io_uring: acquire 'mm' for task_work for SQPOLL

If we're unlucky with timing, we could be running task_work after
having dropped the memory context in the sq thread. Since dropping
the context requires a runnable task state, we cannot reliably drop
it as part of our check-for-work loop in io_sq_thread(). Instead,
abstract out the mm acquire for the sq thread into a helper, and call
it from the async task work handler.

Cc: stable@vger.kernel.org # v5.7
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4256,6 +4256,28 @@ static void io_async_queue_proc(struct file *file, struct wait_queue_head *head,
 	__io_queue_proc(&pt->req->apoll->poll, pt, head);
 }
 
+static void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
+{
+	struct mm_struct *mm = current->mm;
+
+	if (mm) {
+		kthread_unuse_mm(mm);
+		mmput(mm);
+	}
+}
+
+static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
+				   struct io_kiocb *req)
+{
+	if (io_op_defs[req->opcode].needs_mm && !current->mm) {
+		if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
+			return -EFAULT;
+		kthread_use_mm(ctx->sqo_mm);
+	}
+
+	return 0;
+}
+
 static void io_async_task_func(struct callback_head *cb)
 {
 	struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
@@ -4290,11 +4312,16 @@ static void io_async_task_func(struct callback_head *cb)
 
 	if (!canceled) {
 		__set_current_state(TASK_RUNNING);
+		if (io_sq_thread_acquire_mm(ctx, req)) {
+			io_cqring_add_event(req, -EFAULT);
+			goto end_req;
+		}
 		mutex_lock(&ctx->uring_lock);
 		__io_queue_sqe(req, NULL);
 		mutex_unlock(&ctx->uring_lock);
 	} else {
 		io_cqring_ev_posted(ctx);
+end_req:
 		req_set_fail_links(req);
 		io_double_put_req(req);
 	}
@@ -5841,11 +5868,8 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	if (unlikely(req->opcode >= IORING_OP_LAST))
 		return -EINVAL;
 
-	if (io_op_defs[req->opcode].needs_mm && !current->mm) {
-		if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
-			return -EFAULT;
-		kthread_use_mm(ctx->sqo_mm);
-	}
+	if (unlikely(io_sq_thread_acquire_mm(ctx, req)))
+		return -EFAULT;
 
 	sqe_flags = READ_ONCE(sqe->flags);
 	/* enforce forwards compatibility on users */
@@ -5954,16 +5978,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 	return submitted;
 }
 
-static inline void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
-{
-	struct mm_struct *mm = current->mm;
-
-	if (mm) {
-		kthread_unuse_mm(mm);
-		mmput(mm);
-	}
-}
-
 static int io_sq_thread(void *data)
 {
 	struct io_ring_ctx *ctx = data;