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-wq.c

@@ -60,6 +60,7 @@ struct io_worker {
 	const struct cred *cur_creds;
 	const struct cred *saved_creds;
 	struct files_struct *restore_files;
+	struct nsproxy *restore_nsproxy;
 	struct fs_struct *restore_fs;
 };
 
@@ -153,6 +154,7 @@ static bool __io_worker_unuse(struct io_wqe *wqe, struct io_worker *worker)
 
 		task_lock(current);
 		current->files = worker->restore_files;
+		current->nsproxy = worker->restore_nsproxy;
 		task_unlock(current);
 	}
 
@@ -318,6 +320,7 @@ static void io_worker_start(struct io_wqe *wqe, struct io_worker *worker)
 
 	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
 	worker->restore_files = current->files;
+	worker->restore_nsproxy = current->nsproxy;
 	worker->restore_fs = current->fs;
 	io_wqe_inc_running(wqe, worker);
 }
@@ -454,6 +457,7 @@ static void io_impersonate_work(struct io_worker *worker,
 	if (work->files && current->files != work->files) {
 		task_lock(current);
 		current->files = work->files;
+		current->nsproxy = work->nsproxy;
 		task_unlock(current);
 	}
 	if (work->fs && current->fs != work->fs)

fs/io-wq.h

@@ -88,6 +88,7 @@ struct io_wq_work {
 	struct files_struct *files;
 	struct mm_struct *mm;
 	const struct cred *creds;
+	struct nsproxy *nsproxy;
 	struct fs_struct *fs;
 	unsigned long fsize;
 	unsigned flags;

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);