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 69B20E7F14F for ; Wed, 27 Sep 2023 21:48:08 +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 60AC97BAE8 for ; Wed, 27 Sep 2023 21:48:06 +0000 (UTC) Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id E537A9867C7 for ; Wed, 27 Sep 2023 21:48:05 +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 BF5DE98670A; Wed, 27 Sep 2023 21:48:05 +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 A11B7986381 for ; Wed, 27 Sep 2023 21:48:05 +0000 (UTC) X-Virus-Scanned: amavisd-new at kavi.com X-MC-Unique: CZWJm4B6OjmEvYZHq2WGlQ-1 X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1695851282; x=1696456082; 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=GJdqh1Nsx/AW2uJ9Zd6A54iKOH/w/dukxrlzRRWZsaU=; b=odfj5W4aYtIM8zkb/xw8ujkO+IdCpBFkdonZWe5Uem1P7HMm8wgtCbtgudsbGQEQeE 0R/enpHanYQgQvdjYa9aYgRAov/1nZjTbPya62J5m4vJT0Ou2e4wwnZTBnm19zv25zwV Ir04dHywiu4pACQYsqrX/ZyXBh+vJ0eih25fSh/hBQpQNEteL23pemu5gK9STCFxOZXt XBmu3SLmI0X0vHLteg1hgeTIv04m21g41Qe1BpdE/NC2E1ZwVVoDHrgKeNycldsXaZPt e3RbyzuoF+TSM2RjUs9yWwyb8ThmgMIQ8ztsb9rM3JKmJ59UGTUrTnaJ2aE+sYCh5CzD hCaw== X-Gm-Message-State: AOJu0YzyfiQvuzWEgY5H5K8U+9tPtZL6FcZwoB7/MoJNuoHlkrm08sdv mfQBp4b0jUCIk0zNB269trPRG9kb2sr7gd++rPmBMihaPnQWaTbC2dUqHiRIsJmOz5Q/5+0fDUW KZsxhm3uOBlOi/5jIOn/2dSqTQj6lagvkelJk X-Received: by 2002:a7b:cb97:0:b0:405:4daa:6e3d with SMTP id m23-20020a7bcb97000000b004054daa6e3dmr3132410wmi.39.1695851281713; Wed, 27 Sep 2023 14:48:01 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHhIZf5FYxTkQ4ZGupijATRptuihR/7qjoQQuuMFgEfbRe9N6GufSfF/r3opEMaJPAK/3mw1Q== X-Received: by 2002:a7b:cb97:0:b0:405:4daa:6e3d with SMTP id m23-20020a7bcb97000000b004054daa6e3dmr3132400wmi.39.1695851281325; Wed, 27 Sep 2023 14:48:01 -0700 (PDT) Date: Wed, 27 Sep 2023 17:47:57 -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: <20230927174611-mutt-send-email-mst@kernel.org> References: <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> <20230922115920-mutt-send-email-mst@kernel.org> <19967fde-8893-4fe8-9f9a-7ce49bf38772@amazon.es> MIME-Version: 1.0 In-Reply-To: <19967fde-8893-4fe8-9f9a-7ce49bf38772@amazon.es> 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 Wed, Sep 27, 2023 at 12:43:20PM +0200, Babis Chalios wrote: > On 22/9/23 18:01, Michael S. Tsirkin wrote: > > 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. > > > > > > > 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 > > > > Heh no I just came up with the name. Will write up in a couple > > of days, but the idea is that driver does get_user_pages, > > adds buffer to queue, and device will remember the address > > and change that memory on a snapshot. If there are buffers > > in the queue it will also use these to tell driver, > > but if there are no buffers then it won't. > > That sounds like a nice mechanism. However in our case the page > holding the counter that gets increased by the hardware is a kernel > page. > > The reason for that is that things other than us (virtio-rng) might > want to notify for leak events. For example, I think that Jason > intended to use this mechanism to periodically notify user-space > PRNGs that they need to reseed. > > Cheers, > Babis Now I'm lost. when you write, e.g.: 4. vcpu 0: get getrandom() entropy and cache epoch value how does vcpu access the epoch? -- MST --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org