aio: restore locking of ioctx list on removal

Commit 36f55889
"aio: refcounting cleanup" resulted in ioctx_lock not being held
during ctx removal, leaving the list susceptible to corruptions.

In mainline kernel the issue went away as a side effect of
db446a08 "aio: convert the ioctx list to
table lookup v3".

Fix the problem by restoring appropriate locking.
Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Reported-by: Eryu Guan <eguan@redhat.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/aio.c

@@ -423,10 +423,12 @@ static void kill_ioctx_rcu(struct rcu_head *head)
  *	when the processes owning a context have all exited to encourage
  *	the rapid destruction of the kioctx.
  */
-static void kill_ioctx(struct kioctx *ctx)
+static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
 {
 	if (!atomic_xchg(&ctx->dead, 1)) {
+		spin_lock(&mm->ioctx_lock);
 		hlist_del_rcu(&ctx->list);
+		spin_unlock(&mm->ioctx_lock);
 
 		/*
 		 * It'd be more correct to do this in free_ioctx(), after all
@@ -494,7 +496,7 @@ void exit_aio(struct mm_struct *mm)
 		 */
 		ctx->mmap_size = 0;
 
-		kill_ioctx(ctx);
+		kill_ioctx(mm, ctx);
 	}
 }
 
@@ -852,7 +854,7 @@ SYSCALL_DEFINE2(io_setup, unsigned, nr_events, aio_context_t __user *, ctxp)
 	if (!IS_ERR(ioctx)) {
 		ret = put_user(ioctx->user_id, ctxp);
 		if (ret)
-			kill_ioctx(ioctx);
+			kill_ioctx(current->mm, ioctx);
 		put_ioctx(ioctx);
 	}
 
@@ -870,7 +872,7 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
 {
 	struct kioctx *ioctx = lookup_ioctx(ctx);
 	if (likely(NULL != ioctx)) {
-		kill_ioctx(ioctx);
+		kill_ioctx(current->mm, ioctx);
 		put_ioctx(ioctx);
 		return 0;
 	}