io_uring: initialize io_buffer_list head when shared ring is unregistered

We use ->buf_pages != 0 to tell if this is a shared buffer ring or a
classic provided buffer group. If we unregister the shared ring and
then attempt to use it, buf_pages is zero yet the classic list head
isn't properly initialized. This causes io_buffer_select() to think
that we have classic buffers available, but then we crash when we try
and get one from the list.

Just initialize the list if we unregister a shared buffer ring, leaving
it in a sane state for either re-registration or for attempting to use
it. And do the same for the initial setup from the classic path.

Fixes: c7fb19428d ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4969,6 +4969,8 @@ static int __io_remove_buffers(struct io_ring_ctx *ctx,
 		kvfree(bl->buf_pages);
 		bl->buf_pages = NULL;
 		bl->buf_nr_pages = 0;
+		/* make sure it's seen as empty */
+		INIT_LIST_HEAD(&bl->buf_list);
 		return i;
 	}
 
@@ -5156,6 +5158,7 @@ static int io_provide_buffers(struct io_kiocb *req, unsigned int issue_flags)
 			ret = -ENOMEM;
 			goto err;
 		}
+		INIT_LIST_HEAD(&bl->buf_list);
 		ret = io_buffer_add_list(ctx, bl, p->bgid);
 		if (ret) {
 			kfree(bl);