io_uring: fix possible race condition against REQ_F_NEED_CLEANUP

to #28736503

commit 6f2cc1664db20676069cff27a461ccc97dbfd114 upstream

In io_read() or io_write(), when io request is submitted successfully,
it'll go through the below sequence:

    kfree(iovec);
    req->flags &= ~REQ_F_NEED_CLEANUP;
    return ret;

But clearing REQ_F_NEED_CLEANUP might be unsafe. The io request may
already have been completed, and then io_complete_rw_iopoll()
and io_complete_rw() will be called, both of which will also modify
req->flags if needed. This causes a race condition, with concurrent
non-atomic modification of req->flags.

To eliminate this race, in io_read() or io_write(), if io request is
submitted successfully, we don't remove REQ_F_NEED_CLEANUP flag. If
REQ_F_NEED_CLEANUP is set, we'll leave __io_req_aux_free() to the
iovec cleanup work correspondingly.

Cc: stable@vger.kernel.org
Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

fs/io_uring.c

@@ -2657,8 +2657,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
 		}
 	}
 out_free:
-	kfree(iovec);
-	req->flags &= ~REQ_F_NEED_CLEANUP;
+	if (!(req->flags & REQ_F_NEED_CLEANUP))
+		kfree(iovec);
 	return ret;
 }
 
@@ -2780,8 +2780,8 @@ static int io_write(struct io_kiocb *req, bool force_nonblock)
 		}
 	}
 out_free:
-	req->flags &= ~REQ_F_NEED_CLEANUP;
-	kfree(iovec);
+	if (!(req->flags & REQ_F_NEED_CLEANUP))
+		kfree(iovec);
 	return ret;
 }