io_uring: ensure consistent view of original task ->mm from SQPOLL

Ensure we get a valid view of the task mm, by using task_lock() when
attempting to grab the original task mm.

Reported-by: syzbot+b57abf7ee60829090495@syzkaller.appspotmail.com
Fixes: 2aede0e4 ("io_uring: stash ctx task reference for SQPOLL")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -995,20 +995,33 @@ static void io_sq_thread_drop_mm(void)
 	if (mm) {
 		kthread_unuse_mm(mm);
 		mmput(mm);
+		current->mm = NULL;
 	}
 }
 
 static int __io_sq_thread_acquire_mm(struct io_ring_ctx *ctx)
 {
-	if (!current->mm) {
-		if (unlikely(!(ctx->flags & IORING_SETUP_SQPOLL) ||
-			     !ctx->sqo_task->mm ||
-			     !mmget_not_zero(ctx->sqo_task->mm)))
-			return -EFAULT;
-		kthread_use_mm(ctx->sqo_task->mm);
+	struct mm_struct *mm;
+
+	if (current->mm)
+		return 0;
+
+	/* Should never happen */
+	if (unlikely(!(ctx->flags & IORING_SETUP_SQPOLL)))
+		return -EFAULT;
+
+	task_lock(ctx->sqo_task);
+	mm = ctx->sqo_task->mm;
+	if (unlikely(!mm || !mmget_not_zero(mm)))
+		mm = NULL;
+	task_unlock(ctx->sqo_task);
+
+	if (mm) {
+		kthread_use_mm(mm);
+		return 0;
 	}
 
-	return 0;
+	return -EFAULT;
 }
 
 static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,