driver can obtain decrypted KDBG and
expose its address through BugcheckParameter1 field in raw header.
After this patch, QEMU will be able to use fallback KdDebuggerDataBlock.
Signed-off-by: NViktor Prutyanov <viktor.prutyanov@virtuozzo.com>
Message-Id: <20180517162342.4330-4-viktor.prutyanov@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

win_dump.c

@@ -144,21 +144,37 @@ static void check_kdbg(WinDumpHeader64 *h, Error **errp)
 {
     const char OwnerTag[] = "KDBG";
     char read_OwnerTag[4];
+    uint64_t KdDebuggerDataBlock = h->KdDebuggerDataBlock;
+    bool try_fallback = true;
 
+try_again:
     if (cpu_memory_rw_debug(first_cpu,
-            h->KdDebuggerDataBlock + KDBG_OWNER_TAG_OFFSET64,
+            KdDebuggerDataBlock + KDBG_OWNER_TAG_OFFSET64,
             (uint8_t *)&read_OwnerTag, sizeof(read_OwnerTag), 0)) {
         error_setg(errp, "win-dump: failed to read OwnerTag");
         return;
     }
 
     if (memcmp(read_OwnerTag, OwnerTag, sizeof(read_OwnerTag))) {
-        error_setg(errp, "win-dump: invalid KDBG OwnerTag,"
-                         " expected '%.4s', got '%.4s',"
-                         " KdDebuggerDataBlock seems to be encrypted",
-                         OwnerTag, read_OwnerTag);
-        return;
+        if (try_fallback) {
+            /*
+             * If attempt to use original KDBG failed
+             * (most likely because of its encryption),
+             * we try to use KDBG obtained by guest driver.
+             */
+
+            KdDebuggerDataBlock = h->BugcheckParameter1;
+            try_fallback = false;
+            goto try_again;
+        } else {
+            error_setg(errp, "win-dump: invalid KDBG OwnerTag,"
+                             " expected '%.4s', got '%.4s'",
+                             OwnerTag, read_OwnerTag);
+            return;
+        }
     }
+
+    h->KdDebuggerDataBlock = KdDebuggerDataBlock;
 }
 
 void create_win_dump(DumpState *s, Error **errp)