Merge remote-tracking branch 'remotes/gkurz/tags/for-upstream' into staging

Fixes a potential use-after-free issue that could be triggered by a
misbehaving guest.

# gpg: Signature made Thu 08 Nov 2018 20:36:48 GMT
# gpg:                using RSA key 71D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <groug@kaod.org>"
# gpg:                 aka "Gregory Kurz <gregory.kurz@free.fr>"
# gpg:                 aka "[jpeg image of size 3330]"
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/for-upstream:
  9p: write lock path in v9fs_co_open2()
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/9pfs/cofile.c

@@ -140,10 +140,10 @@ int coroutine_fn v9fs_co_open2(V9fsPDU *pdu, V9fsFidState *fidp,
     cred.fc_gid = gid;
     /*
      * Hold the directory fid lock so that directory path name
-     * don't change. Read lock is fine because this fid cannot
-     * be used by any other operation.
+     * don't change. Take the write lock to be sure this fid
+     * cannot be used by another operation.
      */
-    v9fs_path_read_lock(s);
+    v9fs_path_write_lock(s);
     v9fs_co_run_in_worker(
         {
             err = s->ops->open2(&s->ctx, &fidp->path,