powerpc/powernv/vas: Release reference to tgid during window close

The kernel handles the NX fault by updating CSB or sending
signal to process. In multithread applications, children can
open VAS windows and can exit without closing them. But the
parent can continue to send NX requests with these windows. To
prevent pid reuse, reference will be taken on pid and tgid
when the window is opened and release them during window close.

The current code is not releasing the tgid reference which can
cause pid leak and this patch fixes the issue.

Fixes: db1c08a740635 ("powerpc/vas: Take reference to PID and mm for user space windows")
Cc: [e-mail] # 5.8+
Reported-by: Nicholas Piggin <[e-mail]>
Signed-off-by: Haren Myneni <[e-mail]>
Reviewed-by: Nicholas Piggin <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/platforms/powernv/vas-window.c

···
 		/*
 		 * Process closes window during exit. In the case of
 		 * multithread application, the child thread can open
-		 * window and can exit without closing it. Expects parent
-		 * thread to use and close the window. So do not need
-		 * to take pid reference for parent thread.
+		 * window and can exit without closing it. so takes tgid
+		 * reference until window closed to make sure tgid is not
+		 * reused.
 		 */
 		txwin->tgid = find_get_pid(task_tgid_vnr(current));
 		/*
···
 	/* if send window, drop reference to matching receive window */
 	if (window->tx_win) {
 		if (window->user_win) {
-			/* Drop references to pid and mm */
+			/* Drop references to pid. tgid and mm */
 			put_pid(window->pid);
+			put_pid(window->tgid);
 			if (window->mm) {
 				mm_context_remove_vas_window(window->mm);
 				mmdrop(window->mm);