From mboxrd@z Thu Jan 1 00:00:00 1970 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 ws5-mx01.kavi.com (ws5-mx01.kavi.com [34.193.7.191]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 39C48C4167D for ; Thu, 2 Nov 2023 11:25:23 +0000 (UTC) Received: from lists.oasis-open.org (oasis.ws5.connectedcommunity.org [10.110.1.242]) by ws5-mx01.kavi.com (Postfix) with ESMTP id 741E160D22 for ; Thu, 2 Nov 2023 11:25:22 +0000 (UTC) Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 63D84986C08 for ; Thu, 2 Nov 2023 11:25:22 +0000 (UTC) Received: from host09.ws5.connectedcommunity.org (host09.ws5.connectedcommunity.org [10.110.1.97]) by lists.oasis-open.org (Postfix) with QMQP id 538CE986BF4; Thu, 2 Nov 2023 11:25:22 +0000 (UTC) Mailing-List: contact virtio-dev-help@lists.oasis-open.org; run by ezmlm List-ID: Sender: Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 460B0986BFA for ; Thu, 2 Nov 2023 11:25:22 +0000 (UTC) X-Virus-Scanned: amavisd-new at kavi.com X-MC-Unique: 4hjMaHJENbm_R4qGeBYdSA-1 X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698924316; x=1699529116; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=Ih27gt5hpIV4i/CvAiwBfRZQMYWZSmP6MP4xnpNHSYA=; b=YLdDx0x6M/3U0zhfE+8UKAYhVo7DBhA6mYhDEIeS+OqvclyuKqu6MWbNMmwGYqzvC6 s/4/6ivfpdxZcLdRLaUuiYgt/TEGCkbrBsCIDeo01gy8vWXkP4BsKgwSYO0XYyx0eb2W E5XDU3JmopPt6Lz4CTxt3Ps8nA+mWQY7RWhSIFsRHnQp+mBskkJCCd3SoWC7kn8a4BjG MM/7Mqp0XqB3o79ACc5MuwuL7PzEx+2S5hkGNmu5RM1tt4B2b0/vbfhzMdSbCkMtoiNV Mau9KdmRVS6sWZYxHz/5ciOKaLkupDi+xKv3Dy9Y4QqubIZLt7b+0FRNq3E99AvgypoY qxNQ== X-Gm-Message-State: AOJu0Yx66tri05tr4E0UY0/KaCfT4k+YmWQYzD/jpE93ZmwQgRKRpCSb u6P2X30XtPsK80f2/iix0eRud12/0VDKRZs7iLCh7z0I2rojK8AIJYCsT+KMMjR4RQH95XwGwYT wPAKrd5RrzksFZ+OKoc+q/Z/xRJBG X-Received: by 2002:adf:e94d:0:b0:32d:857c:d51d with SMTP id m13-20020adfe94d000000b0032d857cd51dmr13341349wrn.60.1698924316307; Thu, 02 Nov 2023 04:25:16 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEWKG5LJFxc6gIwenDIXRv11XJDUmqPzKAcJX7oHLr4t2MtogRezs+C6R3ag7ORGInBTuwF1Q== X-Received: by 2002:adf:e94d:0:b0:32d:857c:d51d with SMTP id m13-20020adfe94d000000b0032d857cd51dmr13341335wrn.60.1698924315946; Thu, 02 Nov 2023 04:25:15 -0700 (PDT) Date: Thu, 2 Nov 2023 07:25:10 -0400 From: "Michael S. Tsirkin" To: Babis Chalios Cc: virtio-comment@lists.oasis-open.org, virtio-dev@lists.oasis-open.org, "Cali, Marco" , "Graf (AWS), Alexander" , "Jason A. Donenfeld" , aams@amazon.de Message-ID: <20231102072055-mutt-send-email-mst@kernel.org> References: <20230918091506-mutt-send-email-mst@kernel.org> <20230918100355-mutt-send-email-mst@kernel.org> <4ab1b10e-4524-4b6c-a3df-863360fd225d@amazon.es> <20230919055811-mutt-send-email-mst@kernel.org> <4c6d6710-43c8-402b-9ae6-d2f4cafbe9a7@amazon.es> <20230922110329-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Subject: Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support On Fri, Sep 22, 2023 at 05:40:50PM +0200, Babis Chalios wrote: > > > On 22/9/23 17:06, Michael S. Tsirkin wrote: > > On Tue, Sep 19, 2023 at 12:11:37PM +0200, Babis Chalios wrote: > > > On 19/9/23 12:01, Michael S. Tsirkin wrote: > > > > On Tue, Sep 19, 2023 at 09:32:08AM +0200, Babis Chalios wrote: > > > > > Resending to fix e-mail formatting issues (sorry for the spam) > > > > > > > > > > On 18/9/23 18:30, Babis Chalios wrote: > > > > > > > > > > Yes, that's what the driver does now in the RFC patch. > > > > > > > > > > However, this just > > > > > > > > > > decreases > > > > > > > > > > the race window, it doesn't eliminate it. If a third > > > > > > > > > > leak event happens it > > > > > > > > > > might not > > > > > > > > > > find any buffers to use: > > > > > > > > > > > > > > > > > > > > 1. available buffers to queue 1-X > > > > > > > > > > 2. available buffers to queue X > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 3. poll queue X > > > > > > > > > > 4. used buffers in queue X <- leak event 1 will > > > > > > > > > > use buffers in X > > > > > > > > > > 5. avail buffers in queue X > > > > > > > > > > 6. poll queue 1-X <- leak event 2 will > > > > > > > > > > use buffers in 1-X > > > > > > > > > > 7. used buffers in queue 1-X > > > > > > > > > > 8. avail buffers in queue 1-X > > > > > > > > > > <- leak event 3 (it > > > > > > > > > > needs buffers in X, race with step 5) > > > > > > > > > > 9. goto 3 > > > > > > > > > I don't get it. we added buffers in step 5. > > > > > > > > What if the leak event 3 arrives before step 5 had time to > > > > > > > > actually add the > > > > > > > > buffers in X and make > > > > > > > > them visible to the device? > > > > > > > Then it will see a single event in 1-X instead of two events. A leak is > > > > > > > a leak though, I don't see does it matter how many triggered. > > > > > > > > > > > > So the scenario I have in mind is the following: > > > > > > > > > > (Epoch here is terminology that I used in the Linux RFC. It is a value > > > > > maintained by random.c > > > > > that changes every time a leak event happens). > > > > > > > > > > 1. add buffers to 1-X > > > > > 2. add buffers to X > > > > > 3. poll queue X > > > > > 4. vcpu 0: get getrandom() entropy and cache epoch value > > > > > 5. Device: First snapshot, uses buffers in X > > > > > 6. vcpu 1: sees used buffers > > > > > 7. Device: Second snapshot, uses buffers in 1-X > > > > > 8. vcpu 0: getrandom() observes new epoch value & caches it > > > > > 9. Device: Third snapshot, no buffers in either queue, (vcpu 1 from step 6 > > > > > has not yet finished adding new buffers). > > > > > 10. vcpu 1 adds new buffer in X > > > > > 11. vcpu 0: getrandom() will not see new epoch and gets stale entropy. > > > > > > > > > > > > > > > In this succession of events, when the third snapshot will happen, the > > > > > device won't find > > > > > any buffers in either queue, so it won't increase the RNG epoch value. So, > > > > > any entropy > > > > > gathered after step 8 will be the same across all snapshots. Am I missing > > > > > something? > > > > > > > > > > Cheers, > > > > > Babis > > > > > > > > > Yes but notice how this is followed by: > > > > > > > > 12. vcpu 1: sees used buffers in 1-X > > > > > > > > Driver can notify getrandom I guess? > > > It could, but then we have the exact race condition that VMGENID had, > > > userspace has already consumed stale entropy and there's nothing we > > > can do about that. > > > > > > Although this is indeed a corner case, it feels like it beats the purpose > > > of having the hardware update directly userspace (via copy on leak). > > > > > > How do you feel about the proposal a couple of emails back? It looks to > > > me that it avoids completely the race condition. > > > > > > Cheers, > > > Babis > > It does. The problem of course is that this means that e.g. > > taking a snapshot of a guest that is stuck won't work well. > > That is true, but does it matter? The intention of the proposal > is that if it is not safe to take snapshots (i.e. no buffers in the > queue) don't take snapshots. OK. Basically I think if there's a way for device to detect that guest is stuck and not refilling the queue in a timely manner, then we are ok - host will make its own decisions on whether to snapshot or not. However, I feel in that case we need a way to create a big backlog of buffers for guest to fill such that this ring empty condition is very unlikely. One or even 2 queues does not seem enough then. For example, I can see a "stop" command that will tell device: "stop consuming buffers" and device will stop consuming buffers until the next leak event. > > I have been thinking of adding MAP/UNMAP descriptors for > > a while now. Thus it will be possible to modify > > userspace memory without consuming buffers. > > Would something like this solve the problem? > > I am not familiar with MAP/UNMAP descriptors. Is there > a link where I can read about them? > > Cheers, > Babis --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org