io_uring: reference ->nsproxy for file table commands

If we don't get and assign the namespace for the async work, then certain
paths just don't work properly (like /dev/stdin, /proc/mounts, etc).
Anything that references the current namespace of the given task should
be assigned for async work on behalf of that task.

Cc: stable@vger.kernel.org # v5.5+
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5678,6 +5678,7 @@ static void io_req_drop_files(struct io_kiocb *req)
 	spin_unlock_irqrestore(&ctx->inflight_lock, flags);
 	req->flags &= ~REQ_F_INFLIGHT;
 	put_files_struct(req->work.files);
+	put_nsproxy(req->work.nsproxy);
 	req->work.files = NULL;
 }
 
@@ -6086,6 +6087,8 @@ static int io_grab_files(struct io_kiocb *req)
 		return 0;
 
 	req->work.files = get_files_struct(current);
+	get_nsproxy(current->nsproxy);
+	req->work.nsproxy = current->nsproxy;
 	req->flags |= REQ_F_INFLIGHT;
 
 	spin_lock_irq(&ctx->inflight_lock);