hw/9pfs/local: harden local_fid_fd() on FID types

local_fid_fd() returns fs->fd for any FID type that is not P9_FID_DIR.

Since P9_FID_XATTR and P9_FID_NONE share union V9fsFidOpenState, calling
local_fid_fd() on these types misinterprets xattr state as a file
descriptor, potentially leading to undefined behaviour or information
disclosure.

Even though we are catching these FID type mismatches on protocol level
in 9p.c already, previous patches proofed this to be error prone.

So let's add another safety layer in local_fid_fd() that would return -1
if the FID type would not possess a valid file descriptor, to prevent
wrong file descriptors from reaching fs backend calls.

Link: https://lore.kernel.org/qemu-devel/531f6b81bc1bf1a48c3d4afaa60a65db10511041.1781621428.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

hw/9pfs/9p-local.c

@@ -775,8 +775,11 @@ static int local_fid_fd(int fid_type, V9fsFidOpenState *fs)
 {
     if (fid_type == P9_FID_DIR) {
         return dirfd(fs->dir.stream);
-    } else {
+    } else if (fid_type == P9_FID_FILE) {
         return fs->fd;
+    } else {
+        errno = EBADF;
+        return -1;
     }
 }