From patchwork Thu May 4 03:59:32 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Herbert Xu X-Patchwork-Id: 13230757 X-Patchwork-Delegate: herbert@gondor.apana.org.au Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ACFB9C77B78 for ; Thu, 4 May 2023 03:59:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229553AbjEDD7r (ORCPT ); Wed, 3 May 2023 23:59:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59046 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229514AbjEDD7r (ORCPT ); Wed, 3 May 2023 23:59:47 -0400 Received: from 167-179-156-38.a7b39c.syd.nbn.aussiebb.net (167-179-156-38.a7b39c.syd.nbn.aussiebb.net [167.179.156.38]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 64A6FE45; Wed, 3 May 2023 20:59:45 -0700 (PDT) Received: from loth.rohan.me.apana.org.au ([192.168.167.2]) by formenos.hmeau.com with smtp (Exim 4.94.2 #2 (Debian)) id 1puQ7v-0051wS-Ps; Thu, 04 May 2023 11:59:33 +0800 Received: by loth.rohan.me.apana.org.au (sSMTP sendmail emulation); Thu, 04 May 2023 11:59:32 +0800 Date: Thu, 4 May 2023 11:59:32 +0800 From: Herbert Xu To: "Michael S. Tsirkin" Cc: Dmitry Vyukov , syzbot , davem@davemloft.net, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, olivia@selenic.com, syzkaller-bugs@googlegroups.com, Jason Wang , Laurent Vivier , Rusty Russell Subject: [v2 PATCH] hwrng: virtio - Fix race on data_avail and actual data Message-ID: References: <00000000000050327205f9d993b2@google.com> <20230503073220-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20230503073220-mutt-send-email-mst@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On Wed, May 03, 2023 at 07:37:00AM -0400, Michael S. Tsirkin wrote: > > On the surface of it, it looks like you removed this store > which isn't described in the commit log. > I do not, offhand, remember why we stored 0 in data_idx here > when we also zero it in request_entropy. > It was added with Yes I removed because it's redundant. But you're right I'll add a note about it in the log: ---8<--- The virtio rng device kicks off a new entropy request whenever the data available reaches zero. When a new request occurs at the end of a read operation, that is, when the result of that request is only needed by the next reader, then there is a race between the writing of the new data and the next reader. This is because there is no synchronisation whatsoever between the writer and the reader. Fix this by writing data_avail with smp_store_release and reading it with smp_load_acquire when we first enter read. The subsequent reads are safe because they're either protected by the first load acquire, or by the completion mechanism. Also remove the redundant zeroing of data_idx in random_recv_done (data_idx must already be zero at this point) and data_avail in request_entropy (ditto). Reported-by: syzbot+726dc8c62c3536431ceb@syzkaller.appspotmail.com Fixes: f7f510ec1957 ("virtio: An entropy device, as suggested by hpa.") Signed-off-by: Herbert Xu Acked-by: Michael S. Tsirkin diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c index f7690e0f92ed..e41a84e6b4b5 100644 --- a/drivers/char/hw_random/virtio-rng.c +++ b/drivers/char/hw_random/virtio-rng.c @@ -4,6 +4,7 @@ * Copyright (C) 2007, 2008 Rusty Russell IBM Corporation */ +#include #include #include #include @@ -37,13 +38,13 @@ struct virtrng_info { static void random_recv_done(struct virtqueue *vq) { struct virtrng_info *vi = vq->vdev->priv; + unsigned int len; /* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */ - if (!virtqueue_get_buf(vi->vq, &vi->data_avail)) + if (!virtqueue_get_buf(vi->vq, &len)) return; - vi->data_idx = 0; - + smp_store_release(&vi->data_avail, len); complete(&vi->have_data); } @@ -52,7 +53,6 @@ static void request_entropy(struct virtrng_info *vi) struct scatterlist sg; reinit_completion(&vi->have_data); - vi->data_avail = 0; vi->data_idx = 0; sg_init_one(&sg, vi->data, sizeof(vi->data)); @@ -88,7 +88,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait) read = 0; /* copy available data */ - if (vi->data_avail) { + if (smp_load_acquire(&vi->data_avail)) { chunk = copy_data(vi, buf, size); size -= chunk; read += chunk;